C# 反射完全指南：运行时类型发现、动态编程与特性读取

反射对象用于在运行时获取类型信息。提供访问运行中程序元数据的类位于 System.Reflection 命名空间。该命名空间包含类，允许获取应用程序信息、动态添加类型、值、对象。

反射应用场景

• 运行时查看特性信息
• 检查程序集中的各种类型并实例化
• 方法和属性的晚绑定
• 运行时创建新类型并执行任务

查看元数据

初始化 System.Reflection.MemberInfo 对象发现与类关联的特性：

System.Reflection.MemberInfo info = typeof(MyClass);

示例：读取类上的特性

using System;
[AttributeUsage(AttributeTargets.All)]
public class HelpAttribute : System.Attribute {
    public readonly string Url;
    public string Topic { get { return topic; } set { topic = value; } }
    public HelpAttribute(string url) { this.Url = url; }
    private string topic;
}

[HelpAttribute("Information on the class MyClass")]
class MyClass { }

namespace AttributeAppl {
    class Program {
        static void Main(string[] args) {
            System.Reflection.MemberInfo info = typeof(MyClass);
            object[] attributes = info.GetCustomAttributes(true);
            for (int i = 0; i < attributes.Length; i++) {
                System.Console.WriteLine(attributes[i]);  // HelpAttribute
            }
            Console.ReadKey();
        }
    }
}

完整实战：读取 DeBugInfo 特性

using System;
using System.Reflection;
namespace BugFixApplication {
    [AttributeUsage(
        AttributeTargets.Class | AttributeTargets.Constructor | 
        AttributeTargets.Field | AttributeTargets.Method | AttributeTargets.Property,
        AllowMultiple = true)]
    public class DeBugInfo : System.Attribute {
        private int bugNo;
        private string developer;
        private string lastReview;
        public string message;
        
        public DeBugInfo(int bg, string dev, string d) {
            bugNo = bg; developer = dev; lastReview = d;
        }
        public int BugNo { get { return bugNo; } }
        public string Developer { get { return developer; } }
        public string LastReview { get { return lastReview; } }
        public string Message { get { return message; } set { message = value; } }
    }
    
    [DeBugInfo(45, "Zara Ali", "12/8/2012", Message = "Return type mismatch")]
    [DeBugInfo(49, "Nuha Ali", "10/10/2012", Message = "Unused variable")]
    class Rectangle {
        protected double length, width;
        public Rectangle(double l, double w) { length = l; width = w; }
        
        [DeBugInfo(55, "Zara Ali", "19/10/2012", Message = "Return type mismatch")]
        public double GetArea() { return length * width; }
        
        [DeBugInfo(56, "Zara Ali", "19/10/2012")]
        public void Display() {
            Console.WriteLine("Length: {0}", length);
            Console.WriteLine("Width: {0}", width);
            Console.WriteLine("Area: {0}", GetArea());
        }
    }
    
    class ExecuteRectangle {
        static void Main(string[] args) {
            Rectangle r = new Rectangle(4.5, 7.5);
            r.Display();
            
            Type type = typeof(Rectangle);
            
            // 遍历类级特性
            foreach (Object attributes in type.GetCustomAttributes(false)) {
                DeBugInfo dbi = (DeBugInfo)attributes;
                if (dbi != null) {
                    Console.WriteLine("Bug no: {0}", dbi.BugNo);
                    Console.WriteLine("Developer: {0}", dbi.Developer);
                    Console.WriteLine("Last Reviewed: {0}", dbi.LastReview);
                    Console.WriteLine("Remarks: {0}", dbi.Message);
                }
            }
            
            // 遍历方法级特性
            foreach (MethodInfo m in type.GetMethods()) {
                foreach (Attribute a in m.GetCustomAttributes(true)) {
                    DeBugInfo dbi = (DeBugInfo)a;
                    if (dbi != null) {
                        Console.WriteLine("Bug no: {0}, for Method: {1}", dbi.BugNo, m.Name);
                        Console.WriteLine("Developer: {0}", dbi.Developer);
                        Console.WriteLine("Last Reviewed: {0}", dbi.LastReview);
                        Console.WriteLine("Remarks: {0}", dbi.Message);
                    }
                }
            }
            Console.ReadLine();
        }
    }
}

输出：

Length: 4.5
Width: 7.5
Area: 33.75
Bug No: 49
Developer: Nuha Ali
Last Reviewed: 10/10/2012
Remarks: Unused variable
Bug No: 45
Developer: Zara Ali
Last Reviewed: 12/8/2012
Remarks: Return type mismatch
Bug No: 55, for Method: GetArea
Developer: Zara Ali
Last Reviewed: 19/10/2012
Remarks: Return type mismatch
Bug No: 56, for Method: Display
Developer: Zara Ali
Last Reviewed: 19/10/2012
Remarks: 

反射核心类

常用反射操作

获取 Type 对象的三种方式

Type t1 = typeof(string);           // 编译时已知类型
Type t2 = "hello".GetType();        // 运行时实例
Type t3 = Type.GetType("System.String");  // 字符串名称

创建实例

object obj = Activator.CreateInstance(type);           // 无参构造
object obj = Activator.CreateInstance(type, args);     // 有参构造
object obj = type.GetConstructor(types).Invoke(args);  // 精确控制

调用方法

MethodInfo mi = type.GetMethod("MethodName");
object result = mi.Invoke(instance, parameters);

读写属性

PropertyInfo pi = type.GetProperty("PropertyName");
pi.SetValue(instance, value);
object val = pi.GetValue(instance);

性能与安全考量

• 反射比直接调用慢（MethodInfo.Invoke 有开销），热路径缓存 MethodInfo/PropertyInfo
• 使用 Delegate.CreateDelegate 将反射调用转为委托提升性能
• DynamicMethod / Expression Trees 生成 IL 进一步优化
• 反射可访问私有成员（BindingFlags.NonPublic），破坏封装，慎用
• 部分信任环境受 ReflectionPermission 限制

Source Generators 替代方案

C# 9.0+ 引入 Source Generators，编译时生成代码替代运行时反射，适合序列化、依赖注入、ORM 映射等场景，零运行时开销。