使用Delegate来调用构造函数 [英] Using a Delegate to call a constructor

问题描述

我发现这个，但试图使用它并且失败。

I found this but tried to use it and failed.

我如何使用反射创建对象，并将其放在委托中快速进行？

How can i create an object using reflections and make it fast by putting it in a delegate?

        DynamicMethod dm = new DynamicMethod("MyCtor", t, new Type[] { });            
        var ctor = t.GetConstructor(new Type[] { });
        ILGenerator ilgen = dm.GetILGenerator();
        ilgen.Emit(OpCodes.Ldarg_0);
        ilgen.Emit(OpCodes.Newobj, ctor);
        ilgen.Emit(OpCodes.Ret);
        var d = (Func<T>)dm.CreateDelegate(t);
        dm.Invoke(null, new object[] { });

在把它放在一个元数据之前，我试图至少调用它，当我上面我得到错误

Before putting it in a deleage i tried to at least invoke it and when i did above i get the error

An unhandled exception of type 'System.Reflection.TargetInvocationException' occurred in mscorlib.dll

其他信息：调用目标抛出异常。

Additional information: Exception has been thrown by the target of an invocation.

如果我调用d（），我得到异常

If i call d() instead i get the exception

An unhandled exception of type 'System.ArgumentException' occurred in mscorlib.dll

Additional information: Type must derive from Delegate.

如何将一个无参数构造函数放入代理并调用？

How do i put a no param constructor into a delegate and call it?

推荐答案

如果您可以访问.NET 3.5（您使用 Func< T> ），您可能会发现表达式比 ILGenerator 更容易：

If you have access to .NET 3.5 (which your use of Func<T> suggests), you may find Expression easier than ILGenerator:

class Foo { }
static void Main() {
    Func<Foo> func = GetCtor<Foo>(); // cache this somewhere!
    Foo foo = func();
}
static Func<T> GetCtor<T>() {
    Type type = typeof(T);
    Expression body = Expression.New(type);
    return Expression.Lambda<Func<T>>(body).Compile();        
}

很容易扩展，以使用特定的构造函数，传递参数或添加后构造函数属性绑定;转换等等（请参阅此相关答案）。如果你有一个具体的场景，我会很高兴地添加一个例子。

Pretty easy to extend that to use a specific constructor, passing arguments, or adding post-constructor property bindings; casts, conversions, etc (see this related answer). If you have a specific scenario, I'll happily add an example.

还要注意，你应该缓存并重新使用任何这样的构造函数 - 否则你会失去好处即不要重新创建每个呼叫的委托）。

Note also that you should cache and re-use any such constructors - otherwise you lose the benefit (i.e. don't recreate the delegate per-call).

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