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CA1003: Use generic event handler instances

  • Article
Property Value
Rule ID CA1003
Title Use generic event handler instances
Category Design
Fix is breaking or non-breaking Breaking
Enabled by default in .NET 9 No

Cause

A type contains a delegate that returns void and 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.

By default, this rule only looks at externally visible types, but this is configurable.

Rule description

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 System.EventArgs class. In .NET Framework 2.0 and later versions, the generic System.EventHandler<TEventArgs> delegate allows any class that's derived from EventArgs to be used together with the event handler.

How to fix violations

To fix a violation of this rule, remove the delegate and replace its use by using the System.EventHandler<TEventArgs> delegate.

If the delegate is autogenerated by the Visual Basic compiler, change the syntax of the event declaration to use the System.EventHandler<TEventArgs> delegate.

When to suppress warnings

Do not suppress a warning from this rule.

Configure code to analyze

Use the following option to configure which parts of your codebase to run this rule on.

You can configure this option for just this rule, for all rules it applies to, or for all rules in this category (Design) that it applies to. For more information, see Code quality rule configuration options.

Include specific API surfaces

You can configure which parts of your codebase to run this rule on, based on their accessibility. For example, to specify that the rule should run only against the non-public API surface, add the following key-value pair to an .editorconfig file in your project:

dotnet_code_quality.CAXXXX.api_surface = private, internal

Example

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.

Imports System

Namespace ca1003

    Public Class CustomEventArgs
        Inherits EventArgs

        Public info As String = "data"

    End Class

    Public Class ClassThatRaisesEvent

        ' This statement creates a new delegate, which violates the rule.
        Event SomeEvent(sender As Object, e As CustomEventArgs)

        ' To satisfy the rule, comment out the previous line 
        ' and uncomment the following line.
        'Event SomeEvent As EventHandler(Of CustomEventArgs)

        Protected Overridable Sub OnSomeEvent(e As CustomEventArgs)
            RaiseEvent SomeEvent(Me, e)
        End Sub

        Sub SimulateEvent()
            OnSomeEvent(New CustomEventArgs())
        End Sub

    End Class

    Public Class ClassThatHandlesEvent

        Sub New(eventRaiser As ClassThatRaisesEvent)
            AddHandler eventRaiser.SomeEvent, AddressOf HandleEvent
        End Sub

        Private Sub HandleEvent(sender As Object, e As CustomEventArgs)
            Console.WriteLine("Event handled: {0}", e.info)
        End Sub

    End Class

    Class Test

        Shared Sub Main1003()

            Dim eventRaiser As New ClassThatRaisesEvent()
            Dim eventHandler As New ClassThatHandlesEvent(eventRaiser)

            eventRaiser.SimulateEvent()

        End Sub

    End Class

End Namespace

// 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)
    {
        SomeEvent?.Invoke(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 MainEvent()
    {
        var eventRaiser = new ClassThatRaisesEvent();
        var eventHandler = new ClassThatHandlesEvent(eventRaiser);

        eventRaiser.SimulateEvent();
    }
}

The following code snippet removes the delegate declaration from the previous example, which satisfies the rule. It replaces its use in the ClassThatRaisesEvent and ClassThatHandlesEvent methods by using the System.EventHandler<TEventArgs> delegate.

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

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

    protected virtual void OnSomeEvent(CustomEventArgs e)
    {
        SomeEvent?.Invoke(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 MainEvent()
    {
        var eventRaiser = new ClassThatRaisesEvent();
        var eventHandler = new ClassThatHandlesEvent(eventRaiser);

        eventRaiser.SimulateEvent();
    }
}

See also