AssemblyBuilder以及Activator双剑合璧

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。

巴特西

AssemblyBuilder以及Activator双剑合璧

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