Bing Maps in an MVC Application

Here is a simple example of how to use Bing Maps in an MVC application.

I am using Bing Maps V8 which is the current version of Microsoft.

Also, nothing that document.ready will fire long before the map script loads as it loads asynchronously.  Sometimes document.ready will fire before the page is loaded which means the map div might not even be available. To overcome this, we are using the callback parameter of the map script UR: for example:

http://www.bing.com/api/maps/mapcontrol?callback=LoadMap
Continue reading Bing Maps in an MVC Application

Async Unit Tests

In theory, Async unit testing seems easy, run the Async, wait until it is finished and look at the results.  But as you will find out it is not that easy.

Here is the official approach to Async unit testing

[TestMethod]
public void FourDividedByTwoIsTwo()
{
    GeneralThreadAffineContext.Run(async () =>
    {
        int result = await MyClass.Divide(4, 2);
        Assert.AreEqual(2, result);
    });
}
    
[TestMethod]
[ExpectedException(typeof(DivideByZeroException))]
public void DenominatorIsZeroThrowsDivideByZero()
{
    GeneralThreadAffineContext.Run(async () =>
    {
        await MyClass.Divide(4, 0);
    });
}

Hang on what is GeneralThreadAffineContext, it a Utility code originally distributed as part of the Async CTP, and the project file can be found here: AsyncTestUtilities

Original article: Async Unit Tests, Part 2: The Right Way

Generate C# Classes from JSON Responses

Well, as web developers, we are always dealing with JSON data which are coming from different sources. Either we are serialising our entities, or it’s coming from the external sources like third-party services and so on.

If the data is coming from an external source, one standard requirement is to always de-serialise it back to the data model to be able to process the data. Creating data model for JSON data is not the most exciting work in the world, especially when the data model is a bit nested and complex.

Fortunately, there is a very nice feature in Visual Studio which makes the life much more manageable. This feature is called Paste Special.

To take advantage, you first need to Copy the JSON data. Imaging there is JSON data as follow:

{
 "glossary": {
 "title": "example glossary",
 "GlossDiv": {
 "title": "S",
 "GlossList": {
 "GlossEntry": {
 "ID": "SGML",
 "SortAs": "SGML",
 "GlossTerm": "Standard Generalized Markup Language",
 "Acronym": "SGML",
 "Abbrev": "ISO 8879:1986",
 "GlossDef": {
 "para": "A meta-markup language, used to create markup languages such as DocBook.",
 "GlossSeeAlso": ["GML", "XML"]
 },
 "GlossSee": "markup"
 }
 }
 }
 }
}

And we require creating the data model for that. To do so, just Copy the data in memory and go to Visual Studio, create a new class (or an existing one where we intend to have our data model).

From the Edit menu in Visual Studio, select the Paste Special and from the submenu, Paste JSON As Classes.

Then, the data model will generate as follow:

public class Rootobject
 {
 public Glossary glossary { get; set; }
 }

public class Glossary
 {
 public string title { get; set; }
 public Glossdiv GlossDiv { get; set; }
 }

public class Glossdiv
 {
 public string title { get; set; }
 public Glosslist GlossList { get; set; }
 }

public class Glosslist
 {
 public Glossentry GlossEntry { get; set; }
 }

public class Glossentry
 {
 public string ID { get; set; }
 public string SortAs { get; set; }
 public string GlossTerm { get; set; }
 public string Acronym { get; set; }
 public string Abbrev { get; set; }
 public Glossdef GlossDef { get; set; }
 public string GlossSee { get; set; }
 }

public class Glossdef
 {
 public string para { get; set; }
 public string[] GlossSeeAlso { get; set; }
 }

As if by magic you see the initial data model has been created for us

As you may have noticed, there is one more option under Paste Special menu item, named Paste XML As Classes. This item does the same thing but for XML data. That means you need to copy your XML data to the memory and from the Paste Special menu item, choose Paste XML As Classes this time, to have your data model generated.

Original post ‘Paste Special’: a less well-known feature in Visual Studio

Repository Pattern – for the REST API

The Repository Pattern used to be the next big thing when it was used, but over time it got replaced by frameworks such as the Entity Framework and LINQ, which provided much more functionality and flexibility.

I started working on an external customers REST API then I realised that the Repository Pattern would work perfectly here.

Let’s recap the Repository Pattern.

The Repository Pattern has gained quite a bit of popularity since it was first introduced as a part of Domain-Driven Design in 2004. Primarily, it provides an abstraction of data, so that your application can work with a pure abstraction that has an interface approximating that of a collection. Adding, removing, updating, and selecting items from this collection is done through a series of straightforward methods, without the need to deal with database concerns like connections, commands, cursors, or readers. Using this pattern can help achieve loose coupling and can keep domain objects persistence ignorant. Although the pattern is prevalent (or perhaps because of this), it is also frequently misunderstood and misused. There are many different ways to implement the Repository pattern. Let’s consider a few of them, and their merits and drawbacks.

Repository Per Entity or Business Object

The most straightforward approach, especially with an existing system, is to create a new Repository implementation for each business object you need to store to or retrieve from your persistence layer. Further, you should only implement the specific methods you are calling in your application. Avoid the trap of creating a “standard” repository class, base class, or default interface that you must implement for all repositories. Yes, if you need to have an Update or a Delete method, you should strive to make its interface consistent (does Delete take an ID, or does it take the object itself?). Don’t implement a Delete method on your LookupTableRepository that you’re only ever going to be calling List(). The most significant benefit of this approach is YAGNI – you won’t waste any time implementing methods that never get called.

Generic Repository Interface

Another approach is to go ahead and create a simple, generic interface for your Repository. You can constrain what kind of types it works with to be of a specific type or to implement a particular interface (e.g. ensuring it has an Id property, as is done below using a base class). An example of a generic C# repository interface might be:

public interface IRepository<T> where T : EntityBase
{
    T GetById(int id);
    IEnumerable<T> List();
    IEnumerable<T> List(Expression<Func<T, bool>> predicate);
    void Add(T entity);
    void Delete(T entity);
    void Edit(T entity);
}
 
public abstract class EntityBase
{
   public int Id { get; protected set; }
}

The advantage of this approach is that it ensures you have a common interface for working with any of your objects. You can also simplify the implementation by using a Generic Repository Implementation (below). Note that taking in a predicate eliminates the need to return an IQueryable since any filter details can be passed into the repository. This can still lead to leaking of data access details into calling code, though. Consider using the Specification pattern (described below) to alleviate this issue if you encounter it.

Generic Repository Implementation

Assuming you create a Generic Repository Interface, you can implement the interface generically as well. Once this is done, you can quickly develop repositories for any given type without having to write any new code, and your classes the declare dependencies can merely specify IRepository<Item> as the type, and it’s easy for your IoC container to match that up with a Repository<Item> implementation. You can see an example Generic Repository Implementation, using Entity Framework, here.

public class Repository<T> : IRepository<T> where T : EntityBase
{
private readonly ApplicationDbContext _dbContext;

public Repository(ApplicationDbContext dbContext)
{
_dbContext = dbContext;
}

public virtual T GetById(int id)
{
return _dbContext.Set<T>().Find(id);
}

public virtual IEnumerable<T> List()
{
return _dbContext.Set<T>().AsEnumerable();
}

public virtual IEnumerable<T> List(System.Linq.Expressions.Expression<Func<T, bool>> predicate)
{
return _dbContext.Set<T>()
.Where(predicate)
.AsEnumerable();
}

public void Insert(T entity)
{
_dbContext.Set<T>().Add(entity);
_dbContext.SaveChanges();
}

public void Update(T entity)
{
_dbContext.Entry(entity).State = EntityState.Modified;
_dbContext.SaveChanges();
}

public void Delete(T entity)
{
_dbContext.Set<T>().Remove(entity);
_dbContext.SaveChanges();
}
}

Note that in this implementation, all operations are saved as they are performed; there is no Unit of Work being applied. There are a variety of ways in which Unit of Work behaviour can be added to this implementation, the simplest of which being to add an explicit Save() method to the IRepository<T> method, and to only call the underlying SaveChanges() method from this method.

IQueryable?

Another common question with Repositories has to do with what they return. Should they return data, or should they return queries that can be further refined before execution (IQueryable)? The former is safer, but the latter offers a great deal of flexibility. In fact, you can simplify your interface only to provide a single method for reading data if you go the IQueryable route since from there any number of items can be returned.

A problem with this approach is that it tends to result in business logic bleeding into higher application layers and becoming duplicated there. If the rule for returning valid customers is that they’re not disabled and they’ve bought something in the last year, it would be better to have a method ListValidCustomers() that encapsulates this logic rather than specifying these criteria in lambda expressions in multiple different UI layer references to the repository. Another typical example in real applications is the use of “soft deletes” represented by an IsActive or IsDeleted property on an entity. Once an item has been deleted, 99% of the time it should be excluded from display in any UI scenario, so nearly every request will include something like

.Where(foo => foo.IsActive)

in addition to whatever other filters are present. This is better achieved within the repository, where it can be the default behaviour of the List() method, or the List() method might be renamed to something like ListActive(). If it’s essential to view deleted/inactive items, a unique List method can be used for just this (probably rare) purpose.

Specification

Repositories that follow the advice of not exposing IQueryable can often become bloated with many custom query methods. The solution to this is to separate queries into their types, using the Specification Design Pattern. The Specification can include the expression used to filter the query, any parameters associated with this expression, as well as how much data the query should return (i.e. “.Include()” in EF/EF Core). Combining the Repository and Specification patterns can be a great way to ensure you follow the Single Responsibility Principle in your data access code. See an example of how to implement a generic repository along with a generic specification in C#.

Repository Pattern for the REST API

Now let’s see if this can work for the REST API, first the HTTP verbs that are used, the primary or most-commonly-used HTTP verbs (or methods, as they are properly called) are POST, GET, PUT, PATCH, and DELETE. These correspond to create, read, update, and delete (or CRUD) operations, respectively. There are a number of other verbs, too, but are utilized less frequently. Of those less-frequent methods, OPTIONS and HEAD are used more often than others.

HTTP Verb CRUD
POST Create
GET Read
PUT Update/Replace
PATCH Update/Modify
DELETE Delete

One of the main difference is that the calls to REST API are Asynchronous, and the interface needs to reflect this.

public interface IRepository
 {
    Task AddAsync<T>(T entity, string requestUri);
    Task<HttpStatusCode> DeleteAsync(string requestUri);
    Task EditAsync<T>(T t, string requestUri);
    Task<T> GetAsync<T>(string path);
 }

In the concrete implementation I am using the HttpClient to connect to the REST API, which needs a few parameters:

  • Uri of the end point
  • Authorization – type of authorization to be used, default NoAuthHttpSample
  • username – username for basic authorization – default null
  • password – the password for basic authorization – default null

A sample application can be found here:

Original reference Repository Pattern A data persistence abstraction

Defining and Using Custom Attribute Classes in C#

Attributes are advantageous in C# to provide none business-related functionality in an application or to abstract out the code to make it easier to follow.

The issue is when does the attribute code get called and how do you force it to run?

I set the task on to write a simple console application showing different ways you can use attributes.

I found a fascinating article for my first sample application on Defining and Using Custom Attribute Class in C# by David Tansey, below is an edited version of this article:

The complex, component-style development that businesses expect out of modern software developers requires greater design flexibility than the design methodologies of the past.

Microsoft’s .NET Framework makes extensive use of attributes to provide added functionality through what is known as “declarative” programming. Attributes enhance flexibility in software systems because they promote loose coupling of functionality. Because you can create your custom attribute classes and then act upon them, you can leverage the loose coupling power of attributes for your purposes.

The .NET Framework makes many aspects of Windows programming much more straightforward. In many cases, the Framework’s use of metadata that .NET compilers bind to assemblies at compile time causes tricky programming easier. Indeed, the use of intrinsic metadata makes it possible for .NET to relieve us from “DLL Hell.”

Lucky for us, the designers of the .NET framework did not choose to keep these metadata “goodies” hidden away under the covers. The designers gave us the Reflection API through which a .NET application can programmatically investigate this metadata. An application can “reflect” upon any imaginable aspect of a given assembly or on its contained types and their members.

Binding the metadata to the executable provides many advantages. It makes the assemblies in .NET fully self-describing. This allows developers to share components across languages and eliminates the need for header files. (They can become out-of-date relative to the implementation code.)

With all this positive news about .NET metadata, it seems hard to believe there could be anything more to the story? But there is. You can create your application-specific metadata in .NET and then use that metadata for any purpose you can imagine.

Developers define their application-specific metadata through the use of Custom Attributes. Because these attribute values become just another part of the metadata bound into an assembly, the custom attribute values are available for examination by the Reflection API.

In this article, you’ll learn how to define custom attribute classes, how to apply attributes to classes and methods in your source code, and you’ll learn how to use the Reflection API to retrieve and act upon these values.

How Does .NET Use Attributes in the Common Language Runtime?

Before you start to consider what you can accomplish with your custom attribute classes, let’s examine some of the standard attributes that the Common Language Runtime already makes available.

The [WebService] attribute provides a simple example? It lets you turn any public method of a WebService subclass into a method that you can expose as part of the Web Service merely by attaching the [WebMethod] attribute to the method definition.

public class SomeWebService : System.Web.Services.WebService
{

   [WebMethod]
   public DataSet GetDailySales( )
   {
      // code to process the request...
   }
}

You just attach the [WebMethod] attribute to the method, and .NET handles everything else for you behind the scenes.

Using the [Conditional] attribute allows you to make a given method conditional based on the presence or absence of the specified preprocessing symbol. For example, the following code:

public class SomeClass
{
   [Conditional( "DEBUG" )]
   public void UnitTest( )
   {
      // code to do unit testing...
   }
}

Indicates that the UnitTest( ) method of this class is “conditional” based on the presence of the preprocessing symbol “DEBUG”. The fascinating part is what happens. The compiler stubs out all calls to the method when the condition fails rather than attempt to nullify the behaviour of the method the way an #if…#endif pre-processing directive does. This is a much cleaner approach, and again we didn’t have to do much of anything to utilise this functionality.

Attributes utilise positional and/or named parameters. In the example using the [Conditional] attribute, the symbol specification is a positional parameter. You must always supply positional parameters.

To look at named parameters, let’s return to the [WebMethod] attribute example. This attribute has a named parameter called Description. To use it you would change the line to read:

[WebMethod(Description = "Sales volume" )]

Named parameters are optional, and you write them using the name of the parameter followed by the assignment of a value. Named parameters follow after you’ve specified all positional parameters.

I will talk more about named and positional parameters later in this article when I show you how to create and apply your Attribute class.

Run-Time, Design-Time

The examples provided in this article are involved in run-time activities. But Binaries (assemblies) aren’t just for run-time. In .NET, the metadata you describe isn’t limited to being available only at runtime. You can query the metadata at any time after you’ve compiled an assembly.

Think about some design-time possibilities. The open nature of the IDE in Visual Studio.NET allows you to create tools (using .NET languages) that facilitate development and design (wizards, builders, etc.) Thus, one module’s run-time environment (the IDE tool) is another module’s design-time environment (the source code being developed). This presents an excellent opportunity to implement some custom attributes. You could allow the IDE tool to reflect and then act upon the source classes/types you develop. Unfortunately, due to the additional subject of the IDE tool code, exploring such an example is beyond the scope of a single article.

The standard .NET attributes contain a similar example. When a developer creates custom controls to include in the Toolbox of the Visual Studio .NET IDE, they have attributes available to them to indicate how to handle the control in the property sheet. Table 1 lists and describes the four standard .NET attributes that the property sheet uses.

These property sheet-related attributes make it clear that you can use attributes and their values in the design-time as well as in the run-time environment.

Custom Attributes vs. Class Properties

Apparent similarities exist between attributes and regular member properties of a class. This can make it difficult to decide when and where you might want to utilise a custom attribute class. Developers commonly refer to properties of a class and their values as being “attributes” themselves, so what is the difference between properties and attributes?

An attribute takes the same “shape and form” as a property when you define it, but you can attach it to all manner of different assembly level types?not just Classes. Table 2 lists all the assembly level types that you can apply attributes.

Let’s pick one item from the list as an example. You can apply an attribute to a parameter, which is a little bit like adding a property to a parameter? A very novel and powerful idea indeed, because you just can’t do that with class member properties. This emphasises the most significant way in which attributes and properties are different because properties are just always going to be members of a class?  They can’t be associated with a parameter or any number of other types listed in Table 2 different than Class.

Member properties of a class are also limited in another way in which attributes are not. By definition, a member property is tied to a specific class. That member property can only ever be used through an instance or subclass instance of the class on which the property was defined. On the other hand, you can attach/apply attributes anywhere! The only requirement is that the assembly type the attribute is being assigned to matches the validon definition in the custom attribute. We’ll talk more about the validon property of custom attribute classes in the next section. This characteristic of attributes helps to promote the loose coupling that is so helpful in component-style development.

Another difference between properties and attributes relates to the values you can store in each of them. The values of member properties are instance values and can be changed at run-time. However, in the case of attributes, you set values at design time (in source code) and then compile the attributes (and their values) directly into the metadata contained in an assembly. After that point you cannot change the values of the attributes?you’ve essentially turned the values of the attributes into hard-coded, read-only data.

Consider this when you attach an attribute. If you attach an attribute to a class definition, for example, every instance of the class will have the same values assigned to the attribute regardless of how many objects of this class type you instantiate. You cannot attach an attribute to an instance of a class. You may only attach an attribute to a Type/Class definition.

Creating a Custom Attribute Class

Now we’ll create a more realistic implementation of the ideas presented above. Let’s create a custom attribute class. This will allow us to store some tracking information about code modifications that you would typically record as comments in source code. For example, we’ll mark just a few items: defect id, developer id, the date of the change, the origin of the defect, and a comment about the fix. To keep the example simple we’ll focus on creating a custom attribute class (DefectTrackAttribute) designated for use only with classes and methods.

Listing 1 shows the source code for the DefectTrackAttribute class. You can identify some critical lines of code.

If you haven’t used attributes before, the following line of code might look a bit strange.

[AttributeUsage(AttributeTargets.Class | AttributeTargets.Method, AllowMultiple = true)]

This line attaches an [AttributeUsage] attribute to the attribute class definition. Square bracket syntax identifies the construct as an attribute. So, Attributes classes can have their attributes. This may seem a bit confusing at first, but it should become more evident as we show you what you’ll use it for.

The [AttributeUsage] attribute has one positional parameter and two named parameters. The positional parameter validon specifies which of the various assembly types you can attach this attribute to. The value for this parameter uses a combination of values from the AttributeTargets enumeration. In my example, I allow only classes and methods, so I get the proper specification by OR’ing the two AttributeTargets values together.

The first named parameter of the [AttributeUsage] attribute (and the only one specified in the example) is the AllowMultiple parameter, which indicates whether you can apply this type of attribute multiple times to the same category. The default value is false. However, you want to use the AllowMultiple parameter of the Attribute more than once on a single type because that represents what the example will model. A given method or class potentially goes through many revisions during its lifetime, and you need to be able to denote each of these changes with an individual [DefectTrack] attribute.

The second named parameter of the [AttributeUsage] attribute is the Inherited parameter, which indicates whether or not derived classes inherit the attribute. I’ve made the default value for this parameter false. I opted to take the default value, so I did not specify this named parameter. Why? The source code modification information I want to capture is always related to each class and method individually. Would it confuse the developer for a class to inherit the [DefectTrack] attribute(s) from its parent class? The developer couldn’t distinguish which [DefectTrack] attributes came from the parent and which were specified directly.

Listing1 then lists the class declaration. Attribute classed are subclassed from System.Attribute. You will directly or indirectly subclass all custom Attribute classes from System.Attribute.

Next, Listing 1 shows that I’ve defined five private fields to hold the values for the attribute.

The first method in our Attribute class is the class constructor, which has a call signature with three parameters. The parameters of a constructor for an Attribute class represent the positional parameters for that attribute, which makes these required parameters. If you choose, you can create overloaded constructors and have more than one proper positional / configuration of parameter needed.

The remainder of the Attribute class is a series of public property declarations that correspond to the private fields of the class. You’ll use these properties to access the values of the attribute when you get to the example that examines the metadata. Note that the properties that correspond to the positional parameters only have a get clause and do not have a set clause. This makes these properties read-only and communicates with the fact that these are meant to be positional and not named parameters.

Applying the Custom Attribute

You’ve already seen that you can attach an attribute to a target item in your C# code by putting the attribute name and its parameters in square brackets immediately before the item’s declaration statement.

In Listing 2 you attach the [DefectTrack] attribute to a couple of methods and a couple of classes.

You need to ensure that you have access to the class definition for your custom attribute, so you start by including this line.

using MyAttributeClasses;

Beyond that, you’re merely “adorning” or “decorating” your class declarations and some of your methods with the [DefectTrack] custom attribute.

SomeCustomPricingClass has two uses of the [DefectTrack] attribute attached. The first [DefectTrack] attribute uses only the three positional parameters whereas the second [DefectTrack] attribute also includes a specification for the named parameter Origin.

[DefectTrack( "1377", "12/15/02", "David Tansey" ) ]
[DefectTrack( "1363", "12/12/02", "Toni Feltman", Origin = "Coding: Unhandled Exception")]
public class SomeCustomPricingClass
{ ... }

The PriceIsValid( ) method also uses the [DefectTrack] custom attribute, and it includes a specification for both of the named parameters, Origin and FixComment. Listing 2 contains a couple of additional uses of the [DefectTrack] attribute that you can examine on you own.

Some readers might wonder if you could rely on the old-fashioned approach of using comments for this sort of source modification information. .NET does make tools available for using XML blocks within comments to give them some structure and form.

You can easily see a comment in your source code right at the relevant place. You could process such information by text parsing the comments in the source, but it’s tedious and potentially error-prone. .NET provides tools to process XML blocks in comments that practically eliminate this issue.

Using a custom attribute for the same purpose also provides you with a structured approach to recording and processing the information, but it has an added advantage. Consider that after you compile source code into a binary, you lose your comments?forever removed from the byproduct executable code. By comparison, the values of the attributes become a part of the metadata that you’ve permanently bound to the assembly?you have still had access to the information even without any source code.

Additionally, the way an attribute “reads” in source code allows it to fill the same valuable design-time function still that the original comment did.

Retrieving the Values of the Custom Attributes

At this point, even though you’ve applied your custom attribute to some classes and methods, you haven’t seen it in action. It seems as if nothing indeed occurs whether you attach the attributes or not. But something does happen and you don’t have to take my word for it. You can use the MSIL Disassembler to open an EXE or DLL that contains types you’ve decorated with your custom attributes. The MSIL Disassembler lets you see that .NET included your attributes and their values right there in the IL code. Figure 1 shows an example of ILDASM form with the EXE from the sample code in this article opened.

Despite seeing the attribute values in the disassembly as proof of their existence, you still haven’t seen any action related to them. Now you’ll use the Reflection API to traverse the types/objects of an assembly, query for your custom attribute, and retrieve the attribute values when you find types that have your custom attribute attached to them.

Consider the general structure and intent of the test program in Listing 3. The program loads the specified assembly, gets an array of all members of the assembly, and iterates through each member looking for classes that have the [DefectTrack] attribute attached. For classes that have the attribute, the test program outputs the values of the attribute to the console. The program then performs the same steps and iteration for methods. These loops “walk” their way through the entire assembly.

Now examine some of the more critical lines of code. The first line and second line of the DisplayDefectTrack( ) method retrieve a reference to an Assembly object by loading the specified Assembly and then extracts an array containing all of the types in the assembly.

Assembly loAssembly = Assembly.Load( lcAssembly ) ;
Type[ ] laTypes = loAssembly.GetTypes( ) ;

A FOR…EACH loop iterates through each of the types of the assembly. The program outputs the name of the current type to the console, and then the following line of code queries the present type for an array containing [DefectTrack] attributes.

object[ ] laAttributes = loType.GetCustomAttributes(typeof(DefectTrackAttribute ), false ) ;

You specify the parameter typeof(DefectTrackingAttribute) on the GetCustomAttributes() method so that you can limit the returned custom attributes to be only of the type that you created in the example. The second parameter of false indicates that you do not want to include the type’s inheritance chain when trying to find your attributes.

A FOR…EACH loop iterates through each of the custom attributes and outputs its values to the console. You should recognise that the first line of the FOR…EACH block creates a new variable and does a typecast against the current attribute.

DefectTrackAttribute loDefectTrack = (DefectTrackAttribute)loAtt ;

Why is this necessary? The GetCustomAttributes() method returned an array that contains references that get cast to the generic type Object. You want to gain access to the values from your custom attribute class, and to do so, you must recast these references to their actual concrete type, DefectTrackAttribute. Once you’ve completed this, you can use the attributes, and the program can output the attribute values to the console.

Because you can apply your attribute to either classes or methods, the program then calls the GetMethods() method of the current type object from the assembly.

MethodInfo[ ] laMethods =
  loType.GetMethods(
    BindingFlags.Public |
    BindingFlags.Instance |
    BindingFlags.DeclaredOnly ) ;

For example, we have chosen to pass some values from the BindingFlags enumeration to GetMethods(). These three BindingFlags, when used in combination, limit the methods returned to ones that you defined directly in the current class. I wanted to limit the amount of output in the example, but you probably would not do this in practice because a developer might apply the [DefectTrackAttribute] to an overridden method. My implementation would not catch those attributes.

The remaining code does virtually the same processing for each of the methods that it did for each of the classes? The code queries each method for custom attributes of the [DefectTrack] type and then outputs the values for the ones it finds to the console.

Conclusion

This is just presented the implementation as only one example of how a developer might use .NET attributes to enhance their development process. Custom attributes are a bit like XML in that the significant benefits aren’t related to “what it does.” Custom attributes’ real interests lie in “what you can do with it.” The possibilities are truly limitless, and the open nature of custom attributes makes it likely that some of their most novel and powerful uses have yet to be conceived of.

Listing 1: Custom attribute class for DefectTrack attribute

using System;

namespace DefiningAndUsingCustomAttribute
{
 [AttributeUsage(AttributeTargets.Class |
 AttributeTargets.Method,
 AllowMultiple = true)]
 public class DefectTrackAttribute : System.Attribute
 {
 private string cDefectID;
 private DateTime dModificationDate;
 private string cDeveloperID;
 private string cDefectOrigin;
 private string cFixComment;

public DefectTrackAttribute(string lcDefectID, string lcModificationDate, string lcDeveloperID)
 {
 this.cDefectID = lcDefectID;
 this.dModificationDate = System.DateTime.Parse(lcModificationDate);
 this.cDeveloperID = lcDeveloperID;
 }

public string DefectID
 { get { return cDefectID; } }

public DateTime ModificationDate
 { get { return dModificationDate; } }

public string DeveloperID
 { get { return cDeveloperID; } }

public string Origin
 {
 get { return cDefectOrigin; }
 set { cDefectOrigin = value; }
 }

public string FixComment
 {
 get { return cFixComment; }
 set { cFixComment = value; }
 }
 }
}

Listing 2: Two example classes with attributes attached

namespace DefiningAndUsingCustomAttribute
{
 [DefectTrack("1377", "12/15/02", "David Tansey")]
 [DefectTrack("1363", "12/12/02", "Toni Feltman",
 Origin = "Coding: Unhandled Exception")]
 public class SomeCustomPricingClass
 {
 public double GetAdjustedPrice(double tnPrice, double tnPctAdjust)
 {
 return tnPrice + (tnPrice * tnPctAdjust);
 }

[DefectTrack("1351", "12/10/02", "David Tansey", Origin = "Specification: Missing Requirement", FixComment = "Added PriceIsValid( ) function")]
 public bool PriceIsValid(double tnPrice)
 {
 return tnPrice > 0.00 && tnPrice < 1000.00;
 }
 }
}

Listing 3: Code to walk assembly and output attribute values

using System;
using System.Reflection;

namespace DefiningAndUsingCustomAttribute
{
 public class TestMyAttribute
 {
 public static void Main()
 {
 DisplayDefectTrack("DefiningAndUsingCustomAttribute");
 Console.ReadLine();
 }

public static void DisplayDefectTrack(
 string lcAssembly)
 {
 Assembly loAssembly = Assembly.Load(lcAssembly);

Type[] laTypes = loAssembly.GetTypes();

foreach (Type loType in laTypes)
 {
 Console.WriteLine("*======================*");
 Console.WriteLine("TYPE:\t" + loType.ToString());
 Console.WriteLine("*=====================*");

object[] laAttributes = loType.GetCustomAttributes(typeof(DefectTrackAttribute), false);

if (laAttributes.Length > 0)
 Console.WriteLine("\nMod/Fix Log:");

foreach (Attribute loAtt in laAttributes)
 {
 DefectTrackAttribute loDefectTrack = (DefectTrackAttribute)loAtt;

Console.WriteLine("----------------------");
 Console.WriteLine("Defect ID:\t" + loDefectTrack.DefectID);
 Console.WriteLine("Date:\t\t" + loDefectTrack.ModificationDate);
 Console.WriteLine("Developer ID:\t" + loDefectTrack.DeveloperID);
 Console.WriteLine("Origin:\t\t" + loDefectTrack.Origin);
 Console.WriteLine("Comment:\n" + loDefectTrack.FixComment);
 }

MethodInfo[] laMethods = loType.GetMethods(BindingFlags.Public | BindingFlags.Instance | BindingFlags.DeclaredOnly);

if (laMethods.Length > 0)
 {
 Console.WriteLine("\nMethods: ");
 Console.WriteLine("----------------------");
 }

foreach (MethodInfo loMethod in laMethods)
 {
 Console.WriteLine("\n\t" + loMethod.ToString());

object[] laMethodAttributes = loMethod.GetCustomAttributes(typeof(DefectTrackAttribute), false);

if (laMethodAttributes.Length > 0)
 Console.WriteLine("\n\t\tMod/Fix Log:");

foreach (Attribute loAtt in laMethodAttributes)
 {
 DefectTrackAttribute loDefectTrack = (DefectTrackAttribute)loAtt;
 Console.WriteLine("\t\t----------------");
 Console.WriteLine("\t\tDefect ID:\t" + loDefectTrack.DefectID);
 Console.WriteLine("\t\tDeveloper ID:\t" + loDefectTrack.DeveloperID);
 Console.WriteLine("\t\tOrigin:\t\t" + loDefectTrack.Origin);
 Console.WriteLine("\t\tComment:\n\t\t" + loDefectTrack.FixComment);
 }
 }
 Console.WriteLine("\n\n");
 }
 }
 }
}

Table 1: Standard .NET attributes that the property sheet uses at design-time in the Visual Studio .NET IDE.

Attribute Description
Designer Specifies the class used to implement design-time services for a component.
DefaultProperty Specifies which property to indicate as the default property for a component in the property sheet.
Category Specifies the category in which the property will be displayed in the property sheet.
Description Specifies the description to display in the property sheet for a property.

Table 2: .NET assembly level types that you can apply attributes to.

Type
Assembly
Class
Delegate
Enum
Event
Interface
Method
Module
Parameter
Constructor
Field
Property
ReturnValue
Structure

DefiningAndUsingCustomAttribute.zip

Database Layer

The DatabaseLayer is an abstraction for database access which means the calling application or library does not need to be tight-coupled to the database itself. A set of factory methods is available to call in order to create commands and parameters, all based on the .NET abstract base classes.

Currently supported engines are SQL server and MySQL, represented by “sql” and “mysql” in your web.config or app.config.

The DatabaseLayer does not use a provider pattern but the argument is passed in the factory to tell the library what database engine to use. An example is shown below. It is preferable to create one constant for the database type and share it across multiple places it is used.

The example below is lazy loaded and uses AppSettings and ConnectionStrings from the web.config or app.config file.

private static Database database;

internal static Database GetDb()
{
if ( database == null )
{
database = Database.GetDatabase(
ConfigurationManager.AppSettings["databasetype"],
ConfigurationManager.ConnectionStrings["myconnection"].ConnectionString);
}
return database;
}

DatabaseLayer solution

Cryptography .NET, Avoiding Timing Attack

When comparing two MACs or hashes (also password hashes) for equality you would first think a simple comparison would be okay?  Think again as they are susceptible to timing attacks.

In cryptography, a timing attack is a side channel attack in which the attacker attempts to compromise a cryptosystem by analysing the time taken to execute cryptographic algorithms.

If you want to read more about this take a look at this article.

if ($hash1 == $hash2) {
//mac verification is Okay
return "hashs are equal"
} else {
//something happened
return "hashs verification failed!";
}

Okay, so what is wrong with this?

Both arguments must be of the same length to be compared successfully. When arguments of differing length are supplied, FALSE is returned immediately and the length of the known string may be leaked in the case of a timing attack.  It is done by timing the amount of time it takes to compare a known string length with the HASH, and as you will then find the length of the string based on the time it takes during the comparison.

Compares two-byte arrays in length-constant time. This comparison method is used so that password hashes cannot be extracted from on-line systems using a timing attack and then attacked off-line.

 private static bool SlowEquals(byte[] a, byte[] b)
 {
 uint diff = (uint)a.Length ^ (uint)b.Length;
 for (int i = 0; i < a.Length && i < b.Length; i++)
 diff |= (uint)(a[i] ^ b[i]);
 return diff == 0;
 }

What does the line diff |= (uint)(a[i] ^ b[i]); do?

This sets diff based on whether there’s a difference between a and b.

It avoids a timing attack by always walking through the entirety of the shorter of the two of a and b, regardless of whether there’s a mismatch sooner than that or not.

The diff |= (uint)(a[i] ^ (uint)b[i]) takes the exclusive-or of a byte of a with a corresponding byte of b. That will be 0 if the two bytes are the same, or non-zero if they’re different. It then ors that with diff.

Therefore, diff will be set to non-zero in an iteration if a difference was found between the inputs in that iteration. Once diff is given a non-zero value at any iteration of the loop, it will retain the non-zero value through further iterations.

Therefore, the final result in diff will be non-zero if any difference is found between corresponding bytes of a and b, and 0 only if all bytes (and the lengths) of a and b are equal.

Unlike a typical comparison, however, this will always execute the loop until all the bytes in the shorter of the two inputs have been compared to bytes in the other. A typical comparison would have an early-out where the loop would be broken as soon as a mismatch was found:

Microsoft provides a SecureString to hold sensitive data, and this uses a generic method Equals(Object), which determines whether the specified object is equal to the current object. There is no information saying that Equals method protects against Timing Attacks. Therefore we can only conclude that it is not safe for this operation.

I would suggest research into writing our own String Compare that is protected against Timing Attacks.

I’m not sure about all of this and further research is required, I did find this article which would be worth considering:

https://paragonie.com/blog/2015/11/preventing-timing-attacks-on-string-comparison-with-double-hmac-strategy

SecureString

I’ve come across a class in the System.Security namespace that is often overlooked. The class is SecureString. In this post, I will go over what SecureString is and why it is needed.

Have you ever come across these scenarios before?

  • A password appears in a log file accidentally.
  • A password is being shown at somewhere – once a GUI did show a command line of application that was being run, and the command line consisted of the password.
  • Using memory profiler to profile software with your colleague. Colleague sees your password in memory.
  • Using RedGate software that could capture the “value” of local variables in case of exceptions, amazingly useful. Though, I can imagine that it will log “string passwords” accidentally.
  • A crash dump that includes string password.

Do you know how to avoid all these problems? SecureString. It makes sure you don’t make silly mistakes as such. How does it avoid it? By making sure that password is encrypted in unmanaged memory and the real value can be only accessed when you are 90% sure what you’re doing.

In the sense, SecureString works pretty easily:

  1. Everything is encrypted
  2. User calls AppendChar
  3. Decrypt everything in UNMANAGED MEMORY and add the character
  4. Encrypt everything again in UNMANAGED MEMORY.

What if the user has access to your computer? Would a virus be able to get access to all the SecureStrings? Yes. All you need to do is hook yourself into RtlEncryptMemory then decrypt the memory, and you will get the location of the unencrypted memory address, and read it out. Voila! In fact, you could make a virus that will continuously scan for usage of SecureString and log all the activities with it. I am not saying it will be an easy task, but it can be done. As you can see, the “powerfulness” of SecureString is completely gone once there’s a user/virus on your system.

You have few points in your post. Sure, if you use some of the UI controls that hold “string password” internally, using actual SecureString is not that useful.

The bottom line is; if you have sensitive data(passwords, credit cards, ..), use SecureString. This is what C# Framework is following. For example, NetworkCredential class stores password as SecureString. If you look this, you can see over ~80 different usages in .NET framework of SecureString.

There are many cases when you have to convert SecureString to string because some API expects it.

The usual problem is either:

  • The API is GENERIC. It does not know that there’s a sensitive data.
  • The API knows that it’s dealing with sensitive data and uses “string” – that’s just bad design.

You raised a good point: what happens when you convert SecureString to a string? That can only happen because of the first point. Eg the API does not know that it’s sensitive data. I have personally not seen that happening. Getting string out of SecureString is not that simple.

It’s not simple for a simple reason; it was never intended to let the user convert SecureString to string, as you stated: GC will kick in. If you see yourself doing that, you need to step back and ask yourself: Why am I even doing this, or do I need this, why?

You can always extend the SecureString class with an extension method, such as ToEncryptedString(__SERVER__PUBLIC_KEY), which gives you a string instance of SecureString that is encrypted using server’s public key. The only server can then decrypt it. Problem solved, GC will never see the “original” string, as you never expose it in managed memory. This is precisely what is being done in PSRemotingCryptoHelper (EncryptSecureStringCore(SecureString secureString)).

And as something very almost-related: Mono SecureString does not encrypt at all. The implementation has been commented out because ..wait for it. “It somehow causes nunit test breakage”, which brings to my last point:

Not supported everywhere is SecureString. If the platform/architecture does not support SecureString, you’ll get an exception. The recommended list of deployment platforms is documentation.

Using SecureString for Sensitive Data

The standard System.String class has never been a very secure solution for storing sensitive strings such as passwords or credit card numbers. Using a string for this purposes has numerous problems including it’s not pinned, so the garbage collector can move it around and will leave several copies in memory, it’s not encrypted, so anyone who can read your processor’s memory will be able to see the values of the string easily. Also, if your process gets swapped out to disk, the unencrypted contents of the string will be sitting in your swap file. And it’s not mutable, so whenever you need to modify it there will be an old version and the new version both in memory.

Since it’s not mutable, there’s no effective way to clear it out when you’re done using it. Secure strings are held in encrypted memory by the CLR using the Data Protection API, or DPAPI, and they’re only unencrypted when they are accessed.  This limits the amount of time that your string is in plain text for an attacker to see.

The garbage collector will not move the encrypted string around in memory, so you never have to worry about multiple copies of your string sitting in your address space, unless you make copies of those.

SecureString also implements IDisposable, and when it’s disposed of or finalised if you forget to dispose of it, the memory that was used to hold your encrypted string will be zeroed out.

They also provide a feature that lets you lock them down as read only preventing other code from modifying your string.

You can create a secure string with a pointer to a character array and a length of that array. When constructed this way, the secure string will make a copy of your array, allowing you to zero out your insecure copy.

A secure string can also be constructed without an existing character array, and the data can be copied in one character at a time.

One important thing to note though is that SecureString shouldn’t be used as a blanket replacement for System.String, it should only be used in places where you need to store sensitive information that you do not want to spread around in memory for longer than is required.

To add data or modify data in your string, standard operations are provided. For instance, you’ll find an AppendChar, InsertAt, RemoveAt, and SetAt methods. MakeReadOnly and IsReadOnly allows you to lock down the secure string. Clear, Dispose, and the finaliser takes care of removing any trace of the safe string from memory.

The main idea with SecureString is that you would never store a password or other textual secret in plain text. Unfortunately, SecureString was introduced into the framework only after plenty of APIs were built and shipped using passwords stored in a string, such as the System.Net.NetworkCredential. So any application that must use these APIs had no option but to convert secure strings to strings.

However, the SecureString class itself doesn’t provide any method to get back a plain string with its content, precisely to discourage this type of usage. What a developer has to do is use functions from the System.Runtime.InteropServices.Marshal to get a native buffer with the plain string, marshal a value into managed string, and then very importantly free the native buffer. The best implementation to get a string out of a secure string is to use a try/finally block to free the native buffer.

Here is a sample application showing how you can use SecureString, please remember in production do not use the SecureString to String as this just does not make sense and would make it insecure.

SecureString

Here is a short method to convert a string to a SecureString

public static SecureString ToSecureString(this string source)
{
 if (string.IsNullOrWhiteSpace(source))
 return null;
 else
 {
 SecureString result = new SecureString();
 foreach (char c in source.ToCharArray())
 result.AppendChar(c);
 return result;
 }
}