Raising Multiple Exceptions with AggregateException

There are occasions where you are aware of many exceptions within your code that you want to raise together in one go. Perhaps your system makes a service call to a middleware orchestration component that potentially returns many exceptions detailed in its response, or another scenario might be a batch processing task dealing with many items in one process that require you to collate all exceptions until the end and then throw them together.

Let us look at the batch scenario in more detail. In this situation, if you raised the first exception that you found it would exit the method without processing the remaining items. Alternatively, you could store the exception information in a variable of some sort and once all the elements are processed using the information to construct an exception and throw it. While this approach works, there are some drawbacks. There is the extra effort required to create a viable storage container to hold the exception information, and this may mean modifying existing code not to throw an exception but instead to log the details in this new ‘exception detail helper class’. This solution also lacks the additional benefits you get with creating an exception then, for example, the numerous intrinsic properties that exist within Exception objects that provide valuable additional context information to support the message within the exception. Even when all the relevant information has been collated into a single exception class, then you are still left with one exception holding all that information when you may need to handle the exceptions individually and pass them off to existing error handling frameworks which rely on a type deriving from Exception.

Luckily included in .Net Framework 4.0 is the simple but very useful AggregateException class which lives in the System namespace (within mscorlib.dll). It was created to use the Task Parallel Library, and its use within that library is described on MSDN here. Don’t think that is it’s only used though, as it can be put to good use within your code in situations like those described above where you need to throw many exceptions, so let’s see what it offers.

The AggregateException class is an exception type, inheriting from System.Exception, that acts a wrapper for a collection of child exceptions. Within your code, you can create instances of any exception based type and add them to the AggregateException’s collection. The idea is a simple one, but the AggregateException’s beauty comes in the implementation of this simplicity. As it is a regular exception class, it can handle in the usual way by existing code but also as a unique exception collection by the particular system that cares about all the exceptions nested within its bowels.

The class accepts the child exceptions on one of its seven constructors and then exposes them through its InnerExceptions property. Unfortunately, this is a read-only collection, and so it is not possible to add inner exceptions to the AggregateException after it has instantiated (which would have been nice) and so you will need to store your exceptions in a collection until you’re ready to create the Aggregate and throw it:

// create a collection container to hold exceptions List<Exception> exceptions = new List<Exception>(); // do some stuff here ........ // we have an exception with an innerexception, so add it to the list exceptions.Add(new TimeoutException("It timed out", new ArgumentException("ID missing"))); // do more stuff ..... // Another exception, add to list exceptions.Add(new NotImplementedException("Somethings not implemented")); // all done, now create the AggregateException and throw it AggregateException aggEx = new AggregateException(exceptions); throw aggEx;

The method you use to store the exceptions is up to you as long as you have them all ready at the time you create the AggregateException class. Seven constructors are allowing you to pass combinations of nothing, a string message, collections or arrays of inner exceptions.

Once created you interact with the class as you would any other exception type:

try {    // do task } catch (AggregateException ex) {    // handle it  }

The key as it means that you can make use of existing code and patterns for handling exceptions within your (or third parties) codebase.

In addition to the general Exception members, the class exposes a few custom ones. The common InnerException property is there for compatibility, and this appears to return the first exception added to the AggregateException class via the constructor, so in the example above it would be the TimeoutException instance. All of the child exceptions expose via the InnerExceptions read-only collection property (as shown below).

The Flatten() method is another custom property that might prove useful if you find the need to nest Exceptions as inner exceptions within several AggregateExceptions. The method will iterate the InnerExceptions collection, and if it finds AggregateExceptions nested as InnerExceptions, it will promote their child exceptions to the parent level. As you can see in this example:

AggregateException aggExInner = 
 new AggregateException("inner AggEx", new TimeoutException());
AggregateException aggExOuter1 = 
 new AggregateException("outer 1 AggEx", aggExInner);
AggregateException aggExOuter2 = 
 new AggregateException("outer 2 AggEx", new ArgumentException());
AggregateException aggExMaster =
 new AggregateException(aggExOuter1, aggExOuter2);

If we create this structure above of AggregrateExceptions with inner exceptions of TimeoutException and ArgumentException then the InnerExceptions property of the parent AggregateException (i.e. aggExMaster) shows, as expected, two objects, both being of type AggregrateException and both containing child exceptions of their own:

But if we call Flatten()…

AggregateException aggExFlatterX = aggExMaster.Flatten();

…we get a new ArgumentException instance returned that contains still two objects, but this time the AggregrateException objects have gone, and we have the two child exceptions of TimeoutException and ArgumentException:

A useful feature to discard the AggregateException containers (which are effectively just packaging) and expose the real meat, i.e. the real exceptions that have been thrown and needs to be addressed.

If you’re wondering how the ToString() is implemented then the aggExMaster object in the examples above (without flattening) produces this:

System.AggregateException: One or more errors occurred. ---> System.AggregateException
: outer 1 AggEx ---> System.AggregateException: inner AggEx ---> 
System.TimeoutException: The operation has timed out. --- End of inner exception 
stack trace --- --- End of inner exception stack trace --- --- End of inner exception 
stack trace ------> (Inner Exception #0) System.AggregateException: outer 1 AggEx ---> 
System.AggregateException: inner AggEx ---> System.TimeoutException: The operation
 has timed out. --- End of inner exception stack trace --- --- End of inner 
exception stack trace ------> (Inner Exception #0) System.AggregateException: inner
AggEx ---> System.TimeoutException: The operation has timed out. --- End of inner 
exception stack trace ------> (Inner Exception #0) System.TimeoutException: The 
operation has timed out.<---<---<------> (Inner Exception #1) System.AggregateException
: outer 2 AggEx --- System.ArgumentException: Value does not fall within the expected
 range. --- End of inner exception stack trace ------> (Inner Exception #0) 
System.ArgumentException: Value does not fall within the expected range.

As you can see the data has been formatted in a neat and convenient way for readability, with separators between the inner exceptions.

In summary, this is a very useful class to be aware of and have in your arsenal whether you are dealing with the Parallel Tasks Library or you just need to manage multiple exceptions. I like simple and neat solutions, and to me, this is a good example of that philosophy.

Orginal Article

Using MongoDB

If you want to setup MongoDB with C#, then the best option is to follow the getting started guide on the MongoDB website

https://docs.mongodb.com/getting-started/csharp/client/

You can install the NuGet package for MongoDB by typing into the Package Manager Console

  • “Install-Package MongoDB.Driver”

Although MongoDB does provide an example that you can work through, I did find that the MoveNextAsync command is no longer supported and the case fails.

I built a simple application that uses MongoDB CRUD application that works very well to show how it can be implemented.

MongoDB

Here is a helpful MongoDB tool to administer your Mongo database, Robomogo

https://robomongo.org

On my searching to find good source of material I came across this nice video demo here https://youtu.be/ubwC63DwJ8w

Custom Configuration in your config

This post is to show how to setup a customised configuration section in your app.config or web.conifg of your .NET application

I have written an article on this before, back in 2008, but things have moved on in the libraries and this is the latest and easiest way to create your own customised configuration.

We just need to define our class, inherit from System.Configuration.ConfigurationSection, and add a property per setting we wish to store.

using System;
using System.Configuration;

public class BlogSettings : ConfigurationSection
{
 private static BlogSettings settings = ConfigurationManager.GetSection("BlogSettings") as BlogSettings;
 
 public static BlogSettings Settings
 {
 get
 {
 return settings;
 }
 }

 [ConfigurationProperty("frontPagePostCount" , DefaultValue = 20 , IsRequired = false)]
 [IntegerValidator(MinValue = 1 , MaxValue = 100)]
 public int FrontPagePostCount
 {
 get { return (int)this["frontPagePostCount"]; }
 set { this["frontPagePostCount"] = value; }
 }


 [ConfigurationProperty("title" , IsRequired=true)]
 public string Title
 {
 get { return (string)this["title"]; }
 set { this["title"] = value; }
 }
}

Notice that you use an indexed property to store and retrieve each property value.

Also not a static property named Settings for convenience.

Add your new configuration section to web.config (or app.config).

<configuration>
 <configSections>
 <section name="BlogSettings" type="Fully.Qualified.TypeName.BlogSettings, 
 AssemblyName" />
 </configSections>
 <BlogSettings
 frontPagePostCount="10"
 title="You’ve Been Haacked" />
</configuration>

And to access the new configuration using code it is as simple as:

string title = BlogSettings.Settings.Title;
Response.Write(title); //it works!!!

Here is the source code: Configuration Example

Another article that goes into this in a lot more details can be found on Code Project – Unraveling the Mysteries of .NET 2.0 Configuration

How slow are Exceptions

I’ve seen from time to time applications that wrap a TRY – CATCH around some code, but do nothing except carry on.  So why is this so wrong?

As programmers we want to write quality code that solves problems. Unfortunately, exceptions come as side effects of our code. No one likes side effects, so we soon find our own ways to get around them. I have seen some smart programmers deal with exceptions the following way:

public void consumeAndForgetAllExceptions(){
    try {
        ...some code that throws exceptions
    } catch (Exception ex){
        ex.printStacktrace();
    }
}

What is wrong with the code above?

Once an exception is thrown, normal program execution is suspended and control is transferred to the catch block. The catch block catches the exception and just suppresses it. Execution of the program continues after the catch block, as if nothing had happened.

How about the following?

public void someMethod() 
 {
 throw new Exception();
 }

This method is a blank one; it does not have any code in it. How can a blank method throw exceptions? C# does not stop you from doing this. Recently, I came across similar code where the method was declared to throw exceptions, but there was no code that actually generated that exception. When I asked the programmer, he replied “I know, it is corrupting the API, but I am used to doing it and it works.”

This debate goes around in circles in the C# community. I have seen several C# programmers struggle with the use of exceptions. If not used correctly, exceptions can slow down your program, as it takes memory and CPU power to create, throw, and catch exceptions. If overused, they make the code difficult to read and frustrating for the programmers using the API. We all know frustrations lead to hacks and code smells. The client code may circumvent the issue by just ignoring exceptions or throwing them.

Let’s go back to basics and start with the definition of an Exception:

Exception handling is the process of responding to the occurrence, during computation, of exceptions – anomalous or exceptional conditions requiring special processing – often changing the normal flow of program execution. It is provided by specialized programming language constructs or computer hardware mechanisms.

The important words here I feel is special processing something which is out of your control.

Here is a simple example

 var a = 0;
 int b;
 try
 {
  b = 10 / a;
 }
 catch
 {
 }

So what is wrong with this?

I call it lazy and poormans programming, one correct solution to this could be:

 var a = 0;
 int b;
 if (a != 0)
  b = 10 / a;

Okay this is a very simple example of abusive Exception handling, but does it really matter?

I wrote a small benchmark test to see just what the difference is in performance.  Just a simple 10,000 parallel loop count running the same code over and over again.

I was quite overwhelmed with the performnce hit of the Exception handling, even just in this small example.  The Exception handling caused the code to slow down by over 600 times.

600 times slower code, well that does not matter if this is only happening once I hear you say…!

The performance is just one area, what about the extra memory that the exception is using.  With a few changes to the Benchmark application am able to monitor the Garbage Collector, which although it is not a true representation of how much memory is being used it will provide a very good gauge.

The results of the memory useage were quite staggering

  • 14,416 for the programming logic
  • 2,501,832 for the exception handing

Just another reason why this is not acceptable to use the exception handler in this way.

This is just a short article on Excpetion Handling, but it does frustrate me when I see Exception Handling  being abused.

ExceptionPerformance

Object to CSV extension

Here is a nice little extension that will take an object and convert it into a CSV file.

Also has the option if you want to include a header on the CSV file.

public static class ObjectToCsv
 {
 public static string ToCsv<T>(this IEnumerable<T> items, bool includeHeading = true)
 where T : class
 {
 var csvBuilder = new StringBuilder();
 var properties = typeof(T).GetProperties();

 if (includeHeading)
 {
 csvBuilder.AppendLine(string.Join(",", properties.Select(p => p.Name.ToCsvValue()).ToArray()));
 }

 foreach (var item in items)
 {
 var line = string.Join(",", properties.Select(p => p.GetValue(item, null).ToCsvValue()).ToArray());
 csvBuilder.AppendLine(line);
 }
 return csvBuilder.ToString();
 }

 private static string ToCsvValue<T>(this T item)
 {
 if (item == null) return "\"\"";

 if (item is string)
 {
 return $"\"{item.ToString().Replace("\"", "\\\"")}\"";
 }
 double dummy;
 return double.TryParse(item.ToString(), out dummy) ? $"{item}" : $"\"{item}\"";
 }
 }

Upgrading an ASP.NET MVC 3 Project to ASP.NET MVC 4

 

ASP.NET MVC 4 can be installed side by side with ASP.NET MVC 3 on the same computer, which gives you flexibility in choosing when to upgrade an ASP.NET MVC 3 application to ASP.NET MVC 4.

The simplest way to upgrade is to create a new ASP.NET MVC 4 project and copy all the views, controllers, code, and content files from the existing MVC 3 project to the new project and then to update the assembly references in the new project to match any non-MVC template included assembiles you are using. If you have made changes to the Web.config file in the MVC 3 project, you must also merge those changes into the Web.config file in the MVC 4 project.

To manually upgrade an existing ASP.NET MVC 3 application to version 4, do the following:

In all Web.config files in the project (there is one in the root of the project, one in the Views folder, and one in the Views folder for each area in your project), replace every instance of the following text (note: System.Web.WebPages, Version=1.0.0.0 is not found in projects created with Visual Studio 2012):

System.Web.Mvc, Version=3.0.0.0
System.Web.WebPages, Version=1.0.0.0
System.Web.Helpers, Version=1.0.0.0
System.Web.WebPages.Razor, Version=1.0.0.0

with the following corresponding text:

System.Web.Mvc, Version=4.0.0.0
System.Web.WebPages, Version=2.0.0.0
System.Web.Helpers, Version=2.0.0.0
System.Web.WebPages.Razor, Version=2.0.0.0

In the root Web.config file, update the webPages:Version element to “2.0.0.0” and add a new PreserveLoginUrl key that has the value “true”:

<appSettings>
  <add key="webpages:Version" value="2.0.0.0" />
  <add key="PreserveLoginUrl" value="true" />
</appSettings>

n Solution Explorer, right-click on the References and select Manage NuGet Packages. In the left pane, select Online\NuGet official package source, then update the following:

  • ASP.NET MVC 4
  • (Optional) jQuery, jQuery Validation and jQuery UI
  • (Optional) Entity Framework
  • (Optonal) Modernizr

In Solution Explorer, right-click the project name and then select Unload Project. Then right-click the name again and select Edit ProjectName.csproj.

Locate the ProjectTypeGuids element and replace {E53F8FEA-EAE0-44A6-8774-FFD645390401} with {E3E379DF-F4C6-4180-9B81-6769533ABE47}.

Save the changes, close the project (.csproj) file you were editing, right-click the project, and then select Reload Project.

If the project references any third-party libraries that are compiled using previous versions of ASP.NET MVC, open the root Web.config file and add the following three bindingRedirect elements under the configuration section:

<configuration>
  <!--... elements deleted for clarity ...-->
 
  <runtime>
    <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
      <dependentAssembly>
        <assemblyIdentity name="System.Web.Helpers" 
             publicKeyToken="31bf3856ad364e35" />
        <bindingRedirect oldVersion="1.0.0.0" newVersion="2.0.0.0"/>
      </dependentAssembly>
      <dependentAssembly>
        <assemblyIdentity name="System.Web.Mvc" 
             publicKeyToken="31bf3856ad364e35" />
        <bindingRedirect oldVersion="1.0.0.0-3.0.0.0" newVersion="4.0.0.0"/>
      </dependentAssembly>
      <dependentAssembly>
        <assemblyIdentity name="System.Web.WebPages" 
             publicKeyToken="31bf3856ad364e35" />
        <bindingRedirect oldVersion="1.0.0.0" newVersion="2.0.0.0"/>
      </dependentAssembly>
    </assemblyBinding>
  </runtime>
</configuration>

Original article can be found at: MVC 4 release notes

 

Visual C# and .NET 4.0 samples

I came across some nice C# samples from Microsoft today providing everything from Hello World, to how to use Yield and how to program Attributes.

Official Visual Studio 2010 Samples for C# 4.0

Worth having a look at if you are a beginner or a seasoned programmer.

Language Samples

The C# 4.0-specific samples are called:

  • Named and Optional
  • Office Sample
  • Python Sample
  • Simple Variance.

Here is the complete list of language samples:

  • Anonymous Delegates: Demonstrates the use of unnamed delegates to reduce application complexity.
  • Arrays: Shows how to use arrays.
  • Attributes: Shows how to create custom attribute classes, use them in code, and query them through reflection.
  • Collection Classes: Shows how to make non-generic collection classes that can be used with the foreach statement.
  • COM Interop Part I: Shows how to use C# to interoperate with COM objects.
  • COM Interop Part II: Shows how to a use a C# server together with a C++ COM client.
  • Commandline: Demonstrates simple command-line processing and array indexing.
  • Condiational Methods: Demonstrates conditional methods, which provide a powerful mechanism by which calls to methods can be included or omitted depending on whether a symbol is defined.
  • Delegates: Shows how delegates are declared, mapped to static and instance methods, and combined into multicast delegates.
  • Events: Shows how to declare, invoke, and configure events in C#.
  • Explicit Interface: Demonstrates how to explicitly implement interface members and how to access those members from interface instances.
  • Generics: Shows how to make generic collection classes that can be used with the foreach statement.
  • Hello World: A Hello World application.
  • Indexers Part I: Shows how C# classes can declare indexers to provide array-like access to objects.
  • Indexers Part II: Shows how to implement a class that uses indexed properties. Indexed properties enable you to use a class that represents an array-like collection.
  • Libraries: Shows how to use compiler options to create a DLL from multiple source files; also, how to use the library in other programs
  • Named and Optional (C# 4.0): Demonstrates Named and Optional parameters, an alternative to method overloads
  • Nullable: Demonstrates value types, such as double and bool, that can be set to null
  • Office Sample (C# 4.0): Demonstrates how Dynamic and COM Interop make it easy to call Microsoft Office in C# 4.0
  • OLEDB: Demonstrates how to use a Microsoft Access database from C# by creating a dataset and adding tables to it.
  • Operator Overloading: Shows how user-defined classes can overload operators
  • Partial Types: Demonstrates how classes and structures can be defined in multiple C# source-code files
  • PInvokeShows how to call exported DLL functions from C#
  • Properties: Shows how properties are declared and used; also demonstrates abstract properties
  • Python Sample (C# 4.0): Learn how to call a Python script by using the Dynamic feature in C# 4.0
  • Security: Discusses .NET Framework security and shows two ways to modify security permissions in C#: using permission classes and permission attributes
  • Simple Variance (C# 4.0): See how Covariance and Contravariance are supported in generic interfaces and delegates
  • Structs: Shows how to use structs in C#.
  • Threading: Demonstrates various thread activities such as creating and executing a thread, synchronizing threads, interacting between threads, and using a thread pool
  • Unsafe: Shows how to use unmanaged code (code that uses pointers) in C#
  • User Conversions: Shows how to define conversions to and from user-defined types
  • Versioning: Demonstrates versioning in C# by using the override and new keywords
  • XML Documents: Shows how to document code by using XML
  • Yield: Demonstrates how to use the yield keyword to filter items in a collection

LinqSamples

These samples help you learn LINQ quickly. They also act as a reference and guide for those with existing LINQ experience. A few handy utilities are also included. They are available in the LinqSamples folder.

  • DynamicQuery: Code to create LINQ queries at run time.
  • LinqToNorthwind: A basic example of how to use LINQ To SQL to query a database
  • LinqToXmlDataBinding: Bind LINQ to XML code to WPF controls
  • ObjectDumper: A utility for writing the output from a LINQ query to the screen in text mode
  • PasteXmlAsLinq: A Visual Studio addin that automatically converts XML to LINQ to XML.
  • QueryVisualizer: Allows LINQ to SQL developers to see the SQL for their query, and also to see the results of the query in a grid.
  • Reflector: Use LINQ to right queries against the objects in your code using the Reflection APIs
  • RSS: This sample acts as a tiny web server that aggregates several RSS feeds
  • SampleQueries: This is the most important sample and contains about 500 examples of how to use each of the query operators in LINQ to Objects, LINQ to SQL, LINQ to XML and LINQ to DataSet.
  • SimpleLambdas: Several examples of how to write and use lambda expressions
  • SimpleLinqToObjects: The hello world of the LINQ samples. This shows you how easy it is to do simple queries of in-memory objects using LINQ to Objects.
  • SimpleLinqToXml: Get started using LINQ to XML.
  • WebServiceLinqProvider(TerraService): A custom LINQ provider and client for the TerraServer-USA Web service.
  • Whitepapers: The following papers are stored in this directory in Word format:
    • LINQ Project Overview
    • LINQ to SQL Overview for C# and VB
    • Standard Query Operators
    • LINQ to XML Overview
  • WinFormsDataBinding: Learn how to display LINQ queries with grids in a Windows form. It includes a one to many query example.
  • XQuery: Another simple LINQ to XML sample query. This sample shows the minimal code necessary to write a LINQ to XML query