INSERT ATTRIBUTES

C# provides attributes so you can specify additional information about the functionality of a string of code. For example, you can add an attribute to a

string of code that points to another file for the program to open or to a type in another program.

Attributes retrieve information by using the reflection process. When your C# program runs, the attributes obtain information about program assemblies — the collection of types used in your program — and the types within those assemblies including classes, interfaces, and value types.

When your C# program encounters an attribute in a string of code, the program invokes the attribute. For example, if

your C# program encounters an attribute for accessing an external file, such as a text file, the file will open when you and your users access that string of code.

C# contains three reserved attributes: AttributeUsage, which decribes how a custom attribute class is used; Conditional, which marks a conditional method; and Obsolete, which marks a piece of unusable code. You can also create your own custom attributes by definining an attribute class. C# gives you the building blocks of attribute creation by deriving all custom attributes from the built-in

System.Attribute class.

¤ Click New Project in the

Start page.

■ The New Project window appears.

‹ Click the Console Application icon in the Templates pane.

› Type a name for your file.

ˇ Click OK.

The Visual C# reflection system comes in the form of a built-in method called GetCustomAttributes. The GetCustomAttributes class returns those custom attributes as an array of objects that you can view in your program output.

TYPE THIS:

using System; [Obsolete]

class GetAttribute

{

static void Main(string[] args)

{

Type x = typeof(GetAttribute);

object[] xget = x.GetCustomAttributes(); Console.WriteLine(“The custom attribute is:”); foreach (object y in xget)

Console.WriteLine (y);

}

}

RESULT:

The custom attribute is

System.ObsoleteAtt ribute.

■ The Class1.cs code appears in the parent window.

Á Delete the summary description code for Class1.

C# provides three built-in attributes. One provides the necessary building block for creating custom attributes. The other two attributes provide common

attribute functions in C# programs: making a method conditional and marking a string of code that is obsolete. For example, you can set an attribute that will run a method only if the user-entered password matches the password in your program.

The AttributeUsage attribute lets you define where you can apply your attributes. Visual C# contains built-in attribute values called AttributeTargets that let you determine what elements should include the attribute, such as a class.

The Conditional attribute lets you determine whether a string in your program will let a method execute. The Conditional attribute looks for a preprocessing identifier, such as input from the user, that tells the program whether it should execute a particular method or skip the entire method.

The Obsolete attribute lets you mark a section of code as that not to be used. You can set the Obsolete attribute to display a message when you encounter the code in your program (which is the preferred method so you can inform users about other options), or as a Boolean false attribute that will generate a compiler warning when you access that part of the program.

■ The Main method string appears first followed by the One method string and the

NewMethod string.

¡ Change the NewMethod

code in the Main method to

OldMethod.