This post originated from an RSS feed registered with .NET Buzz by Peter G Provost. Original Post: Unit Testing Events with Anonymous Delegates in .NET 2.0 Feed Title: Peter Provost's Geek Noise Feed URL: /error.aspx?aspxerrorpath=/Rss.aspx Feed Description: Technology news, development articles, Microsoft .NET, and other stuff... Latest .NET Buzz Posts Latest .NET Buzz Posts by Peter G Provost Latest Posts From Peter Provost's Geek Noise

There has been a lot of discussion comparing anonymous delegates to closures and blocks in Ruby, so I’m not going to get into that whole thing…

What I want to talk about for a minute is how cool it is to use anonymous delegates to test that events on your classes fire as expected.

Suppose, for example, that you have a class like this:

public class MyContainerContainer
{
  public event EventHandler<ItemAddedEventArgs> ItemAdded;

  public void Add( object item )
  {
    // Store it in an internal list or something, then
    // fire the event

    if( ItemAdded != null )
      ItemAdded( this, new ItemAddedEventArgs(item) );
  }
}

Now before anonymous delegates, you would have had to introduce a private method in your test fixture to attach to the ItemAdded event. And as shown here, you probably would have had to introduce a private field to pass data between the event handler method and the test method. (Sure, you could have created a mock class to hold the state and the event handler, but that would have been even more code to read/parse/grok.)

[TestFixture]
public class ContainerMonitorFixture
{
  private object addedItem = null;

  [Test]
  public void AddFiresAddedEventWithCorrectArgs()
  {
    MyContainer container = new MyContainer();
    container.ItemAdded +=
      new EventHandler<ItemAddedEventArgs>(this.ItemAdded);

    object item = new object();
    container.Add( item );
    Assert.AreSame( item, this.addedItem );
  }

  private void ItemAdded( object sender, ItemAddedEventArgs e )
  {
    addedItem = e.AddedItem;
  }
}

With anonymous delegates, you can now write very clear, very clean tests like this (forgive the strange line breaks—I had to add them to make it fit nicely on the page):

[TestFixture]
public class ContainerMonitorFixture
{
  [Test]
  public void AddFiresAddedEventWithCorrectArgs()
  {
    object expected = new object();
    object actual = null;
    MyContainer container = new MyContainer();
    container.ItemAdded += delegate( object sender, 
      ItemAddedEvent args e )
    {
      actual = e.AddedItem;
    }

    container.Add( expected );
  
    Assert.AreSame( expected, actual );
  }
}

There are a couple of cool things in this code:

• The delegate can modify variables in the test method, outside the event handler itself
• The entire test’s meaning is clearly stated within the test—you don’t have to look in some other method or class to see what’s happening
• Because the entire test is self encapsulated, it is easier to move/copy/change should you need to do any refactoring to either the class under test or the test fixture itself.

Enjoy!

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