A delegate is a data type that holds a reference to a method with a compatible signature. It is similar to a function pointer in C and C++.
System.Delegate class.The following example declares a delegate named MyDelegate that can encapsulate a method that takes a string as an argument and returns void.
public delegate void MyDelegate(string message);
The delegate type can be declared using the delegate keyword. After a delegate declaration, you can call those methods whose return type and parameter-list matches with the delegate using its instance.
Let's create a simple method that will take a string as a parameter and print that string to the console window.
private static void PrintMessage(string message)
{
Console.WriteLine(message);
}
A delegate object is created with the help of the new keyword or you can assign the method name directly to the delegate object as shown below.
MyDelegate delegate1 = new MyDelegate(PrintMessage);
MyDelegate delegate2 = PrintMessage;
delegate1("This is a C# Tutorial.");
delegate1("You are learning Delegates.");
Once a delegate is instantiated, a method call made to the delegate is passed by the delegate to that method. Let's run the above code and you will see the following output.
This is a C# Tutorial.
You are learning Delegates.
Let's have a loot into another example where we use a single delegate object and different methods with that object. Here are the delegate declaration and MathUtility class implementation.
private delegate int MyMathDelegate(int number1, int number2);
public static class MathUtility
{
public static int Add(int number1, int number2)
{
return (number1 + number2);
}
public static int Subtract(int number1, int number2)
{
return (number1 - number2);
}
public static int Multiply(int number1, int number2)
{
return (number1 * number2);
}
}
The following code first instantiates the delegate object and uses different methods one by one.
MyMathDelegate mathDelegate = new MyMathDelegate(MathUtility.Add);
int num1 = 10;
int num2 = 20;
Console.WriteLine("Add({0}, {1}) = {2}", num1, num2, mathDelegate(num1, num2));
// Assign the Subtract method reference to the delegate object.
mathDelegate = MathUtility.Subtract;
Console.WriteLine("Subtract({0}, {1}) = {2}", num1, num2, mathDelegate(num1, num2));
// Assign the Multiply method reference to the delegate object.
mathDelegate = MathUtility.Multiply;
Console.WriteLine("Multiply({0}, {1}) = {2}", num1, num2, mathDelegate(num1, num2));
Let's run the above code and you will see the following output.
Add(10, 20) = 30
Subtract(10, 20) = -10
Multiply(10, 20) = 200
A useful property of delegate objects is that multiple objects can be assigned to one delegate instance by using the + operator.
The following code shows how you can use multicasting of a delegate.
private delegate void MyDelegate(string message);
private static void Welcome(string message)
{
Console.WriteLine("Welcome: " + message);
}
private static void GoodBye(string message)
{
Console.WriteLine("Bye-bye: " + message);
}
public static void CombineDelegatesExample()
{
MyDelegate welcomeDel = Welcome;
MyDelegate goodByeDel = GoodBye;
MyDelegate welcomeGoodByeDel = welcomeDel + goodByeDel;
Console.WriteLine("Call welcomeDel.");
welcomeDel("Mark");
Console.WriteLine("\nCall goodByeDel.");
goodByeDel("John");
Console.WriteLine("\nCall welcomeGoodByeDel.");
welcomeGoodByeDel("Stella");
}
Let's run the above code and you will see the following output.
Call welcomeDel.
Welcome: Mark
Call goodByeDel.
Bye-bye: John
Call welcomeGoodByeDel.
Welcome: Stella
Bye-bye: Stella
For more information about delegates, visit https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/delegates/
All the examples related to the delegates are available in the Delegates.cs file of the source code. Download the source code and try out all the examples for better understanding.