问题描述

我有一个我想调用的带有私有方法的结构.由于我计划在性能关键部分执行此操作，因此我想缓存一个委托以执行该操作.问题是我似乎无法使用 Delegate.CreateDelegate 绑定到它的方法.有问题的结构不是我的创作，而是用于与第三方库的交互.有问题的结构如下所示::

I have a struct with a private method that I'd like to invoke. Since I plan to do this in a performance critical section, I'd like to cache a delegate to perform the action. The problem is I can't seem to bind to its method with Delegate.CreateDelegate. The struct in question is not my creation and is used in interaction with a third party library. The struct in question looks like this::

public struct A
{
     private int SomeMethod()
     {
        //body go here
     }
}

以下代码将失败并显示错误绑定到目标方法".

And the following code will fail with an "Error binding to target method".

Delegate.CreateDelegate(typeof(Func<A,int>),typeof(A).GetMethod("SomeMethod",BindingFlags.Instance | BindingFlags.NonPublic));

我知道我可以编写一个表达式树来执行该操作，但我不能使用我的正常 goto 来处理这些事情似乎很奇怪 Delegate.CreateDelegate 方法.

I know I can write an expression tree to perform the action, but it seems odd that I can't use my normal goto for these things the Delegate.CreateDelegate method.

如果 A 是一个类，上面的代码就可以正常工作.之所以会出现这个问题，是因为 A 是一个结构.MSDN 文档对于 CreateDelegate 的这种重载是不正确的，因为它确实适用于非静态方法.

The above code works just fine if A were a class. The issue only arises because A is a struct. MSDN documentation is incorrect for this overload of CreateDelegate as it does work on non-static methods.

推荐答案

有趣的问题.从这个错误报告来看，这可能是一个将在未来版本的 .NET 中修复的错误:http://connect.microsoft.com/VisualStudio/feedback/details/574959/cannot-create-open-instance-delegate-for-value-types-methods-which-implement-an-界面#details

Interesting problem. From this bug report, it looks like this might be a bug that will be fixed in a future version of .NET: http://connect.microsoft.com/VisualStudio/feedback/details/574959/cannot-create-open-instance-delegate-for-value-types-methods-which-implement-an-interface#details

实际上，我认为这个错误报告是关于一个不同的问题，所以你看到的行为实际上可能不是错误.

actually, I think this bug report is regarding a different issue, so the behavior you're seeing may not actually be a bug.

从该错误报告中，我了解到如果您将委托的第一个参数指定为通过引用传递，则可以解决此问题.下面是一个完整的工作示例:

From that bug report, I gleaned that there is a work-around if you specify the first argument of your delegate as being passed by reference. Below is a complete working example:

public struct A
{
    private int _Value;

    public int Value
    {
        get { return _Value; }
        set { _Value = value; }
    }

    private int SomeMethod()
    {
        return _Value;
    }
}

delegate int SomeMethodHandler(ref A instance);

class Program
{
    static void Main(string[] args)
    {
        var method = typeof(A).GetMethod("SomeMethod", BindingFlags.Instance | BindingFlags.NonPublic);

        SomeMethodHandler d = (SomeMethodHandler)Delegate.CreateDelegate(typeof(SomeMethodHandler), method);

        A instance = new A();

        instance.Value = 5;

        Console.WriteLine(d(ref instance));
    }
}

Jon Skeet's answer here 也讨论了这个问题.

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