How to get ThreadStatic value of another thread?

Question

Is it possible given the Thread reference to get ThreadStatic value for that thread?

Answer 1

No, that's not possible. As common with attributes like this one, the [ThreadStatic] attribute is recognized by the jitter. It generates a call into the CLR to obtain a pointer to the thread-local storage for a class. There are multiple versions of this helper method, the basic one is JIT_GetSharedGCThreadStaticBase(). But it gets more convoluted for a generic class for example, it can have multiple static variables based on the type parameter. The helper function takes two non-obvious arguments, the module ID and the class ID. Those IDs depend on the AppDomain in which the code was loaded.

Long story short, you don't stand a chance to make this same call, nor does the helper method even take the thread ID, it is implied by the call context.

You can hang arbitrary data off a thread with Thread.AllocateNamedDataSlot(). But note that it is static method and doesn't take a thread ID either, it is again based on the call context.

This is all quite intentional. A very nice property of thread-local storage is that it is always thread-safe. A backdoor that would permit accessing it from another thread would completely destroy that feature. Something that should worry you a great deal if you want to do this. You could, with, say, your own lookup table that's keyed by the ManagedThreadId.