What is the best way to accomplish this: The main thread (Thread A) creates two other threads (Thread B and Thread C). Threads B and C do heavy disk I/O and eventually need to pass in resources they created to Thread A to then call a method in an external DLL file which requires the thread that created it to be called correctly so only Thread A can call it.

The only other time I ever used threads was in a Windows Forms application, and the invoke methods were just what I needed. This program does not use Windows Forms, and as such there are no Control.Invoke methods to use.

I have noticed in my testing that if a variable is created in Thread A, I have no trouble accessing and modifying it from Thread B/C which seems very wrong to me. With Winforms, I was sure it threw errors for trying to access things created on other threads. I know it is unsafe to change things from multiple threads, but I really hoped .NET would forbid it altogether to ensure safe coding. Does .NET do this, and I am just missing the boat, or does it only do it with WinForm apps?

Since it does seemingly allow this, do I do something like an OS would do, create a flag and monitor it from Thread A to see if it changes. If it does, then call the method. Doesnt the event handler essentially do this, so could an event be used somehow called on the main thread?