Programming Concepts

Attributes

Attributes provide a powerful method of associating metadata, or declarative information, with code (assemblies, types, methods, properties, and so forth).
After an attribute is associated with a program entity, the attribute can be queried at run time by using a technique called reflection.
For more information, see Reflection (C# and Visual Basic).

Attributes have the following properties:

• Attributes add metadata to your program. Metadata is information about the types defined in a program. All .NET assemblies contain a specified set of metadata that describes the types and type members defined in the assembly. You can add custom attributes to specify any additional information that is required. For more information, see, Creating Custom Attributes (C# and Visual Basic).
• You can apply one or more attributes to entire assemblies, modules, or smaller program elements such as classes and properties.
• Attributes can accept arguments in the same way as methods and properties.
• Your program can examine its own metadata or the metadata in other programs by using reflection. For more information, see Accessing Attributes by Using Reflection (C# and Visual Basic).

Using Attributes 

Attributes can be placed on most any declaration, though a specific attribute might restrict the types of declarations on which it is valid.
In C#, you specify an attribute by placing the name of the attribute, enclosed in square brackets ([]), above the declaration of the entity to which it applies.
In Visual Basic, an attribute is enclosed in angle brackets (< >).
It must appear immediately before the element to which it is applied, on the same line.

In this example, the SerializableAttribute attribute is used to apply a specific characteristic to a class:

using System; 
[Serializable]
    class SampleClass
    {
        // Objects of this type can be serialized.
    }

A method with the attribute DllImportAttribute is declared like this:

using System.Runtime.InteropServices;
        [DllImport("user32.dll")]
        extern static void SampleMethod();

More than one attribute can be placed on a declaration:

public sealed class InAttribute : Attribute

public sealed class OutAttribute : Attribute

using System.Runtime.InteropServices;
        void MethodA([In][Out] ref double x) { }
        void MethodB([Out][In] ref double x) { }
        void MethodC([In, Out] ref double x) { }

Some attributes can be specified more than once for a given entity. An example of such a multiuse attribute is ConditionalAttribute:

using System.Diagnostics;
[Conditional("DEBUG"), Conditional("TEST1")]
        void TraceMethod() { }

By convention, all attribute names end with the word "Attribute" to distinguish them from other items in the .NET Framework.

However, you do not need to specify the attribute suffix when using attributes in code.

For example, [DllImport] is equivalent to [DllImportAttribute], but DllImportAttribute is the attribute's actual name in the .NET Framework.

Reflection

Reflection provides objects (of type Type) that describe assemblies, modules and types.
You can use reflection to dynamically create an instance of a type, bind the type to an existing object, or get the type from an existing object and invoke its methods or access its fields and properties.
If you are using attributes in your code, reflection enables you to access them.
For more information, see Extending Metadata Using Attributes.

Here's a simple example of reflection using the static method GetType - inherited by all types from the Object base class - to obtain the type of a variable:

// Using GetType to obtain type information: 

using System;

int i = 0;
Type type = i.GetType();
Console.WriteLine(type);

输出结果：System.Int32

The following example uses reflection to obtain the full name of the loaded assembly.

using System;
using System.Reflection;

            Type type = typeof(int);
            Assembly assembly = type.Assembly;
            Console.WriteLine(assembly);

            int i = 0;
            type = i.GetType();
            assembly = type.Assembly;
            Console.WriteLine(assembly);

输出结果：

mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089
mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089

The C# keywords protected and internal have no meaning in IL and are not used in the reflection APIs.
The corresponding terms in IL are Family and Assembly.
To identify an internal method using reflection, use the IsAssembly property.
To identify a protected internal method, use the IsFamilyOrAssembly.

Reflection Overview

Reflection is useful in the following situations:
When you have to access attributes in your program's metadata. For more information, see Retrieving Information Stored in Attributes.
For examining and instantiating types in an assembly.
For building new types at runtime. Use classes in System.Reflection.Emit.
For performing late binding, accessing methods on types created at run time. See the topic Dynamically Loading and Using Types.

Related Sections
Reflection in the .NET Framework
Viewing Type Information
Reflection and Generic Types
System.Reflection.Emit
Retrieving Information Stored in Attributes