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Delegates with Named vs. Anonymous Methods (C# Programming Guide)

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A delegate can be associated with a named method. When you instantiate a delegate by using a named method, the method is passed as a parameter, for example:

// Declare a delegate.
delegate void WorkCallback(int x);

// Define a named method.
void DoWork(int k) { /* ... */ }

// Instantiate the delegate using the method as a parameter.
WorkCallback d = obj.DoWork;

The preceding example uses a named method. Delegates constructed with a named method can encapsulate either a static method or an instance method. Named methods are the only way to instantiate a delegate in earlier versions of C#. C# enables you to instantiate a delegate and immediately specify a code block that the delegate processes when called. The block can contain either a lambda expression or an anonymous method, as shown in the following example:

// Declare a delegate.
delegate void WorkCallback(int x);

// Instantiate the delegate using an anonymous method.
WorkCallback d = (int k) => { /* ... */ };

The method that you pass as a delegate parameter must have the same signature as the delegate declaration. A delegate instance can encapsulate either static or instance method.

Note

Although the delegate can use an out parameter, we do not recommend its use with multicast event delegates because you cannot know which delegate will be called.

Method groups with a single overload have a natural type. The compiler can infer the return type and parameter types for the delegate type:

var read = Console.Read; // Just one overload; Func<int> inferred
var write = Console.Write; // ERROR: Multiple overloads, can't choose

Examples

The following example is a simple example of declaring and using a delegate. Notice that both the delegate, MultiplyCallback, and the associated method, MultiplyNumbers, have the same signature

// Declare a delegate
delegate void MultiplyCallback(int i, double j);

class MathClass
{
    static void Main()
    {
        MathClass m = new MathClass();

        // Delegate instantiation using "MultiplyNumbers"
        MultiplyCallback d = m.MultiplyNumbers;

        // Invoke the delegate object.
        Console.WriteLine("Invoking the delegate using 'MultiplyNumbers':");
        for (int i = 1; i <= 5; i++)
        {
            d(i, 2);
        }

        // Keep the console window open in debug mode.
        Console.WriteLine("Press any key to exit.");
        Console.ReadKey();
    }

    // Declare the associated method.
    void MultiplyNumbers(int m, double n)
    {
        Console.Write(m * n + " ");
    }
}
/* Output:
    Invoking the delegate using 'MultiplyNumbers':
    2 4 6 8 10
*/

In the following example, one delegate is mapped to both static and instance methods and returns specific information from each.

// Declare a delegate
delegate void Callback();

class SampleClass
{
    public void InstanceMethod()
    {
        Console.WriteLine("A message from the instance method.");
    }

    static public void StaticMethod()
    {
        Console.WriteLine("A message from the static method.");
    }
}

class TestSampleClass
{
    static void Main()
    {
        var sc = new SampleClass();

        // Map the delegate to the instance method:
        Callback d = sc.InstanceMethod;
        d();

        // Map to the static method:
        d = SampleClass.StaticMethod;
        d();
    }
}
/* Output:
    A message from the instance method.
    A message from the static method.
*/

See also