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how-to-implement-interface-events.md

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title description ms.topic ms.date helpviewer_keywords ms.assetid
How to implement interface events
Learn how to implement interface events in a class. See code examples and view additional available resources.
how-to
07/20/2015
interfaces [C#], event implementation in classes
events [C#], in interfaces
63527447-9535-4880-8e95-35e2075827df

How to implement interface events (C# Programming Guide)

An interface can declare an event. The following example shows how to implement interface events in a class. Basically the rules are the same as when you implement any interface method or property.

To implement interface events in a class

Declare the event in your class and then invoke it in the appropriate areas.

namespace ImplementInterfaceEvents  
{  
    public interface IDrawingObject  
    {  
        event EventHandler ShapeChanged;  
    }  
    public class MyEventArgs : EventArgs
    {  
        // class members  
    }  
    public class Shape : IDrawingObject  
    {  
        public event EventHandler ShapeChanged;  
        void ChangeShape()  
        {  
            // Do something here before the event…  

            OnShapeChanged(new MyEventArgs(/*arguments*/));  

            // or do something here after the event.
        }  
        protected virtual void OnShapeChanged(MyEventArgs e)  
        {  
            ShapeChanged?.Invoke(this, e);  
        }  
    }  

}

Example

The following example shows how to handle the less-common situation in which your class inherits from two or more interfaces and each interface has an event with the same name. In this situation, you must provide an explicit interface implementation for at least one of the events. When you write an explicit interface implementation for an event, you must also write the add and remove event accessors. Normally these are provided by the compiler, but in this case the compiler cannot provide them.

By providing your own accessors, you can specify whether the two events are represented by the same event in your class, or by different events. For example, if the events should be raised at different times according to the interface specifications, you can associate each event with a separate implementation in your class. In the following example, subscribers determine which OnDraw event they will receive by casting the shape reference to either an IShape or an IDrawingObject.

[!code-csharpcsProgGuideEvents#10]

See also