How do I detect multi-threaded use?

Question

Is it sufficient to compare the ManagedThreadId at the time an object is created and at the time a method is called to verify that it isn't being used in a multithreading scenario?

public class SingleThreadSafe
{
    private readonly int threadId;
    public SingleThreadSafe()
    {
        threadId = Thread.CurrentThread.ManagedThreadId;
    }

    public void DoSomethingUsefulButNotThreadSafe()
    {
        if(threadId!=Thread.CurrentThread.ManagedThreadId)
        {
            throw new InvalidOperationException(
                "This object is being accessed by a thread different than the one that created it. " +
                " But no effort has been made to make this object thread safe.");
        }
        //Do something useful, like use a previously established DbConnection
    }
}

My intuition is often wrong about threading, so I wanted to check to see if there are edge cases I should be keeping in mind.

Answer 1

No it is not sufficient !

A managed thread id can be reused by the CLR, so if(threadId!=Thread.CurrentThread.ManagedThreadId) can return false even is the calling thread is different from the one used to construct the object.

What you are trying to achieve is possible through references comparisons:

if (!object.ReferenceEquals(Thread.CurrentThread, ThreadThatCreatedThis))
// ...

EDIT :

MSDN says however that :

The value of the ManagedThreadId property does not vary over time, even if unmanaged code that hosts the common language runtime implements the thread as a fiber.

http://msdn.microsoft.com/en-us/library/system.threading.thread.managedthreadid%28v=vs.110%29.aspx

Answer 2

That's basically what Windows Forms does. Here's a snippet from Control.InvokeRequired (quoted from the reference source):

public bool InvokeRequired {
    get {
        using (new MultithreadSafeCallScope())
        {
            ...
            int hwndThread = SafeNativeMethods.GetWindowThreadProcessId(hwnd, out pid);
            int currentThread = SafeNativeMethods.GetCurrentThreadId();
            return(hwndThread != currentThread);
        }
    }
}

If comparing thread IDs is good enough for Windows Forms, I suppose it'd be good enough for me...