You can use the Action`1 delegate to pass a method as a parameter without explicitly declaring a custom delegate. The encapsulated method must correspond to the method signature that is defined by this delegate. This means that the encapsulated method must have one parameter that is passed to it by value, and it must not return a value. (In C#, the method must return void. In Visual Basic, it must be defined by the Sub…End Sub construct. It can also be a method that returns a value that is ignored.) Typically, such a method is used to perform an operation.

When you use the Action`1 delegate, you do not have to explicitly define a delegate that encapsulates a method with a single parameter. For example, the following code explicitly declares a delegate named DisplayMessage and assigns a reference to either the Console.WriteLine method or the ShowWindowsMessage method to its delegate instance.

code reference: System.Action~1#1

The following example simplifies this code by instantiating the Action`1 delegate instead of explicitly defining a new delegate and assigning a named method to it.

code reference: System.Action~1#2

You can also use the Action`1 delegate with anonymous methods in C#, as the following example illustrates. (For an introduction to anonymous methods, see Anonymous Methods (C# Programming Guide).)

code reference: System.Action~1#3

You can also assign a lambda expression to an Action`1 delegate instance, as the following example illustrates. (For an introduction to lambda expressions, see Lambda Expressions (C# Programming Guide).)

code reference: System.Action~1#4

The List`1.ForEach(Action<`0>) and Array.ForEach``1(``0[], Action<``0>) methods each take an Action`1 delegate as a parameter. The method encapsulated by the delegate allows you to perform an action on each element in the array or list. The example uses the List`1.ForEach(Action<`0>) method to provide an illustration.

Namespace: System
Assembly: mscorlib (in mscorlib.dll)
Assembly Versions: 4.0.0.0