FakeItEasy - NullRef when using ArgumentValueFormatter concurrently

[TimLovellSmith]

I had a one off failure with xunit 2 which I think is a concurrency bug.

Result Message: System.NullReferenceException : Object reference not set to an instance of an object.
Result StackTrace:  
at System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add)
   at FakeItEasy.Core.ArgumentValueFormatter.GetArgumentValueAsString(Object argumentValue) in c:\repo\FakeItEasy\Source\FakeItEasy\Core\ArgumentValueFormatter.cs:line 37
   at FakeItEasy.Core.DefaultFakeObjectCallFormatter.GetArgumentValueAsString(Object argumentValue) in c:\repo\FakeItEasy\Source\FakeItEasy\Core\DefaultFakeObjectCallFormatter.cs:line 170
   at FakeItEasy.Core.DefaultFakeObjectCallFormatter.AppendArgumentValue(StringBuilder builder, ArgumentValueInfo argument) in c:\repo\FakeItEasy\Source\FakeItEasy\Core\DefaultFakeObjectCallFormatter.cs:line 162
   at FakeItEasy.Core.DefaultFakeObjectCallFormatter.AppendArguments(StringBuilder builder, IEnumerable`1 arguments) in c:\repo\FakeItEasy\Source\FakeItEasy\Core\DefaultFakeObjectCallFormatter.cs:line 180
   at FakeItEasy.Core.DefaultFakeObjectCallFormatter.AppendArgumentsList(StringBuilder builder, IFakeObjectCall call) in c:\repo\FakeItEasy\Source\FakeItEasy\Core\DefaultFakeObjectCallFormatter.cs:line 148
   at FakeItEasy.Core.DefaultFakeObjectCallFormatter.GetDescription(IFakeObjectCall call) in c:\repo\FakeItEasy\Source\FakeItEasy\Core\DefaultFakeObjectCallFormatter.cs:line 40
   at FakeItEasy.Core.CallWriter.WriteCalls(IEnumerable`1 calls, IOutputWriter writer) in c:\repo\FakeItEasy\Source\FakeItEasy\Core\CallWriter.cs:line 40
   at FakeItEasy.Core.FakeAsserter.AppendCallList(IEnumerable`1 calls, CallWriter callWriter, IOutputWriter writer) in c:\repo\FakeItEasy\Source\FakeItEasy\Core\FakeAsserter.cs:line 86
   at FakeItEasy.Core.FakeAsserter.CreateExceptionMessage(IEnumerable`1 calls, CallWriter callWriter, String callDescription, String repeatDescription, Int32 matchedCallCount) in c:\repo\FakeItEasy\Source\FakeItEasy\Core\FakeAsserter.cs:line 46
   at FakeItEasy.Core.FakeAsserter.AssertWasCalled(Func`2 callPredicate, String callDescription, Func`2 repeatPredicate, String repeatDescription) in c:\repo\FakeItEasy\Source\FakeItEasy\Core\FakeAsserter.cs:line 31
   at FakeItEasy.Configuration.RuleBuilder.MustHaveHappened(Repeated repeatConstraint) in c:\repo\FakeItEasy\Source\FakeItEasy\Configuration\RuleBuilder.cs:line 114
   at FakeItEasy.Configuration.RuleBuilder.ReturnValueConfiguration`1.MustHaveHappened(Repeated repeatConstraint) in c:\repo\FakeItEasy\Source\FakeItEasy\Configuration\RuleBuilder.cs:line 168
   at FakeItEasy.AssertConfigurationExtensions.MustHaveHappened(IAssertConfiguration configuration) in c:\repo\FakeItEasy\Source\FakeItEasy\AssertConfigurationExtensions.cs:line 18
   at [testcode]

The only reason I can think of this can nullref, given the FakeItEasy source I compiled from is that it's a plain Dictionary<> being shared across threads without locking, since it appears ArgumentFormatter is a singleton.

Sending a PR for this soon!

[blairconrad]

Edited the description to improve formatting.

[blairconrad]

@TimLovellSmith, thanks very much for your work on this issue. Look for your name in the release notes. 🏆

This issue has been fixed in release 2.0.0.