c# - 解决编译时函数不正确的BUG？

Question

我使用的是 Visual Studio 2012 Update 1 和 .NET 4.5，这里是代码。

void Test(Action a) { }
void Test(Func<int> a) { }
void TestError()
{
    bool throwException = true;
    //Resolves to Test(Action a)
    Test(() =>
    {
    });
    //Resolves to Test(Action a)
    Test(() =>
    {
        if (throwException) throw new Exception();
    });
    //Resolves to Test(Func<int> a)
    //(This seems like a bug since there is no return value)
    Test(() =>
    {
        throw new Exception();
    });
    //Resolves to Test(Action a)
    //(With warning unreachable code detected)
    Test(() =>
    {
        throw new Exception();
        return; //unreachable code detected
    });
}

看起来最后一个函数调用错误地解析为 Func 而不是 Action，它与无条件抛出异常有关。

这是BUG吗？谢谢。

Answer 1

嗯，在我看来，两者都是有效的候选人似乎是合理的。换句话说，我认为可以转换为 () => { throw new Exception(); }。到两个Func<int>和 Action .在这两种情况下，方法的末尾都是无法访问的——将其放入像这样的普通命名方法中是完全有效的:

public int GoBang()
{
    throw new Exception();
}

然后写:

Func<int> foo = GoBang;

因此，从 lambda 表达式到 Func<int> 的转换和 Action有效。我们现在需要选择使用哪种方法。这在规范第 7.5.3.2 节(更好的函数成员)中指定，其中包括:

For at least one argument, the conversion from E_x to P_x is better than the conversion from from E_x to Q_x

此时，第 7.5.3.3 节(更好地从表达式转换)开始:

C₁ is a better conversion than C₂ if at least one of the following holds:

• [...]

• E is an anonymous function, T₁ is either a delegate type D₁ or an expression tree type Expression<D₁>, T₂ is either a delegate type D₂ or an expression tree type Expression<D₂>, and one of the following holds:

  • [...]
  • D₁ and D₂ have identical parameter lists, and one of the following holds:
  • D₁ has a return type Y, and D₂ is void returning.

这就是为什么 Func<int>优先于 Action ...编译器正在做正确的事情。