Understanding C# Delegates (with Examples)

Last updated on August 21st, 2023 at 05:15 pm

In simple terms, a delegate in C# is like a pointer to a method. It allows you to refer to a method by its name and invoke it later. Think of it as a “method container” that holds a reference to a particular method with a specific signature (parameters and return type).

What is Delegate in C#?

In C#, A delegate is a type that represents a reference to a method with a specific signature (parameters and return type).

A delegate is like a container for a method. It’s like having a remote control that can activate a specific function in your code.

Instead of directly calling a method, you can use a delegate to call it indirectly. Delegates are helpful when you want to pass methods as parameters into the other methods as if they were objects.

Delegates are useful for events, callbacks, and handling async operations. They let you define and execute different actions based on conditions or user interactions.

So, a C# delegate is a way to indirectly store and refer to a method. It’s like a remote control or note representing a task to be done later in your program.

The C# delegate shares similarities with the function pointer in C and C++. However, unlike the function pointer in C, the C# delegate is object-oriented, type-safe, and secure.

Declaring a Delegate: 

To declare a delegate, you need to define the delegate type. The delegate type specifies the method signature that the delegate can point to. 

Here is an example of declaring a delegate called “MyDelegate” that can point to a method taking two integers and returning an integer:

delegate int MyDelegate(int x, int y);

Using a Delegate: 

Once you have declared a delegate, you can create an instance of it and assign it a method to invoke. Here is how you can create a delegate to point to a method:

int Add(int a, int b)
{
    return a + b;
}

// Create an instance of MyDelegate and assign it the Add method
MyDelegate del = Add;

Invoking a Delegate:

To invoke a delegate and execute the method it points to, you can use the delegate instance as if it were the method itself:

int result = del(5, 3);
Console.WriteLine(result); // Output: 8

Types of Delegates in C#

There are three types of delegates in C#: 

1. Single cast Delegates
2. Multicast Delegates
3. Generic Delegates ( Func, Action, and Predicate delegate)

• A single-cast delegate holds a reference to a single method.
• A multi-cast delegate holds a reference to multiple methods.
• A generic delegate can hold a reference to a method with any signature.

As per MSDN:

The  Delegate  is an abstract class and it is the base class for all the delegate types.

Even though it is an abstract class only the compilers can derive explicitly from the Delegate class or from the MulticastDelegate   class, not the user.
It is not allowed to derive a new type from a delegate type.

The Delegate class is not considered a delegate type. However, it is used to derive delegate types implicitly by the compiler.

Single cast Delegate:

A single-cast delegate is a type of Delegate that can only point to one method at a time. It is derived from the System.Delegate class. This means that a single-cast delegate holds a reference to a single method and can invoke that method when called.

Following is the example of the single cast delegate in C#.

using System;
namespace DelegateExample
{
    // Defining a delegate to refer the methods having same signature.
    public delegate void MyDelegate(string message);
 
    class Program
    {
        // Defining a method
        public static void Print(string message)
        {
            Console.WriteLine($"{message}");
        }
        static void Main(string[] args)
        {
            // Instantiating delegate and passing the method name
            // as a parameter which have the same signature
 
            MyDelegate myDelegate = new MyDelegate(Print);
 
            myDelegate.Invoke("Hello Shekh");
            // Or
           // myDelegate("Hello Shekh");
            Console.ReadLine();
        }
        // Output: Hello Shekh
    }
}

Multicast Delegate:

A multicast delegate is a type of Delegate that can hold references to multiple methods. It is derived from the System.MulticastDelegate class. This means that a multicast delegate can store and invoke more than one method from its invocation list, executing them in sequence when invoked.

To add multiple methods to a multicast delegate, we can use the Delegate.Combine method or the “+” operator. To remove a method reference from the invocation list, we can use the “-” operator. It’s important to note that when invoking multiple methods using a multicast delegate, all the methods should have the same signature (parameter types and return type must match), and the return type must be void.

Here is an Example of Multicast Delegate:

delegate void MyMultiDelegate();

void Method1()
{
    Console.WriteLine("Method 1");
}

void Method2()
{
    Console.WriteLine("Method 2");
}

// Create a multicast delegate and add both methods to it
MyMultiDelegate multiDel = Method1;
multiDel += Method2;

// Invoke the multicast delegate, which will call both methods
multiDel(); // Output: Method 1
            //         Method 2

Example 2: Multicast delegates

The following program demonstrates the multicasting of a delegate.

Example of multicast delegate using  Delegate.Combine  method.

using System;
namespace MulticastDelegate
{
    class Program
    {
        // Declaring a delegate
        public delegate void MyDelegate();

        static void Main(string[] args)
        {
         // Declares the single delegate that points to MethodA
         MyDelegate delegateA = new MyDelegate(MethodA);

        // Declares the single delegate that points to MethodB
        MyDelegate delegateB = new MyDelegate(MethodB);

        // multicast delegate combining both delegates A and B
        MyDelegate multicastDeligate = (MyDelegate)Delegate.Combine(delegateA, delegateB);
           
        // Invokes the multicast delegate
        multicastDeligate.Invoke();

        }

        static void MethodA()
        {
            Console.WriteLine("Executing method A.");
        }

        static void MethodB()
        {
            Console.WriteLine("Executing method B.");
        }
    }
}

Example of multicast delegate using “+” and “-“ operator.

using System;
namespace MulticastDelegate
{
    class Program
    {
        /// Declaring a delegate
        public delegate void MyDelegate();

        static void Main(string[] args)
        {
            // Add MethodA
            MyDelegate multicastDelegate = Program.MethodA;
            
            // To MethodB
            multicastDelegate += Program.MethodB;

            // To remove MethodA from the invocation list
            // multicastDelegate -= Program.MethodA;

            // Get the list of all delegates added to Multicast Delegate.
            Delegate[] delegateList = multicastDelegate.GetInvocationList();
            Console.WriteLine($"MulticastDelegate contains {delegateList.Length} delegate(s).");
            
            // Invokes the multicast delegate
            multicastDelegate.Invoke();

        }

        static void MethodA()
        {
            Console.WriteLine("Executed method A.");
        }

        static void MethodB()
        {
            Console.WriteLine("Executed method B.");
        }

    }
}

In the above example, a multicast delegate is used to hold a reference to the multiple methods.

C# delegate callback example

The following declaration shows how to use a delegate to declare a callback notification.

using System;
using System.Threading;
namespace DelegateExample
{
    public delegate void MyDelegate(int num);
 
    class DelegateCallBack
    {
        public static void LongRunningMethod(MyDelegate myDelegate)
        {
            for (int num = 1; num <= 5; num++)
            {
                Thread.Sleep(TimeSpan.FromSeconds(2));
 
                myDelegate(num);
            }
        }
    }
    class program
    {
        public static void Main()
        {
 
            DelegateCallBack.LongRunningMethod(Print);
 
            Console.ReadLine();
        }
        public static void Print(int number)
        {
            Console.WriteLine($" num : {number}");
 
        }
    }
}

Output:

C# Anonymous method using the delegate keyword

We can use the delegate to invoke the anonymous method.

An anonymous method is a method without a name and return type.
It can be created using the delegate keyword and optional parameters and can be assigned to a variable of the delegate type.

The following is an example of the anonymous method.

using System;
namespace AnonymousMethodDemo
{
    class Program
    {
        // Declaring a delegate
        public delegate int MyDelegate(int num1, int num2);
 
        static void Main(string[] args)
        {
            // Instantiating the delegate using an anonymous method
 
         MyDelegate myDelegate = delegate (int num1, int num2)
         {
            return num1 + num2;
         };
 
            Console.WriteLine(myDelegate(10,20));
 
        }
    }
    // Output: 30
}

Important points about C# delegates

Here are some important points about C# delegates explained simply:

• C# delegates are like containers that hold the name and signature of a method.
• Delegates allow you to indirectly call methods by invoking the delegate.
• Delegates provide a way to pass methods around as if they were objects.
• They enable flexible and reusable code by decoupling method calls from their implementations.
• Delegates are object-oriented, type-safe, and secure.
• Singlecast delegates can point to only one method at a time.
• Multicast delegates can hold references to multiple methods and invoke them in sequence.
• To add multiple methods to a multicast delegate, you can use the Delegate.Combine method or the “+” operator.
• To remove a method reference from a multicast delegate, you can use the “-” operator.
• All methods in a multicast delegate must have the same signature (parameter types and return type must match).
• Delegates are commonly used in event handling, callback functions, and asynchronous programming.
• Delegates provide a higher level of abstraction and ensure type safety during compilation to prevent runtime errors.

Conclusion

C# delegates provide a powerful way to encapsulate and pass around methods as objects. They enable you to write more flexible and reusable code by separating method invocation from method implementation.

FAQs

I hope you like this article “C# Delegates“, if you have any questions, please ask your questions in the comment section below.

References MSDN: C# Delegates, Delegates, geeksforgeeks- C#|Delegates

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