qr code generator c# example Delegates, Events, and Lambda Expressions in Visual C#

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Delegates, Events, and Lambda Expressions
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kevtKeyPress += (source, arg) => ConsoleWriteLine(" Received keystroke: " + argch);
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Next, another lambda expression based handler is added to kvetKeyPress by the following code It counts the number of keypresses
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kevtKeyPress += (source, arg) => count++; // count is an outer variable
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PART I PART I PART I
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Notice that count is a local variable declared in Main( ) that is initialized to zero Next, a loop is started that calls kevtOnKeyPress( ) when a key is pressed This causes the registered event handlers to be notified When the loop ends, the number of keypresses is displayed Although quite simple, this example illustrates the essence of event handling The same basic approach will be used for other event handling situations Of course, in some cases, anonymous event handlers will not be appropriate and named methods will need to be employed
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CHAPTER
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Namespaces, the Preprocessor, and Assemblies
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his chapter discusses three C# features that give you greater control over the organization and accessibility of a program These are namespaces, the preprocessor, and assemblies
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Namespaces
The namespace was mentioned briefly in 2 because it is a concept fundamental to C# In fact, every C# program makes use of a namespace in one way or another We didn t need to examine namespaces in detail before now because C# automatically provides a default, global namespace for your program Thus, the programs in earlier chapters simply used the global namespace In the real world, however, many programs will need to create their own namespaces or interact with other namespaces Here, they are examined in detail A namespace defines a declarative region that provides a way to keep one set of names separate from another In essence, names declared in one namespace will not conflict with the same names declared in another The namespace used by the NET Framework library (which is the C# library) is System This is why you have included
using System;
near the top of every program As explained in 14, the I/O classes are defined within a namespace subordinate to System called SystemIO There are many other namespaces subordinate to System that hold other parts of the C# library Namespaces are important because there has been an explosion of variable, method, property, and class names over the past few years These include library routines, thirdparty code, and your own code Without namespaces, all of these names would compete for slots in the global namespace and conflicts would arise For example, if your program defined a class called Finder, it could conflict with another class called Finder supplied by a third-party library that your program uses Fortunately, namespaces prevent this type of problem because a namespace restricts the visibility of names declared within it
Part I:
The C# Language
Declaring a Namespace
A namespace is declared using the namespace keyword The general form of namespace is shown here: namespace name { // members } Here, name is the name of the namespace A namespace declaration defines a scope Anything declared immediately inside the namespace is in scope throughout the namespace Within a namespace, you can declare classes, structures, delegates, enumerations, interfaces, or another namespace Here is an example of a namespace that creates a namespace called Counter It localizes the name used to implement a simple countdown counter class called CountDown
// Declare a namespace for counters namespace Counter { // A simple countdown counter class CountDown { int val; public CountDown(int n) { val = n; } public void Reset(int n) { val = n; } public int Count() { if(val > 0) return val--; else return 0; } } } // This is the end of the Counter namespace
Notice how the class CountDown is declared within the scope defined by the Counter namespace To follow along with the example, put this code into a file called Countercs Here is a program that demonstrates the use of the Counter namespace:
// Demonstrate the Counter namespace using System; class NSDemo { static void Main() { // Notice how CountDown is qualified by Counter CounterCountDown cd1 = new CounterCountDown(10); int i; do { i = cd1Count(); ConsoleWrite(i + " ");
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