I have the following code:
internal class FakeInvitationsRepository : InvitationsRepository
{
internal readonly static Dictionary<Guid, InvitationDbEntity> Data = new Dictionary<Guid, InvitationDbEntity>()
{
{ ...
Here is a screenshot of what it's worth:

This is just some fake data in memory, just 3 elements. The class lives inside the ASP.NET WebAPI project, and not in the test DLL, but in the WebAPI library.
An API call comes in and presses a breakpoint in the controller action, where it tries to use this fake data directly, not a single IoC is smart yet.
But the field Datais null. This throws a null ref exception at runtime.

The C # language specification says:
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. . Data - , ( readonly afterall).
- :
System.Diagnostics.StackTrace t = new System.Diagnostics.StackTrace();
{ at Company.Product.WebAPI.Controllers.RenamedController.<GetInvitations>d__14.MoveNext()
at Company.Product.WebAPI.Controllers.RenamedController.GetInvitations(Guid id)
at lambda_method(Closure , Object , Object[] )
at Company.WebAPI.Product.Areas.RouteDebugger.InspectActionInvoker.InvokeActionAsync(HttpActionContext actionContext, CancellationToken cancellationToken)
at Company.WebAPI.Product.Areas.RouteDebugger.InspectHandler.<>n__FabricatedMethod12(HttpRequestMessage , CancellationToken )
at Company.WebAPI.Product.Areas.RouteDebugger.InspectHandler.<SendAsync>d__e.MoveNext()
at Company.WebAPI.Product.Areas.RouteDebugger.InspectHandler.SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
}
FrameCount: 84
frames: {System.Diagnostics.StackFrame[87]}
m_iMethodsToSkip: 3
m_iNumOfFrames: 84
, . .
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Data .
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NullReferenceException - this.Invitations .
, , , , , , , null .
.