[英]Lambda of “x => { throw .. }” inferred to match Func<T,Task> in overloaded method?
I don't understand why C# ends up executing an incorrect extension method in the following LINQPad code: 我不明白为什么C#最终在以下LINQPad代码中执行不正确的扩展方法:
void Main()
{
// Actual: Sync Action
"Expected: Sync Action".Run(x => { x.Dump(); });
// Actual: Async Task
"Expected: Async Task".Run(async x => { await System.Threading.Tasks.Task.Run(() => x.Dump()); });
// Actual: Async Task!!
"Expected: Sync Action".Run(x => { throw new Exception("Meh"); });
}
static class Extensions
{
public static void Run<T>(this T instance, Action<T> action)
{
"Actual: Sync Action".Dump();
action(instance);
}
public static void Run<T>(this T instance, Func<T, System.Threading.Tasks.Task> func)
{
"Actual: Async Task".Dump();
func(instance).Wait();
}
}
Why does the compiler think that the lambda returns a Task here? 为什么编译器认为lambda在这里返回一个Task?
I expected to see "Actual: Sync Action" in the third call to Run() since nothing in the lambda indicates that this is a Func returning Task. 我希望在第三次调用Run()时看到“Actual:Sync Action”,因为lambda中没有任何内容表明这是一个Func返回Task。
This is simply an overload resolution issue. 这只是一个重载解决问题。 Clearly, the lambda
x => { throw new Exception("Meh"); }
显然,lambda
x => { throw new Exception("Meh"); }
x => { throw new Exception("Meh"); }
can be converted to either an Action<T>
or to a Func<T, SomeNonVoidType>
(as well as to many other delegate types irrelevant to this question). x => { throw new Exception("Meh"); }
可以被转化到任何一个Action<T>
或一个Func<T, SomeNonVoidType>
以及许多其他委托类型无关这个问题)。 It's simply C#'s overload resolution rules that prefer the latter in this case. 这只是C#的重载决策规则,在这种情况下更喜欢后者。
Here's a more representative example: 这是一个更具代表性的例子:
void Main()
{
// Output: Func<T, int>
"Test".WhatsThis(x => { throw new Exception("Meh"); });
}
static class Extensions
{
public static void WhatsThis<T>(this T dummy, Action<T> action)
{
"Action<T>".Dump();
}
public static void WhatsThis<T>(this T dummy, Func<T, int> func)
{
"Func<T, int>".Dump();
}
}
As for why this is the case, I'm not 100% sure, but a casual look at the spec shows me the below likely explanation (emphasis mine): 至于为什么会出现这种情况,我不是百分百肯定,但随便看一下这个规格就会向我展示下面可能的解释(强调我的):
7.5.3 Overload resolution
7.5.3过载分辨率
[...]
[...]
7.5.3.3 Better conversion from expression
7.5.3.3更好地从表达转换
Given an implicit conversion C1 that converts from an expression E to a type T1, and an implicit conversion C2 that converts from an expression E to a type T2, C1 is a better conversion than C2 if at least one of the following holds:
给定从表达式E转换为类型T1的隐式转换C1,以及从表达式E转换为类型T2的隐式转换C2,如果以下至少一个成立,则C1是比C2更好的转换:
[...]
[...]
• 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 typeExpression<D2>
and one of the following holds:•E是匿名函数,T1是委托类型D1或表达式树类型
Expression<D1>
,T2是委托类型D2或表达式树类型Expression<D2>
并且以下之一成立:[...]
[...]
• D1 has a return type Y, and D2 is void returning
•D1的返回类型为Y,D2返回无效
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