C# Delegates are similar to pointers to functions, in C or C++. A delegate is a reference type variable that holds the reference to a method. The reference can be changed at runtime.
Delegates are specially used for implementing events and call-back methods. All delegates are implicitly derived from the System. Delegate class.
Declaring C# Delegates
Delegate declaration determines the methods that can be referenced by the delegate. A delegate can refer to a method, which has the same signature as that of the delegate.
For example, consider a delegate −
public delegate int MyDelegate (string s);
The preceding delegate can be used to reference any method that has a single string parameter and returns an int-type variable.
The syntax for delegate declaration is −
delegate <return type> <delegate-name> <parameter list>
Instantiating Delegates
Once a delegate type is declared, a delegate object must be created with the new keyword and be associated with a particular method. When creating a delegate, the argument passed to the new expression is written similar to a method call, but without the arguments to the method. For example −
public delegate void printString(string s); ... printString ps1 = new printString(WriteToScreen); printString ps2 = new printString(WriteToFile);
The following example demonstrates declaration, instantiation, and use of a delegate that can be used to reference methods that take an integer parameter and returns an integer value.
using System;
delegate int NumberChanger(int n);
namespace DelegateAppl {
class TestDelegate {
static int num = 10;
public static int AddNum(int p) {
num += p;
return num;
}
public static int MultNum(int q) {
num *= q;
return num;
}
public static int getNum() {
return num;
}
static void Main(string[] args) {
//create delegate instances
NumberChanger nc1 = new NumberChanger(AddNum);
NumberChanger nc2 = new NumberChanger(MultNum);
//calling the methods using the delegate objects
nc1(25);
Console.WriteLine("Value of Num: {0}", getNum());
nc2(5);
Console.WriteLine("Value of Num: {0}", getNum());
Console.ReadKey();
}
}
}
When the above code is compiled and executed, it produces the following result −
Value of Num: 35 Value of Num: 175
Multicasting of a Delegate
Delegate objects can be composed using the “+” operator. A composed delegate calls the two delegates it was composed of. Only delegates of the same type can be composed. The “-” operator can be used to remove a component delegate from a composed delegate.
Using this property of delegates you can create an invocation list of methods that will be called when a delegate is invoked. This is called the multicasting of a delegate. The following program demonstrates multicasting of a delegate −
using System;
delegate int NumberChanger(int n);
namespace DelegateAppl {
class TestDelegate {
static int num = 10;
public static int AddNum(int p) {
num += p;
return num;
}
public static int MultNum(int q) {
num *= q;
return num;
}
public static int getNum() {
return num;
}
static void Main(string[] args) {
//create delegate instances
NumberChanger nc;
NumberChanger nc1 = new NumberChanger(AddNum);
NumberChanger nc2 = new NumberChanger(MultNum);
nc = nc1;
nc += nc2;
//calling multicast
nc(5);
Console.WriteLine("Value of Num: {0}", getNum());
Console.ReadKey();
}
}
}
When the above code is compiled and executed, it produces the following result −
Value of Num: 75
Using Delegates
The following example demonstrates the use of delegates. The delegate printing can be used to reference a method that takes a string as input and returns nothing.
We use this delegate to call two methods, the first prints the string to the console, and the second one prints it to a file −
using System;
using System.IO;
namespace DelegateAppl {
class PrintString {
static FileStream fs;
static StreamWriter sw;
// delegate declaration
public delegate void printString(string s);
// this method prints to the console
public static void WriteToScreen(string str) {
Console.WriteLine("The String is: {0}", str);
}
//this method prints to a file
public static void WriteToFile(string s) {
fs = new FileStream("c:\\message.txt",
FileMode.Append, FileAccess.Write);
sw = new StreamWriter(fs);
sw.WriteLine(s);
sw.Flush();
sw.Close();
fs.Close();
}
// this method takes the delegate as parameter and uses it to
// call the methods as required
public static void sendString(printString ps) {
ps("Hello World");
}
static void Main(string[] args) {
printString ps1 = new printString(WriteToScreen);
printString ps2 = new printString(WriteToFile);
sendString(ps1);
sendString(ps2);
Console.ReadKey();
}
}
}
When the above code is compiled and executed, it produces the following result −
The String is: Hello World
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