C# 8.0 in a Nutshell

Chapter 4. Advanced C#

In this chapter, we cover advanced C# topics that build on concepts explored in Chapters 2 and 3. You should read the first four sections sequentially; you can read the remaining sections in any order.

Delegates

A delegate is an object that knows how to call a method.

A delegate type defines the kind of method that delegate instances can call. Specifically, it defines the method’s return type and its parameter types. The following defines a delegate type called Transformer:

delegate int Transformer (int x);

Transformer is compatible with any method with an int return type and a single int parameter, such as this:

static int Square (int x) { return x * x; }

Or, more tersely:

static int Square (int x) => x * x;

Assigning a method to a delegate variable creates a delegate instance:

Transformer t = Square;

You can invoke a delegate instance in the same way as a method:

int answer = t(3);    // answer is 9

Here’s a complete example:

delegate int Transformer (int x);

class Test
{
  static void Main()
  {
    Transformer t = Square;          // Create delegate instance
    int result = t(3);               // Invoke delegate
    Console.WriteLine (result);      // 9
  }
  static int Square (int x) => x * x;
}

A delegate instance literally acts as a delegate for the caller: the caller invokes the delegate and then the delegate calls the target method. This indirection decouples the caller from the target method.

The statement:

Transformer t = Square;

is shorthand for:

 Transformer t = new Transformer ...