Delegate event handlers (i.e. delegates used as type of an event) should have a very specific signature:

• Return type void.
• First argument of type System.Object and named 'sender'.
• Second argument of type System.EventArgs (or any derived type) and is named 'e'.

This rule raises an issue whenever a delegate declaration doesn’t match that signature.

Noncompliant Code Example

public delegate void AlarmEventHandler(object s);

public class Foo
{
    public event AlarmEventHandler AlarmEvent; // Noncompliant
}

Compliant Solution

public delegate void AlarmEventHandler(object sender, AlarmEventArgs e);

public class Foo
{
    public event AlarmEventHandler AlarmEvent; // Compliant
}

See

Handling and Raising Events