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CA1003: Use generic event handler instances

TypeName

UseGenericEventHandlerInstances

CheckId

CA1003

Category

Microsoft.Design

Breaking Change

Breaking

A type contains a delegate that returns void, whose signature contains two parameters (the first an object and the second a type that is assignable to EventArgs), and the containing assembly targets .NET Framework 2.0.

Before .NET Framework 2.0, in order to pass custom information to the event handler, a new delegate had to be declared that specified a class that was derived from the EventArgs class. This is no longer true in .NET Framework 2.0, which introduced the EventHandler delegate. This generic delegate allows any class that is derived from EventArgs to be used together with the event handler.

To fix a violation of this rule, remove the delegate and replace its use by using the EventHandler delegate. If the delegate is autogenerated by the Visual Basic compiler, change the syntax of the event declaration to use the EventHandler delegate.

Do not suppress a warning from this rule.

The following example shows a delegate that violates the rule. In the Visual Basic example, comments describe how to modify the example to satisfy the rule. For the C# example, an example follows that shows the modified code.

using System;

namespace DesignLibrary
{
   // This delegate violates the rule. 
   public delegate void CustomEventHandler(
      object sender, CustomEventArgs e);

   public class CustomEventArgs : EventArgs
   {
      public string info = "data";
   }

   public class ClassThatRaisesEvent
   {
      public event CustomEventHandler SomeEvent;

      protected virtual void OnSomeEvent(CustomEventArgs e)
      {
         if(SomeEvent != null)
         {
            SomeEvent(this, e);
         }
      }

      public void SimulateEvent()
      {
         OnSomeEvent(new CustomEventArgs());
      }
   }

   public class ClassThatHandlesEvent
   {
      public ClassThatHandlesEvent(ClassThatRaisesEvent eventRaiser)
      {
         eventRaiser.SomeEvent += 
            new CustomEventHandler(HandleEvent);
      }

      private void HandleEvent(object sender, CustomEventArgs e)
      {
         Console.WriteLine("Event handled: {0}", e.info);
      }
   }

   class Test
   {
      static void Main()
      {
         ClassThatRaisesEvent eventRaiser = new ClassThatRaisesEvent();
         ClassThatHandlesEvent eventHandler = 
            new ClassThatHandlesEvent(eventRaiser);

         eventRaiser.SimulateEvent();
      }
   }
}

The following example removes the delegate declaration from the previous example, which satisfies the rule, and replaces its use in the ClassThatRaisesEvent and ClassThatHandlesEvent methods by using the EventHandler delegate.

using System;

namespace DesignLibrary
{
   public class CustomEventArgs : EventArgs
   {
      public string info = "data";
   }

   public class ClassThatRaisesEvent
   {
      public event EventHandler<CustomEventArgs> SomeEvent;

      protected virtual void OnSomeEvent(CustomEventArgs e)
      {
         if(SomeEvent != null)
         {
            SomeEvent(this, e);
         }
      }

      public void SimulateEvent()
      {
         OnSomeEvent(new CustomEventArgs());
      }
   }

   public class ClassThatHandlesEvent
   {
      public ClassThatHandlesEvent(ClassThatRaisesEvent eventRaiser)
      {
         eventRaiser.SomeEvent += 
            new EventHandler<CustomEventArgs>(HandleEvent);
      }

      private void HandleEvent(object sender, CustomEventArgs e)
      {
         Console.WriteLine("Event handled: {0}", e.info);
      }
   }

   class Test
   {
      static void Main()
      {
         ClassThatRaisesEvent eventRaiser = new ClassThatRaisesEvent();
         ClassThatHandlesEvent eventHandler = 
            new ClassThatHandlesEvent(eventRaiser);

         eventRaiser.SimulateEvent();
      }
   }
}
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