为什么 VS2017 说 await 是一个未知的关键字？

Question

    internal /*async*/ void RunAsync()
    {
        var tasks = ...;
        await Task.WhenAll(tasks);
    }

In the above example, the IDE informs me "the ''await operator can only be used within an async method". It offers me fixes to make my method signature async void or async Task

    internal /*async*/ void RunAsync()
    {
        var tasks = ...;
        var t = Task.WhenAll(tasks);
        await t;
    }

In the second example, it gives the error "the type or namespace await cannot be found"... await is not color-coded as a language keyword and the IDE offers me no automatic fixes.

Is this a VS bug?

Answer 1

The code is invalid C# code in both cases. So the compiler/IDE has to do its best to figure out what you're trying to do and give you an appropriate error message.

In the above example, the IDE informs me "the ''await operator can only be used within an async method". It offers me fixes to make my method signature async void or async Task

VS is parsing the code and is pretty sure that the await here is intended to be the await operator. The code isn't valid C#, but that's what it looks like you're trying to do. So it gives you a helpful suggestion that you have to have the async keyword to make await a keyword.

In the second example, it gives the error "the type or namespace await cannot be found"... await is not color-coded as a language keyword and the IDE offers me no automatic fixes.

VS is parsing the code and is not sure whether the await here is an await operator or a type. await t; looks a lot like int t; to the compiler.

Is this a VS bug?

It's not a bug, since the code is invalid C# to begin with. But you could ask for a better error message in the second case, and the C# compiler team may or may not decide it's worth doing that instead of working on other features.

For context, note that await is a contextual keyword . It's only a keyword if the async keyword is present. await is not always a keyword, and it would be a bug if VS were to always treat it as a keyword. Specifically, in both the code examples here, await is not a keyword. VS goes out of its way to do extra fallback parsing in the first example to detect a common scenario where the developer intended to use the await keyword.