Question

I bumped into this C# compiler error and while I’m ok with the compiler not being able to infer the type of the lambda the error message seems wrong. Here’s the relevant piece of code:

Func<object> lambda = async () => { return await Task.FromResult(1); };

and here’s the compiler error:

error CS4010: Cannot convert async lambda expression to delegate type ‘System.Func<object>‘. An async lambda expression may return void, Task or Task<T>, none of which are convertible to ‘System.Func<object>‘.

The part I don’t quite understand is the last sentence. The lambda is indeed returning Task<int>, but why does the compiler think that it should try to convert it to System.Func<object>?

On the other hand, if what the error message is trying to convey is that System.Func<Task<int>> cannot be assigned to System.Func<object>, that doesn’t seem to be the case thanks to generic delegate covariance which means that this works just fine instead:

Func<object> lambda = new Func<Task<int>>(async () => { return await Task.FromResult(1); });

Answer 1

The lambda is indeed returning Task<int>, but why does the compiler think that it should try to convert it to System.Func<object>?

I believe the error message is poorly worded. It’s trying to derive a return type for the lambda (which can be void, Task, or Task<T>), and convert the lambda to Func<object>. I recommend that you raise an issue on Microsoft Connect requesting a clearer error message.

On the other hand, if what the error message is trying to convey is that System.Func<Task<int>> cannot be assigned to System.Func<object>, that doesn’t seem to be the case thanks to generic delegate covariance

True, but the compiler doesn’t determine the type of lambda expressions until “later” than most expressions. The compiler doesn’t see Func<Task<int>> (it just sees a lambda expression taking no arguments and returning Task or Task<int>), and so it won’t use generic delegate variance.

I’m hoping that Eric Lippert will do a blog post on how async lambda expressions are resolved, particularly in the case of method overload selection.