What steps will reproduce the problem? 1. Compile attached BreakMoq.cs 2. Run it

What is the expected output?

It should print "Start", "Stop" and then hang (because for simplicity I've omitted the events necessary to stop the extra threads).

What do you see instead?

It prints "Start" and then hits an exception:

Unhandled Exception: System.InvalidOperationException: Collection was modified; enumeration operation may not execute. at System.Collections.Generic.List1.Enumerator.MoveNextRare() at System.Linq.Enumerable.WhereListIterator1.MoveNext() at System.Linq.Enumerable.Count[TSource](IEnumerable1 source) at Moq.Mock.VerifyCalls(Interceptor targetInterceptor, MethodCall expected, Expression expression, Times times) at Moq.Mock.Verify[T](Mock mock, Expression1 expression, Times times, String failMessage) at Moq.Mock1.Verify(Expression1 expression) at Program.Main() in w:\moqtesting\BreakMoq.cs:line 40

What version of the product are you using? On what operating system?

Moq 4.0.10827, Windows 7.0 using .NET 4.0.

Please provide any additional information below.

It may be that this scenario isn't supported - verifying a Moq concurrently with other threads still accessing it, but it would be helpful. I'm trying to test a component that operates asynchronously, and it's easy to guarantee that it should have called the mock at least once before verifying, but it's hard to guarantee that it won't continue to call it multiple times without restructuring the test teardown significantly. Before I do that it would be good to know whether this is expected behaviour or not.

I had this problem when I was triggering some "BeginInvoke" delegates within a Moq callback. You need to wait for all threads to finish before running the Verify.