Do async methods throw exceptions on call or on await?

Question

When I call an async method and get a task back, will that immediately throw or will it wait until I await the task?

In other words, will this code work? Or will I have to wrap the method call in the try-block as well?

Task task = ThisMethodWillThrow();

try
{
    await task;
}
catch (Exception e)
{
    Console.WriteLine("oops");
}

Answer 1

Both are possible. If the method is actually async (ie uses the C# async keyword in the declaration), then the C# compiler wraps it up in such a way that it will always reliably throw on the await , but it is important to note that this is not the only way to write a method that can be await -ed, so: if you don't control the method being called ( ThisMethodWillThrow ) and can't rely on knowledge of the implementation, it would be better for the try to include the initial invoke, as well as the await .

As an example of a method that will throw immediately rather than in the await :

Task ThisMethodWillThrow() { // note that this is **not** "async", but is awaitable
    if (thingsAreBad) throw new SomeException();
    return SomeInnerMethod();
}
async Task SomeInnerMethod() { ... }

It might be tempting to think "well, just make all awaitable methods async , to avoid this" - like:

async Task ThisMethodWillThrowToo() { // note that this is "async"
    if (thingsAreBad) throw new SomeException();
    await SomeInnerMethod();
}

However: there are scenarios where the async machinery is a very measurable performance overhead in the "often sync, sometimes async" case - and so a common optimization in performance critical awaitable code (IO/network code, for example) is to actively avoid the async machinery unless we know that we're actually falling into the asynchronous path.