If you're already familier with the concept of delegate, then you can skip the next section titled "What is a delegate?."

In C#, a delegate is a way to encapsulate a method reference that allows you to treat the method as an objects. Let's take a look at an example here:

// Declare your delegate
public delegate void MyDelegate(string message);

static void Main(string[] args)
{
  // Instantiate the delegate by assigning the `PrintMessage` method
  MyDelegate myDelegate = PrintMessage;

  // Call the delegate with the message "Hello, world!".
  myDelegate("Hello, world!");
}

static void PrintMessage(string message)
{
  Console.WriteLine(message);
} 

• Action
• Func

The Action delegate is a generic type delegate that represents a method that does not return a value. It can take up to 16 input parameters of any type. Here are a few examples:

// Action with no parameters
Action print1 = () => Console.WriteLine("Hello World!");
print1(); // Output: Hello World!

// Action with one parameter
Action<string> print2 = (message) => Console.WriteLine(message);
print2("Hello World!"); // Output: Hello World!

// Action with multiple parameters
Action<string,string> print3 = (message1, message2) => Console.WriteLine($"{message1} {message2}");
print3("Hello", "World!"); // Output: Hello World!

In the first example, we define an Actoion delegate named print1 that takes no parameters and simply writes "Hello World!" to the console when invoked. We can then call print1() to execute the method.

In the second example, we define an Action delegate named print2 that takes a string (message) as a parameter. When we invoke it, it prints the message to the console.

In the third example, we define an Action delegate named print3 that takes two string messages as parameters. When we invoke it, it prints those messages in sequence.

The Func delegate is a generic type delegate that represents a method that takes input parameters and returns a value. The last type argument of Func represents the return type. It can take up to 16 input parameters.

• Using Delegates (C# Programming Guide)
• Action Delegate
• Func Delegate