如何从一个结构体的实例方法创建一个开放的委托？ [英] How can I create an open Delegate from a struct's instance method?

问题描述

我有一个我想要调用的私有方法的结构体。由于我计划在性能关键部分执行此操作，因此我希望缓存代理来执行该操作。问题是我似乎无法使用Delegate.CreateDelegate绑定到它的方法。所讨论的结构不是我的创建，并且用于与第三方库的交互。
该问题的结构看起来像这样::

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

以下代码将失败，并显示绑定到目标方法的错误。

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

我知道我可以写一个表达式的树来执行这个动作，但是我可以'使用我的正常goto这些东西 Delegate.CreateDelegate 方法。

上面的代码工作正常，如果 A 是一个类。这个问题只是因为 A 是一个结构体。
对于CreateDelegate的这种重载，MSDN文档不正确，因为它在非静态方法上有效。

解决方案

有趣的问题。从这个错误报告，看起来这可能是将来版本的.NET中修复的错误：
http://connect.microsoft.com/VisualStudio/feedback/详细信息/ 574959 / can not-create-open-instance-delegate-for-value-types-methods-which-implement-an-interface＃details

编辑：实际上，我认为这个错误报告是针对一个不同的问题，所以你看到的行为可能不是一个错误。

从该错误报告中我收集如果您指定代理的第一个参数作为参考传递，那么有一个解决方法。以下是一个完整的工作示例：

  public struct A 
 {
 private int _Value; 
 
 public int Value 
 {
 get {return _Value; } 
 set {_Value = value; } 
} 
 
 private int SomeMethod（）
 {
 return _Value; 
} 
} 
 
委托int SomeMethodHandler（ref A instance）; 
 
类程序
 {
 static void Main（string [] args）
 {
 var method = typeof（A）.GetMethod（SomeMethod ，BindingFlags.Instance | BindingFlags.NonPublic）; 
 
 SomeMethodHandler d =（SomeMethodHandler）Delegate.CreateDelegate（typeof（SomeMethodHandler），方法）; 
 
 A instance = new A（）; 
 
 instance.Value = 5; 
 
 Console.WriteLine（d（ref instance））; 
} 
} 
  

编辑： Jon Skeet在这里的回答也讨论了这个问题。

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
     }
}

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));

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.

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.

解决方案

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

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

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));
    }
}

EDIT: Jon Skeet's answer here also discusses this issue.

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