AssemblyBuilder和Activator两个类是DispatchProxy类实现动态代理以及AOP的根本，示例demo可参考 DispatchProxy实现动态代理及AOP 。AssemblyBuilder的命名空间是System.Reflection.Emit，没错就是你听过的Emit。那么它是干什么用的？先看看 官方 的示例代码：

// ↓ AssemblyBuilder构造自定义类
AssemblyName aName = new AssemblyName("DynamicAssemblyExample");
AssemblyBuilder ab =AssemblyBuilder.DefineDynamicAssembly(aName,AssemblyBuilderAccess.Run);ModuleBuilder mb =ab.DefineDynamicModule(aName.Name);TypeBuilder tb = mb.DefineType("MyDynamicType",TypeAttributes.Public);FieldBuilder fbNumber = tb.DefineField("m_number",typeof(int),FieldAttributes.Private);Type[] parameterTypes = { typeof(int) };
ConstructorBuilder ctor1 = tb.DefineConstructor(MethodAttributes.Public,CallingConventions.Standard,parameterTypes);ILGenerator ctor1IL = ctor1.GetILGenerator();
ctor1IL.Emit(OpCodes.Ldarg_0);
ctor1IL.Emit(OpCodes.Call,typeof(object).GetConstructor(Type.EmptyTypes));
ctor1IL.Emit(OpCodes.Ldarg_0);
ctor1IL.Emit(OpCodes.Ldarg_1);
ctor1IL.Emit(OpCodes.Stfld, fbNumber);
ctor1IL.Emit(OpCodes.Ret);ConstructorBuilder ctor0 = tb.DefineConstructor(MethodAttributes.Public,CallingConventions.Standard,Type.EmptyTypes);ILGenerator ctor0IL = ctor0.GetILGenerator();
ctor0IL.Emit(OpCodes.Ldarg_0);
ctor0IL.Emit(OpCodes.Ldc_I4_S, 42);
ctor0IL.Emit(OpCodes.Call, ctor1);
ctor0IL.Emit(OpCodes.Ret);PropertyBuilder pbNumber = tb.DefineProperty("Number",PropertyAttributes.HasDefault,typeof(int),null);MethodAttributes getSetAttr = MethodAttributes.Public |MethodAttributes.SpecialName | MethodAttributes.HideBySig;MethodBuilder mbNumberGetAccessor = tb.DefineMethod("get_Number",getSetAttr,typeof(int),Type.EmptyTypes);ILGenerator numberGetIL = mbNumberGetAccessor.GetILGenerator();
numberGetIL.Emit(OpCodes.Ldarg_0);
numberGetIL.Emit(OpCodes.Ldfld, fbNumber);
numberGetIL.Emit(OpCodes.Ret);MethodBuilder mbNumberSetAccessor = tb.DefineMethod("set_Number",getSetAttr,null,new Type[] { typeof(int) });ILGenerator numberSetIL = mbNumberSetAccessor.GetILGenerator();
numberSetIL.Emit(OpCodes.Ldarg_0);
numberSetIL.Emit(OpCodes.Ldarg_1);
numberSetIL.Emit(OpCodes.Stfld, fbNumber);
numberSetIL.Emit(OpCodes.Ret);pbNumber.SetGetMethod(mbNumberGetAccessor);
pbNumber.SetSetMethod(mbNumberSetAccessor);MethodBuilder meth = tb.DefineMethod("MyMethod",MethodAttributes.Public,typeof(int),new Type[] { typeof(int) });ILGenerator methIL = meth.GetILGenerator();
methIL.Emit(OpCodes.Ldarg_0);
methIL.Emit(OpCodes.Ldfld, fbNumber);
methIL.Emit(OpCodes.Ldarg_1);
methIL.Emit(OpCodes.Mul);
methIL.Emit(OpCodes.Ret);Type t = tb.CreateType();//↑ AssemblyBuilder类代码
MethodInfo mi = t.GetMethod("MyMethod");
PropertyInfo pi = t.GetProperty("Number");//↓ Activator调用自定义的类
object o1 = Activator.CreateInstance(t);Console.WriteLine("o1.Number: {0}", pi.GetValue(o1, null));
pi.SetValue(o1, 127, null);
Console.WriteLine("o1.Number: {0}", pi.GetValue(o1, null));object[] arguments = { 22 };
Console.WriteLine("o1.MyMethod(22): {0}",mi.Invoke(o1, arguments));object o2 = Activator.CreateInstance(t,new object[] { 5280 });
Console.WriteLine("o2.Number: {0}", pi.GetValue(o2, null));
// ↑Activator 类代码
Console.Read();

先看看输出结果吧：

我猜您看的迷糊吧，这都啥玩意，不急。对AssemblyBuilder以及Activator的概念是：动态创建类型。上面的代码中的AssemblyBuilder相关代码是动态的构造一个完整的类型，包括了类的行为（方法）、状态（字段、属性），构造函数等。相当于就是一个类型的定义：

public class MyDynamicType
{private int m_number;public MyDynamicType() : this(42) {}public MyDynamicType(int initNumber){m_number = initNumber;}public int Number{get { return m_number; }set { m_number = value; }}public int MyMethod(int multiplier){return m_number * multiplier;}
}

这就是AssemblyBuilder类的作用了，那么Activator类的作用呢？其实你也应该猜到了，就是一个实例化类型及调用对象的过程。
AssemblyBuidler类 + Activator类 两者的结合可以实现动态代理以及AOP，功能及其强大，园子里蒋老大的Dora AOP框架的代码中也有这两个类。