qr code c# asp.net Delegates, Events, and Lambda Expressions in C#

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Delegates, Events, and Lambda Expressions
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of the anonymous method and is now the preferred approach to creating an anonymous function However, anonymous methods are still used in legacy C# code Therefore, it is important to know how they work Furthermore, anonymous methods are the precursor to lambda expressions and a clear understanding of anonymous methods makes it easier to understand aspects of the lambda expression Also, there is a narrow set of cases in which an anonymous method can be used, but a lambda expression cannot Therefore, both anonymous methods and lambda expressions are described in this chapter
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PART I
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Anonymous Methods
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An anonymous method is one way to create an unnamed block of code that is associated with a specific delegate instance An anonymous method is created by following the keyword delegate with a block of code To see how this is done, let s begin with a simple example The following program uses an anonymous method that counts from 0 to 5
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// Demonstrate an anonymous method using System; // Declare a delegate type delegate void CountIt(); class AnonMethDemo { static void Main() { // Here, the code for counting is passed // as an anonymous method CountIt count = delegate { // This is the block of code passed to the delegate for(int i=0; i <= 5; i++) ConsoleWriteLine(i); }; // notice the semicolon count(); } }
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This program first declares a delegate type called CountIt that has no parameters and returns void Inside Main( ), a CountIt instance called count is created, and it is passed the block of code that follows the delegate keyword This block of code is the anonymous method that will be executed when count is called Notice that the block of code is followed by a semicolon, which terminates the declaration statement The output from the program is shown here:
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0 1 2 3 4 5
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Part I:
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Pass Arguments to an Anonymous Method
It is possible to pass one or more arguments to an anonymous method To do so, follow the delegate keyword with a parenthesized parameter list Then, pass the argument(s) to the delegate instance when it is called For example, here is the preceding program rewritten so that the ending value for the count is passed:
// Demonstrate an anonymous method that takes an argument using System; // Notice that CountIt now has a parameter delegate void CountIt(int end); class AnonMethDemo2 { static void Main() { // Here, the ending value for the count // is passed to the anonymous method CountIt count = delegate (int end) { for(int i=0; i <= end; i++) ConsoleWriteLine(i); }; count(3); ConsoleWriteLine(); count(5); } }
In this version, CountIt now takes an integer argument Notice how the parameter list is specified after the delegate keyword when the anonymous method is created The code inside the anonymous method has access to the parameter end in just the same way it would if a named method were being created The output from this program is shown next:
0 1 2 3 0 1 2 3 4 5
Return a Value from an Anonymous Method
An anonymous method can return a value The value is returned by use of the return statement, which works the same in an anonymous method as it does in a named method As you would expect, the type of the return value must be compatible with the return type
15:
Delegates, Events, and Lambda Expressions
specified by the delegate For example, here the code that performs the count also computes the summation of the count and returns the result:
PART I
// Demonstrate an anonymous method that returns a value using System; // This delegate returns a value delegate int CountIt(int end); class AnonMethDemo3 { static void Main() { int result; // Here, the ending value for the count // is passed to the anonymous method // A summation of the count is returned CountIt count = delegate (int end) { int sum = 0; for(int i=0; i <= end; i++) { ConsoleWriteLine(i); sum += i; } return sum; // return a value from an anonymous method }; result = count(3); ConsoleWriteLine("Summation of 3 is " + result); ConsoleWriteLine(); result = count(5); ConsoleWriteLine("Summation of 5 is " + result); } }
In this version, the value of sum is returned by the code block that is associated with the count delegate instance Notice that the return statement is used in an anonymous method in just the same way that it is used in a named method The output is shown here:
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