C# ПІДРУЧНИКИ / c# / MS Press - Msdn Training Programming Net Framework With C#

Demonstration: Hello World

Topic Objective

To demonstrate how to build a simple application in C#.

Lead-in

In this demonstration, you will learn how to build a simple application in C#.

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2. Save the file as HelloDemoCS.cs

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You can fully reference classes, as in the following example, in which an instance of System.IO.FileStream is declared by using C#:

System.IO.Filestream aFileStream;

However, it is more convenient to reference the required namespaces in your program. Using the namespace effectively disposes of the need to qualify all class library references, as in the following example:

using System.IO;

...

FileStream aFileStream;

For example, in order to have convenient access to System objects, you must use the System namespace.

6Module 2: Introduction to a Managed Execution Environment

Defining a Namespace and a Class

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C# supports the creation of custom namespaces and classes within those namespaces.

Tip The following is the general rule for naming namespaces:

CompanyName.TechnologyName

For example:

Microsoft.Office

This is merely a guideline. Third-party companies can choose other names.

Namespaces in C#

In C#, you use the namespace statement to define a new namespace, which encapsulates the classes that you create, as in the following example:

namespace CompCS {

public class StringComponent {

...

}

}

Note that a namespace may be nested in other namespaces, and a single namespace may be defined in multiple files. A single source code file may also define multiple namespaces.

Entry Points, Scope, and Declarations

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Every executable program must contain an external entry point, where the application begins its execution. In C#, all code must be contained in methods of a class.

Entry Points in C#

To accommodate the entry point code in C#, you must first specify the class, as in the following example:

class MainApp {...}

Next, you specify the entry point for your program. The compiler requires this entry point to be a public static method called Main, as in the following example:

public static void Main () {...}

Scope

C# uses the period as a scope resolution operator. For example, you use the syntax Console.WriteLine when referencing the WriteLine method of the

Console object.

Declaring and Instantiating Variables

In C#, you must declare a variable before it can be used. To instantiate the object, use the new keyword. The following example in C# shows how to declare an object of type Comp, in namespace Lib, with the name myComp:

Lib.Comp myComp = new Lib.Comp();

8Module 2: Introduction to a Managed Execution Environment

Console Input and Output

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You can use the common language runtime Console class of the System namespace for input and output to the console of any string or numeric value by using the Read, ReadLine, Write, and WriteLine methods.

The following example shows a C# program that outputs a string to the console:

using System;

class MainApp {

public static void Main() {

//Write text to the console Console.WriteLine(“Hello World using C#!”);

}

}

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Microsoft Visual C++® and C# are case-sensitive, but Microsoft Visual Basic® is not case-sensitive. To ensure that a program is compliant with the Common Language Specification (CLS), however, you must take special care with public names. You cannot use case to distinguish between identifiers within a single name scope, such as types within assemblies and members within types.

The following examples show situations to avoid:

! Do not have two classes or namespaces whose names differ only by case.

class customer { ... } class Customer { ... }

!Do not have a function with two parameters whose names differ only by case.

void foo(int X, int x)

This constraint enables Visual Basic (and potentially other case-insensitive languages) to produce and use components that have been created in other casesensitive languages. This constraint does not apply to your definitions of private classes, private methods on public classes, or local variables.

" Compiling and Running a .NET Application

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Key Points

String literals in an application are stored and transported as clear text. Therefore, you should avoid putting sensitive information such as passwords in string literals.

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The .NET Framework includes a command line compiler for C#. The file name of the compiler is Csc.exe.

Compiling in C#

To compile the source code for the Hello World application presented in the

Hello World demonstration at the beginning of this module, type the following:

csc HelloDemoCS.cs

This syntax invokes the C# compiler. In this example, you only need to specify the name of the file to be compiled. The compiler generates the program executable, HelloDemoCS.exe.