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[Charlie Poole <charlie@xxxxxxxxx>]

Hi Simone,

I like your overload provided it doesn't introduce any ambiguity in
the existing Assert.That overloads. I don't think it will, since the
argument is so specific.

BTW, we are using C# 3.0 to build our .NET 2.0 target, so C# 3.0
compiler tricks are OK, under the control of an #if if necessary.

Charlie

On Sat, Nov 3, 2012 at 6:53 PM, Simone Busoli
<1074568@xxxxxxxxxxxxxxxxxx> wrote:
> Hi Charlie, I'm afraid we're going to have some problems with this one.
>
> Here's what we want to accomplish:
>
> Assert.That(async () => { return await Task.FromResult(1); },
> Is.EqualTo(1));
>
> The signature of our delegate is "delegate object
> ActualValueDelegate(void)" but the .NET async feature supports only
> methods returning void Task or Task<T>.  In fact, if you try to compile
> the above you'll get:
>
> <snip>
> Cannot convert lambda expression to type 'object' because it is not a delegate type
>
> Cannot convert async lambda expression to delegate type
> 'NUnit.Framework.Constraints.ActualValueDelegate'. An async lambda
> expression may return void, Task or Task<T>, none of which are
> convertible to 'NUnit.Framework.Constraints.ActualValueDelegate'.
>
> Since 'NUnit.Framework.TestDelegate' returns void, a return keyword must not be followed by an object expression
> </snip>
>
> The second paragraph in the compiler error is what we need to fix, the
> issue being that there is no delegate contravariance, which has been
> introduced in, IIRC, C# 3 for generic delegates. In other words,
> assigning an instance of a delegate returning Task<T> to a delegate type
> returning object is not allowed.
>
> So you would assume that introducing an overload for Assert.That as this
> one would fix it:
>
> public static void That(Func<object> method, ...)
>
> But it's not the case as you get this error now:
>
> <snip>
> 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>'.
> </snip>
>
> which is quite misleading to the point of being wrong. Look at the
> second sentence, it says that void, Task or Task<T> are not convertible
> to System.Func<object>, which is indeed the case but not really what we
> are doing here. It cannot infer the type of the lambda but complaining
> about incompatible return types which don't make any sense. In fact,
> keeping the above overload of Assert.That and changing the test code to
> this, it'll stop the compiler complaints:
>
> Assert.That(new Func<Task<int>>(async () => { return await ReturnOne();
> }), Is.EqualTo(1));
>
> But nonetheless this is not what we want our users to need to write.
> Therefore given the constraint of compiling for 2.0 which in turn
> prevents us from exposing types which were not in 2.0, like tasks, I
> think the only solution would be to add a generic overload, which won't
> require any special changes to the test code:
>
> public static void That<T>(Func<T> method, ...)
>
> How does this sound?
>
> --
> You received this bug notification because you are subscribed to the bug
> report.
> https://bugs.launchpad.net/bugs/1074568
>
> Title:
>   Assert/Assume should support an async method for the
>   ActualValueDelegate
>
> To manage notifications about this bug go to:
> https://bugs.launchpad.net/nunit-3.0/+bug/1074568/+subscriptions

-- 
You received this bug notification because you are a member of NUnit
Developers, which is subscribed to NUnit V2.
https://bugs.launchpad.net/bugs/1074568

Title:
  Assert/Assume should support an async method for the
  ActualValueDelegate

Status in NUnit Test Framework:
  Triaged
Status in NUnitLite Testing Framework:
  Triaged
Status in NUnit V2 Test Framework:
  Triaged

Bug description:
  Just as Assert.Throws can take an async method as it's argument, it
  should be possible to use an async method returning a value for
  Assert.That and Assume.That.

To manage notifications about this bug go to:
https://bugs.launchpad.net/nunit-3.0/+bug/1074568/+subscriptions