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C# delegate KeywordUse delegate methods. Understand lambda expressions and types such as Predicate.
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Delegate. This keyword (delegate) is used to create method objects. With the Invoke() method, we can call a delegate as a method.

Event

Notes, higher-order. We can pass the delegate as an argument to another method. The function is an object—a delegate is a higher-order procedure.

Example. This program declares a delegate type D that receives a string, and returns no value. In Main, we create an instance of type D.

Lambda We specify the method body with a lambda expression. We can use lambda expressions with delegate types.

Lambda

Then We call the delegate D method implementation with the argument "cat" using the Invoke method.

Tip This program uses the "delegate" keyword. But many programs use higher-order procedures with just lambda expressions.

C# program that uses delegate
using System; class Program { delegate void D(string value); static void Main() { // ... Specify delegate with lambda expression. D d = v => Console.WriteLine(v); // ... Invoke delegate. d.Invoke("cat"); } }
cat

Example 2. Next, we build upon the delegate concept. We declare a delegate type UppercaseDelegate. It receives a string, and returns a string.

Main We create instances of this delegate Type. We need to declare a delegate type before creating instances of it.

Important When you construct instances of the delegate type, the target method must have the same return value and arguments.

WriteOutput The second argument is of type UppercaseDelegate. When called, WriteOutput invokes the function argument.

Note The target method is specified as the sole argument. It is a method name, not a variable.

C# program that uses UppercaseDelegate
using System; class Program { delegate string UppercaseDelegate(string input); static string UppercaseFirst(string input) { char[] buffer = input.ToCharArray(); buffer[0] = char.ToUpper(buffer[0]); return new string(buffer); } static string UppercaseLast(string input) { char[] buffer = input.ToCharArray(); buffer[buffer.Length - 1] = char.ToUpper(buffer[buffer.Length - 1]); return new string(buffer); } static string UppercaseAll(string input) { return input.ToUpper(); } static void WriteOutput(string input, UppercaseDelegate del) { Console.WriteLine("Your string before: {0}", input); Console.WriteLine("Your string after: {0}", del(input)); } static void Main() { // Wrap the methods inside delegate instances and pass to the method. WriteOutput("perls", new UppercaseDelegate(UppercaseFirst)); WriteOutput("perls", new UppercaseDelegate(UppercaseLast)); WriteOutput("perls", new UppercaseDelegate(UppercaseAll)); } }
Your string before: perls Your string after: Perls Your string before: perls Your string after: perlS Your string before: perls Your string after: PERLS

Anonymous function. Anonymous functions include lambda expressions. You can pass a lambda expression as the implementation of the delegate.

Anonymous Functions

Tip You can construct the UppercaseDelegate by specifying a lambda expression inside the constructor itself.

Info This is functionally equivalent to using a static method with the same return value and argument.

C# program that uses lambda expression, delegate
using System; class Program { // ... // ... (Paste other methods here) // ... static void Main() { WriteOutput("perls", new UppercaseDelegate(x => x.ToUpper())); } }
PERLS

Lambda. The syntax for delegate functions can be complex. Lambda expressions provide a simple and terse way of specifying functions.

Tip Lambdas use the => syntax to separate formal parameters and the method body.

Info Whenever you see a method that receives a Predicate, Action, Func or Comparison, a lambda can be passed.

Predicate

Action

Func

Comparison

Events. Events allow you to specify that when external event occurs, such as a mouse click, a delegate method should always be executed.

But You can define custom events in any program. They are occasionally useful in complex programs.

LINQ. In LINQ extensions, delegate methods are extensively used. Even query expressions are implemented with methods that receive delegates.

A summary. Delegate syntax is at first confusing. Once you understand the instantiation model and type system, it offers immense benefits to certain programs.

With the type system, the language offers the ability to specify actions as data. Complicated (and possibly hard-to-understand) programs can be written.

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