为什么C＃编译器声称在“yield return”和动态之前使用未分配的变量？ [英] Why is the C# compiler claiming 'use of an unassigned variable' prior to 'yield return' and dynamic?

问题描述

编译器会在分配之前使用下面的代码中的 resultingThing 。

  private IEnumerable< IThing> FindThings（动态规范）
 {
 if（spec == null）
 yield break; 
 
 
 if（spec.Something> 0&&&&&& amp;& 
 else 
 // ... 
} 
  

我试过一个不同版本的方法，其中没有 yield 的用法（例如只是 return IEnumerable< IThing> ），但使用了动态参数，并且我尝试了不传递动态 （即我们在以前的C＃版本中所做的）。这些编译。

解决方案

我似乎是一个编译器错误（或限制，如果你喜欢）
$ b

我将最小失败的情况减少到：

  static private IThing FindThings b $ b {
结果Thing; 
 if（（null！= spec）&&&& dictionary.TryGetValue（spec，out resultThing））
 return resultThing; 
 return null; 
} 
  

这给出了相同的编译器诊断，

$ b

$ p> 使用System;
using System.Collections.Generic;

public static class X
{
public interface IThing {}

private static readonly IDictionary< string，IThing> dictionary = new Dictionary< string，IThing>（）;

static private IThing FindThings（动态规格）
{
IThing resultThing;
if（（null！= spec）&&&& dictionary.TryGetValue（spec，out resultThing））
return resultThing;
return null;
}

public static void Main（string [] s）
{

}
}


编译：

  dmcs  - v -warnaserror -warn：4 t.cs 
  

无警告

The compiler complains that resultingThing in the code below is being used before being assigned to.

private IEnumerable<IThing> FindThings(dynamic spec)
{
    if (spec == null)
        yield break;

    IThing resultingThing;
    if (spec.Something > 0 && dictionary.TryGetValue(spec.Something, out resultingThing))
        yield return resultingThing;
    else
        // ...
}

Why does it claim this?

I have tried a different version of the method in which there are no yield usages (e.g. just return IEnumerable<IThing>) but with the dynamic parameter, and I have tried a version of the method in which dynamic is not passed in (i.e. what we've done in prior versions of C#). These compile.

解决方案

I appears to be a compiler bug (or limitation, if you prefer).

I reduced the minimal failing case to:

static private IThing FindThings(dynamic spec)
{
    IThing resultingThing;
    if ((null!=spec) && dictionary.TryGetValue(spec, out resultingThing))
        return resultingThing;
return null;
}

Which gives the same compiler diagnostic, without involving member lookup on dynamics, nor iterator blocks.

For reference the mono compiler does not trip over that:

using System;
using System.Collections.Generic;

public static class X
{
    public interface IThing { }

    private static readonly IDictionary<string, IThing> dictionary = new Dictionary<string, IThing>();

    static private IThing FindThings(dynamic spec)
    {
        IThing resultingThing;
        if ((null!=spec) && dictionary.TryGetValue(spec, out resultingThing))
            return resultingThing;
        return null;
    }

    public static void Main(string[] s)
    {

    }
}

Compiling that:

dmcs -v -warnaserror -warn:4 t.cs

No warnings

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