"C #" delegate-delegate

C#delegate (Delegate)

A delegate in C # (Delegate) is similar to a pointer to a function in C or C + +. A delegate (Delegate) is a reference type variable that holds a reference to a method. References can be changed at run time.

Delegates (Delegate) are specifically used to implement events and callback methods. All delegates (Delegate) are derived from the System.Delegate class.

Declaration Delegate (Delegate)

A delegate declaration determines the method that can be referenced by the delegate. A delegate can point to a method that has the same label as its.

For example, suppose you have a delegate:

public delegate int MyDelegate (string s);

　　

The above delegate can be used to refer to any one method with a single string parameter and return an int type variable.

The syntax for declaring a delegate is as follows:

Delegate <return type> <delegate-name> <parameter list>

　　

Instantiating a delegate (Delegate)

Once a delegate type is declared, the delegate object must be created using the new keyword and is related to a specific method. When a delegate is created, the arguments passed to the new statement are written like a method call, but without parameters. For example:

public delegate void Printstring (string s); printstring PS1 = new Printstring (writetoscreen);p rintstring ps2 = new Print String (WriteToFile);

　　

The following example demonstrates the declaration, instantiation, and use of a delegate that can be used to reference a method with an integer parameter and return an integer value.

1 usingSystem;
2 3 Delegate intNumberchanger (intn);4 namespaceDELEGATEAPPL5 {6    classtestdelegate7    {8       Static intnum =Ten;9        Public Static intAddnum (intp)Ten       { Onenum + =p; A          returnnum; -       } -  the        Public Static intMultnum (intq) -       { -Num *=Q; -          returnnum; +       } -        Public Static intGetnum () +       { A          returnnum; at       } -  -       Static voidMain (string[] args) -       { -          //Create a delegate instance -Numberchanger NC1 =NewNumberchanger (addnum); inNumberchanger NC2 =NewNumberchanger (multnum); -          //To invoke a method using a delegate object toNC1 ( -); +Console.WriteLine ("Value of Num: {0}", Getnum ()); -NC2 (5); theConsole.WriteLine ("Value of Num: {0}", Getnum ()); * Console.readkey (); $       }Panax Notoginseng    } -}

When the above code is compiled and executed, it produces the following results:

Value of Num:35value of num:175

Delegated Multicast (multicasting of a Delegate)

Delegate objects can be merged using the "+" operator. A merge delegate invokes the two delegates it merges. Only delegates of the same type can be merged. The "-" operator can be used to remove a component delegate from a merged delegate.

Using this useful feature of delegates, you can create a call list of methods to invoke when a delegate is called. This is called a delegated multicast (multicasting), also called multicast. The following program demonstrates multicast for a delegate:

1 usingSystem;2 3 Delegate intNumberchanger (intn);4 namespaceDELEGATEAPPL5 {6    classtestdelegate7    {8       Static intnum =Ten;9        Public Static intAddnum (intp)Ten       { Onenum + =p; A          returnnum; -       } -  the        Public Static intMultnum (intq) -       { -Num *=Q; -          returnnum; +       } -        Public Static intGetnum () +       { A          returnnum; at       } -  -       Static voidMain (string[] args) -       { -          //Create a delegate instance - Numberchanger NC; inNumberchanger NC1 =NewNumberchanger (addnum); -Numberchanger NC2 =NewNumberchanger (multnum); toNC =nc1; +NC + =Nc2; -          //Call Multicast thenc5); *Console.WriteLine ("Value of Num: {0}", Getnum ()); $ Console.readkey ();Panax Notoginseng       } -    } the}

When the above code is compiled and executed, it produces the following results:

Value of num:75

Purpose of the delegate (Delegate)

The following example demonstrates the use of a delegate. The delegate printstring can be used to refer to a method with a string as input and does not return anything.

We use this delegate to invoke two methods, the first to print a string to the console, and the second to print the string to a file:

1 usingSystem;2 usingSystem.IO;3 4 namespaceDELEGATEAPPL5 {6    classprintstring7    {8       StaticFileStream FS;9       StaticStreamWriter SW;Ten       //Delegate Declaration One        Public Delegate voidPrintstring (strings); A  -       //This method prints to the console -        Public Static voidWritetoscreen (stringstr) the       { -Console.WriteLine ("The String is: {0}", str); -       } -       //This method prints to a file +        Public Static voidWriteToFile (strings) -       { +FS =NewFileStream ("C:\\message.txt", A filemode.append, FileAccess.Write); atSW =NewStreamWriter (FS); - SW. WriteLine (s); - SW. Flush (); - SW. Close (); - FS. Close (); -       } in       //The method takes the delegate as an argument and uses it to invoke the method -        Public Static voidsendstring (printstring PS) to       { +ps"Hello World"); -       } the       Static voidMain (string[] args) *       { $printstring PS1 =Newprintstring (writetoscreen);Panax Notoginsengprintstring PS2 =Newprintstring (writetofile); - sendstring (PS1); the sendstring (PS2); + Console.readkey (); A       } the    } +}

When the above code is compiled and executed, it produces the following results:

The String Is:hello World

　　

"C #" delegate-delegate