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Reflection and Attributes
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Metadata and Reflection The Type Class Getting a Type Object What Is an Attribute Applying an Attribute Predefined, Reserved Attributes More About Applying Attributes Custom Attributes Accessing an Attribute
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CHAPTER 24 REFLECTION AND ATTRIBUTES
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Most programs are written to work on data. They read, write, manipulate, and display data. (Graphics are a form of data.) The types that you as the programmer create and use are designed for these purposes, and it is you, at design time, that must understand the characteristics of the types you use. For some types of programs, however, the data they manipulate is not numbers, text, or graphics, but information about programs and program types themselves. Data about programs and their types is called metadata, and is stored in the programs assemblies. A program can look at the metadata of other assemblies or of itself, while it is running. When a running program looks at its own metadata, or that of other programs, it is called reflection. An object browser is an example of a program that displays metadata. It can read assemblies and display the types they contain, along with all the characteristics and members. This chapter will look at how your programs can reflect on data using the Type class and how you can add metadata to your types using attributes.
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Note To use reflection, you must use the System.Reflection namespace.
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The Type Class
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Throughout this text I have described how to declare and use the types available in C#. These include the predefined types (int, long, string, etc.), types from the BCL (Console, IEnumerable, etc.), and user-defined types (MyClass, MyDel, etc.). Every type has its own members and characteristics. The BCL declares an abstract class called Type, which is designed to contain the characteristics of a type. Using objects of this class allows you to get information about the types your program is using. Since Type is an abstract class, it cannot have actual instances. Instead, at run time, the CLR creates instances of a class derived from Type (RuntimeType) that contains the type information. When you access one of these instances, the CLR returns a reference, not of the derived type, but of the base class Type. For simplicity s sake, though, throughout the rest of the chapter, I will call the object pointed at by the reference an object of type Type, although technically it is an object of a derived type, with a reference of type Type.
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CHAPTER 24 REFLECTION AND ATTRIBUTES
Important things to know about Type are the following: For every type used in a program, the CLR creates an object of type Type that contains the information about the type. Every type used in a program is associated with a separate object of class Type. Regardless of the number of instances of a type that are created, there is only a single Type object associated with all the instances. Figure 24-1 shows a running program with two MyClass objects and an OtherClass object. Notice that although there are two instances of MyClass, there is only a single Type object representing it.
Figure 24-1. The CLR instantiates objects of type Type for every type used in a program. You can get almost anything you need to know about a type from its Type object. Some of the more useful members of the class are listed in Table 24-1. Table 24-1. Selected Members of Class System.Type
Member
Name Namespace GetFields GetProperties GetMethods
Member Type
Property Property Method Method Method
Description
Returns the name of the type Returns the namespace containing the type declaration Returns a list of the type s fields Returns a list of the type s properties Returns a list of the type s methods
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CHAPTER 24 REFLECTION AND ATTRIBUTES
Getting a Type Object
There are several ways to get a Type object. We will look at using the GetType method and using the typeof operator. Type object contains a method called GetType, which returns a reference to an instance s Type object. Since every type is ultimately derived from object, you can call the GetType method on an object of any type to get its Type object, as shown here: Type t = myInstance.GetType(); The following code shows the declarations of a base class and a class derived from it. Method Main creates an instance of each class and places the references in an array called bca for easy processing. Inside the outer foreach loop, the code gets the Type object and prints out the name of the class. It then gets the fields of the class and prints them out. Figure 24-2 illustrates the objects in memory. class BaseClass { public int BaseField = 0; } class DerivedClass : BaseClass { public int DerivedField = 0; } class Program { static void Main( ) { var bc = new BaseClass(); var dc = new DerivedClass(); BaseClass[] bca = new BaseClass[] { bc, dc }; foreach (var v in bca) { Type t = v.GetType();
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