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Using Variance in Delegates (C#)

  • Article

When you assign a method to a delegate, covariance and contravariance provide flexibility for matching a delegate type with a method signature. Covariance permits a method to have return type that is more derived than that defined in the delegate. Contravariance permits a method that has parameter types that are less derived than those in the delegate type.

Example 1: Covariance

Description

This example demonstrates how delegates can be used with methods that have return types that are derived from the return type in the delegate signature. The data type returned by DogsHandler is of type Dogs, which derives from the Mammals type that is defined in the delegate.

Code

class Mammals {}  
class Dogs : Mammals {}  
  
class Program  
{  
    // Define the delegate.  
    public delegate Mammals HandlerMethod();  
  
    public static Mammals MammalsHandler()  
    {  
        return null;  
    }  
  
    public static Dogs DogsHandler()  
    {  
        return null;  
    }  
  
    static void Test()  
    {  
        HandlerMethod handlerMammals = MammalsHandler;  
  
        // Covariance enables this assignment.  
        HandlerMethod handlerDogs = DogsHandler;  
    }  
}

Example 2: Contravariance

Description

This example demonstrates how delegates can be used with methods that have parameters whose types are base types of the delegate signature parameter type. With contravariance, you can use one event handler instead of separate handlers. The following example makes use of two delegates:

  • A KeyEventHandler delegate that defines the signature of the Button.KeyDown event. Its signature is:

    public delegate void KeyEventHandler(object sender, KeyEventArgs e)

  • A MouseEventHandler delegate that defines the signature of the Button.MouseClick event. Its signature is:

    public delegate void MouseEventHandler(object sender, MouseEventArgs e)

The example defines an event handler with an EventArgs parameter and uses it to handle both the Button.KeyDown and Button.MouseClick events. It can do this because EventArgs is a base type of both KeyEventArgs and MouseEventArgs.

Code

// Event handler that accepts a parameter of the EventArgs type.  
private void MultiHandler(object sender, System.EventArgs e)  
{  
    label1.Text = System.DateTime.Now.ToString();  
}  
  
public Form1()  
{  
    InitializeComponent();  
  
    // You can use a method that has an EventArgs parameter,  
    // although the event expects the KeyEventArgs parameter.  
    this.button1.KeyDown += this.MultiHandler;  
  
    // You can use the same method
    // for an event that expects the MouseEventArgs parameter.  
    this.button1.MouseClick += this.MultiHandler;  
  
}

See also