System.Action

Added in .NET 3.5
The Action delegate can be used for methods that accept one or more arguments and do not return a value.

System.Action<string, string> delegate_variable = delegate(string param1, string param2); 
{ //do something } 

instead of

delegate void Delegate_Type(string x, string y); 

Delegate_Type delegate_variable delegate(string param1, string param2) 
{ // do something } 

This delegate can take zero (or more) parameters and must return void.
We have a class that contains a method that we want to call.

public class MyClass 
{ 
   public void MyMethod() { MessageBox.Show("something"); } 
} 

Before .NET 3.5 we had to declare the delegate.

public delegate void MyDelegate(); 

public class WithDeclaration 
{ 
   public static void Main() 
   { 
      MyClass class1 = new MyClass(); 
      MyDelegate delegate1 = class1.MyMethod; 
      delegate1(); 
   } 
} 

With .NET 3.5 the custom declaration can now be replaced with the built-in data type new System.Action
There are several ways this could be written.

public class WithoutDeclaration 
{ 
   public static void Main() 
   { 
      MyClass class2 = new MyClass(); 

      System.Action delegate2 = class2.MyMethod; 
      System.Action delegate2 = new System.Action(class2.MyMethod); 
      delegate2(); 

      // other ways of writing the same thing 
      System.Action delegate3 = delegate() { MessageBox.Show("something"); } 
      System.Action delegate3 = new System.Action( () => MessageBox.Show("something"); ); 
      delegate3(); 
   } 
}