how to generate barcode in c# Lesson 3: Constructing Classes in Visual C#

Create QR in Visual C# Lesson 3: Constructing Classes

Lesson 3: Constructing Classes
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What Are Attributes
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Attributes describe a type, method, or property in a way that can be programmatically queried using a technique called Reflection. Some common uses for attributes are to
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Specify which security privileges a class requires Specify security privileges to refuse to reduce security risk Declare capabilities, such as supporting serialization Describe the assembly by providing a title, description, and copyright notice
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Attribute types derive from the System.Attribute base class and are specified using <> or [] notation. The following code sample demonstrates how to add assembly attributes:
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' VB - AssemblyInfo.vb <Assembly: AssemblyTitle("ch01vb")> <Assembly: AssemblyDescription(" 1 Samples")> <Assembly: AssemblyCompany("Microsoft Learning")> <Assembly: AssemblyProduct("ch01vb")> <Assembly: AssemblyCopyright("Copyright 2006")> <Assembly: AssemblyTrademark("")> // C# - AssemblyInfo.cs [assembly: AssemblyTitle("ch01cs")] [assembly: AssemblyDescription(" 1 Samples")] [assembly: AssemblyConfiguration("")] [assembly: AssemblyCompany("Microsoft Learning")] [assembly: AssemblyProduct("ch01cs")] [assembly: AssemblyCopyright("Copyright 2006")] [assembly: AssemblyTrademark("")]
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Visual Studio automatically creates some standard attributes for your assembly when you create a project, including a title, description, company, guide, and version. You should edit these attributes for every project you create because the defaults do not include important information such as the description. Attributes do more than describe an assembly to other developers, they can also declare requirements or capabilities. For example, to enable a class to be serialized, you must add the Serializable attribute, as the following code demonstrates:
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' VB <Serializable()> Class ShoppingCartItem End Class // C# [Serializable] class ShoppingCartItem { }
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Without the Serializable attribute, a class is not serializable. Similarly, the following code uses attributes to declare that it needs to read the C:\boot.ini file. Because of this attribute, the runtime will throw an exception prior to execution if it lacks the specified privilege:
' VB Imports System.Security.Permissions <Assembly: FileIOPermissionAttribute(SecurityAction.RequestMinimum, Read := "C:\boot.ini")> Module Module1 Sub Main() Console.WriteLine("Hello, World!") End Sub End Module // C# using System; using System.Security.Permissions; [assembly:FileIOPermissionAttribute(SecurityAction.RequestMinimum, Read=@"C:\boot.ini")] namespace DeclarativeExample { class Class1 { [STAThread] static void Main(string[] args) { Console.WriteLine("Hello, World!"); } } }
What Is Type Forwarding
Type forwarding is an attribute (implemented in TypeForwardedTo) that allows you to move a type from one assembly (assembly A) into another assembly (assembly B), and to do so in such a way that it is not necessary to recompile clients that consume assembly A. After a component (assembly) ships and is being used by client applications, you can use type forwarding to move a type from the component (assembly) into another assembly and ship the updated component (and any additional assemblies required), and the client applications will still work without being recompiled. Type forwarding works only for components referenced by existing applications. When you rebuild an application, there must be appropriate assembly references for any types used in the application.
Lesson 3: Constructing Classes
To move a type from one class library to another, follow these steps:
NOTE .NET 2.0
Type forwarding is a new feature in .NET 2.0.
1. Add a TypeForwardedTo attribute to the source class library assembly. 2. Cut the type definition from the source class library. 3. Paste the type definition into the destination class library. 4. Rebuild both libraries. The following code shows the attribute declaration used to move TypeA to the DestLib class library:
' VB Imports System.Runtime.CompilerServices <Assembly:TypeForwardedTo(GetType(DestLib.TypeA))]> // C# using System.Runtime.CompilerServices; [assembly:TypeForwardedTo(typeof(DestLib.TypeA))]
Lab: Create a Derived Class with Delegates
The following exercises demonstrate inheritance and events. If you encounter a problem completing an exercise, the completed projects are available on the companion CD in the Code folder.
Exercise 1: Derive a New Class from an Existing Class
In this exercise, you will derive a new class from the Person class you created in Lesson 1. 1. Copy the 01\Lesson3-Person folder from the companion CD to your hard disk, and open either the C# version or the Visual Basic version of the CreateStruct project. 2. Change the Person structure to a class. 3. Create a new class definition named Manager that inherits from the base Person class.
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