Assert.That is blocking and might lead to deadlock when used with WCF.

[MikiKaufhold]

Problem 1)

Suppose you are asserting an asynchronous WCF service call - for the sake of simple example just its completion (for complete implementation see NUnitUsingWCFClientDeadlockReproducer.cs.txt)

Assert.That(async () =>
{
  await AsyncCall();
  return true;
}, Is.True);

In certain cases this lead to deadlock…

Analysis 1)

There is a great article where Alois explains a case of similar WCF deadlock

The problem is that WCF runs asynchronous method completions on the WCF channel dispatcher which seems to be single threaded just like a UI application with a message pump.
…
 A sync/async mixture of remoted methods will likely cause deadlocks

Interestingly, our WCF service has only Async methods…
However, Assert.That ends up with a blocking Task.Wait() call and (likely) causes the deadlock.

Solution 1)

To resolve this we have introduced an async variant for Assert.That using DelayedConstraint that looks like this:
public static Task DelayedAsync<T>(Func<Task<T>> actualValueDelegate, IResolveConstraint expression, int delayInMilliseconds)
(for complete implementation see DelayedAsync.cs.txt)

Problem 2) - Considering upgrade of NUnit from 2.6.6 -> 3.11

We are running into the issue generally known as "Catching AssertionException fails tests" #2758 but also mentioned in #2043, #2040, #2007.

Analysis 2)

As suggested in those threads,
using (new NUnit.Framework.Internal.TestExecutionContext.IsolatedContext()) in the most inner DelayedAsync call resolves the issue.

However, since catching AssertionExceptions is not recommended practise and TestExecutionContext is Internal, I am hesitant to adopt this as a final solution.

Instead I would wish for a native awaitable Assert.That that would implement async all the way pattern as recommended by Stephen Cleary.

Looking forward to your feedback and suggestions.

[jnm2]

We mitigate deadlocks for known synchronization contexts, currently just Windows Forms and WPF message loops. #2917 tracks making this extensible.

Does WCF have a synchronization context and a MessageLoop.Run() and MessageLoop.Exit() equivalent? We might be able to detect it and do everything in the box.

Instead I would wish for a native awaitable Assert.That that would implement async all the way pattern as recommended by Stephen Cleary.

I'm with you 110%. #2843 tracks this.

[OsirisTerje]

@uecasm @MikiKaufhold Can you guys confirm https://www.myget.org/feed/nunit/package/nuget/NUnit/4.0.0-dev-07733 is working for you?

[siprbaum]

Cc @z002Holpp, who I know was also affected by this.

[OsirisTerje]

This has not been confirmed by any of the reporters, so I'll close this issue with the assumption it works. The PR has been merged.