ReturnsAsync() with factory method does not behave as expected

[sellotape]

There is a bug in ReturnsAsync<TMock, TResult>(this IReturns<TMock, Task<TResult>> mock, Func<TResult> value):

Returns(() => f()) behaves as one would expect; i.e. call f() each time the method that was setup is called, to determine the value.

ReturnsAsync(() => f()), however, returns the same value every time the setup method is called. Looking at the code, the reason why is clear: ReturnsAsync() evaluates f() at Setup() time, rather than when the setup method is called, as one would expect:

	public static IReturnsResult<TMock> ReturnsAsync<TMock, TResult>(this IReturns<TMock, Task<TResult>> mock, Func<TResult> value) where TMock : class
	{
		return mock.ReturnsAsync(value());
	}

Workaround: drop back to .Returns(() => Task.FromResult(f()))

(That's the code fix as well, BTW).

[SaroTasciyan]

I experienced the same issue yesterday, digging into the code I realized that the func is evaluated right away and Task.FromResult(...) is built with given value.

However, in the documentation for Returns(Func<T, TResult>) it's stated that "returns a calculated value which is evaluated lazily at the time of the invocation"

I agree with your fix suggestion, PR is ready to go.