C#图解教程 第二十四章 反射和特性

using System;
using System.Reflection;
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();

            Console.WriteLine("Object type : {0}",t.Name);

            FieldInfo[] fi=t.GetFields();
            foreach(var f in fi)
            {
                Console.WriteLine("    Field : {0}",f.Name);
            }
            Console.WriteLine();
        }
    }
}

Type t = typeof(DerivedClass);
            ↑        ↑
         运算符 希望的Type对象的类型

using System;
using System.Reflection;

namespace SimpleReflection
{
    class BaseClass
    {
        public int MyFieldBase;
    }
    class DerivedClass:BaseClass
    {
        public int MyFieldDerived;
    }
    class Program
    {
        static void Main()
        {
            Type tbc=typeof(DerivedClass);
            Console.WriteLine("Result is {0}.",tbc.Name);

            Console.WriteLine("It has the following fields:");
            FieldInfo[] fi=tbc.GetFields();
            foreach(var f in fi)
            {
                Console.WriteLine("    {0}",f.Name);
            }
        }
    }
}

[Serializable]
public class MyClass
{
    …
}
[MyAttribute("Simple class","Version 3.57")]
public class MyOtherClass
{
    …
}

class Program
{
    //应用特性
    [Obsolete("User method SuperPrintOut")]
    static void PrintOut(string str)
    {
        Console.WriteLine(str);
    }
    static void Main(string[] args)
    {
        PrintOut("Start of Main");
    }
}

                                   标记为错误
                                       ↓
[Obsolete("User method SuperPrintOut",true)]
static void PrintOut(string str)
{
    …
}

[Conditional("DoTrace")]
static void TraceMessage(string str)
{
    Console.WriteLine(str);
}

#define DoTrace
using System;
using System.Diagnostics;
namespace AttributeConditional
{
    class Program
    {
        [Conditional("DoTrace")]
        static void TraceMessage(string str)
        {
            Console.WriteLine(str);
        }
        static void Main()
        {
            TraceMessage("Start of Main");
            Console.WriteLine("Doing work in Main.");
            TraceMessage("End of Main");
        }
    }
}

using System;
using System.Runtime.CompilerServices;

public static class Program
{
    public static void MyTrace(string message,
                                [CallerFilePath] string fileName="",
                                [CallerLineNumber] int lineNumber=0,
                                [CallerMemberName] string callingMember="")
    {
        Console.WriteLine("File:         {0}",fileName);
        Console.WriteLine("Line:         {0}",lineNumber);
        Console.WriteLine("Called From:  {0}",callingMember);
        Console.WriteLine("Message:      {0}",message);
    }
    public static void Main()
    {
        MyTrace("Simple message");
    }
}

using System;
using System.Diagnostics;

class Program
{
    int _x=1;
    int X
    {
        get{return _x;}
        [DebuggerStepThrough]
        set
        {
            _x=_x*2;
            _x+=value;
        }
    }

    public int Y{get;set;}

    public static void Main()
    {
        var p=new Program();
        p.IncrementFields();
        p.X=5;
        Console.WriteLine("X = {0}, Y = {1}",p.X,p.Y);
    }
    [DebuggerStepThrough]
    void IncrementFields()
    {
        X++;
        Y++;
    }
}

[Serializable ]                       //多层结构
[MyAttribute("Simple class", "Version 3.57")]

[MyAttribute("Simple class", "Version 3.57"),Serializable]    //逗号分隔

[MyAttribute("Holds a value", "Version 3.2")]    //字段上的特性
public int MyField;

[Obsolete]                                       //方法上的特性
[MyAttribute("Prints out a message.", "Version 3.6")]
public void Printout()
{
    …
}

显式目标说明符
   ↓
[method: MyAttribute("Prints out a message.", "Version 3.6")]
[return: MyAttribute("This value represents …", "Version 2.3")]
public long ReturnSetting()
{
    …
}

[assembly: AssemblyTitle("SuperWidget")]
[assembly: AssemblyDescription("Implements the SuperWidget product.")]
[assembly: AssemblyConfiguration("")]
[assembly: AssemblyCompany("McArthur Widgets, Inc.")]
[assembly: AssemblyProduct("Super Widget Deluxe")]
[assembly: AssemblyCopyright("Copyright ? McArthur Widgets 2012")]
[assembly: AssemblyTrademark("")]
[assembly: AssemblyCulture(")]

                         特性名                 基类
                           ↓                     ↓
public sealed class MyAttributeAttribute : System.Attribute
{
…
}

public MyAttributeAttribute(string desc,string ver)
{
    Description=desc;
    VersionNumber=ver;
}

[MyAttribute("Holds a value")]    //使用一个字符串的构造函数
public int MyField;

[MyAttribute("version 1.3", "Sal Martin")]    //使用两个字符串的构造函数
public void MyMethod()
{
    …
}

[MyAttr]
class SomeClass …

[MyAttr()]
class OtherClass …

                   位置参数            命名参数              命名参数
                      ↓                   ↓                   ↓
[MyAttribute("An excellent class",Reviewer="Amy McArthur"，Ver="0.7.15.33")]

public sealed class MyAttributeAttribute : System.Attribute
{
    public string Description;
    public string Ver;
    public string Reviewer;

    public MyAttributeAttribute(string desc) //一个形参
    {
        Description = desc;
    }
}
             //三个实参
[MyAttribute("An excellent class”, Reviewer="Amy McArthur", Ver="7.15.33")]
class MyClass
{
    …
}

                   只针对方法
                       ↓
[AttributeUsage( AttributeTarget.Method )]
public sealed class MyAttributeAttribute : System.Attribute
{
    …
}

                                     目标
                                       ↓
[AttributeUsage( AttributeTarget.Method| AttributeTarget.Constructor )]
public sealed class MyAttributeAttribute : System.Attribute

[AttributeUsage( AttributeTarget.Class,     //必需的，位置参数
                Inherited = false,          //可选的，命名参数
                AllowMultiple = false )]    //可选的，命名参数
public sealed class MyAttributeAttribute : System.Attribute
{
    …
}

[AttributeUsage( AttributeTargets.Class )]
public sealed class ReviewCommentAttribute : System.Attribute
{
    public    string    Description     {get;set;}
    public    string    VersionNumber   {get;set;}
    public    string    ReviewerID      {get;set;}

    public ReviewCommentAttribute(string desc, string ver)
    {
        Description = desc;
        VersionNumber = ver;
    }
}

[AttributeUsage(AttributeTargets.Class)]
public sealed class ReviewCommentAttribute:System.Attribute
{…}

[ReviewComment("Check it out","2.4")]
class MyClass{}

class Program
{
    static void Main()
    {
        var mc=new MyClass();
        Type t=mc.GetType();
        bool isDefined=
            t.IsDefined(typeof(ReviewCommentAttribute),false);
        if(isDefined)
            Console.WriteLine("ReviewComment is applied to type {0}",t.Name);
    }
}

using System;

[AttributeUsage(AttributeTargets.Class)]
public sealed class MyAttributeAttribute:System.Attribute
{
    public  string  Description  {get;set;}
    public  string  VersionNumber{get;set;}
    public  string  ReviewerID   {get;set;}

    public MyAttributeAttribute(string desc,string ver)
    {
        Description=desc;
        VersionNumber=ver;
    }
}

[MyAttribute("Check it out","2.4")]
class MyClass
{
}
class Program
{
    static void Main()
    {
        Type t=typeof(MyClass);
        object[] AttArr=t.GetCustomAttributes(false);

        foreach(Attribute a in AttArr)
        {
            var attr=a as MyAttributeAttribute;
            if(null!=attr)
            {
                Console.WriteLine("Description    :{0}",attr.Description);
                Console.WriteLine("Version Number :{0}",attr.VersionNumber);
                Console.WriteLine("Reviewer ID    :{0}",attr.ReviewerID);
            }
        }
    }
}