Exception thrown randomly when testing.

Im using xUnit and WebApplicationFactory to create a test server. I wrote some test. When I run all one or two randomly fails. Run again and they succeed.

Here is my exception:

Message: 
System.InvalidOperationException : Collection was modified; enumeration operation may not execute.

  Stack Trace: 
Enumerator.MoveNextRare()
Enumerable.Any[TSource](IEnumerable`1 source, Func`2 predicate)
EndpointDefinition.<PostProcessors>b__139_0(IGlobalPostProcessor p)
WhereArrayIterator`1.MoveNext()
List`1.InsertRange(Int32 index, IEnumerable`1 collection)
EndpointDefinition.PostProcessors(IGlobalPostProcessor[] postProcessors)
<>c__DisplayClass0_0.<<Main>$>b__7(EndpointDefinition ep) line 58
MainExtensions.MapFastEndpoints(IEndpointRouteBuilder app, Action`1 configAction)
MainExtensions.UseFastEndpoints(WebApplication app, Action`1 configAction)
Program.<Main>$(String[] args) line 51
Program.<Main>(String[] args)
InvokeStub_Program.<Main>(Object, Object, IntPtr*)
MethodInvoker.Invoke(Object obj, IntPtr* args, BindingFlags invokeAttr)
--- End of stack trace from previous location ---
DeferredHost.StartAsync(CancellationToken cancellationToken)
HostingAbstractionsHostBuilderExtensions.StartAsync(IHostBuilder hostBuilder, CancellationToken cancellationToken)
HostingAbstractionsHostBuilderExtensions.Start(IHostBuilder hostBuilder)
TraffiCaptureFactoryBase.CreateHost(IHostBuilder builder) line 47
WebApplicationFactory`1.ConfigureHostBuilder(IHostBuilder hostBuilder)
WebApplicationFactory`1.EnsureServer()
WebApplicationFactory`1.get_Services()

Any idea whats going wrong?

[dj-nitehawk]

@szekelymatyas that was actually helpful to find the culprit. thank you very much! fixed in v5.2.0-beta10

[szekelymatyas]

Im using visual studio with xunit right know. But the problem has occured in rider too, if i remember correctly. Later im gonna pull de source, but right know i have a lot of think to do. The biggest problem is that its only happens 2-3 times in about 100. So im not sure how to reproduce it deterministicly.