How to tell if a thread is the main thread in C#

You could do it like this:

// Do this when you start your application
static int mainThreadId;

// In Main method:
mainThreadId = System.Threading.Thread.CurrentThread.ManagedThreadId;

// If called in the non main thread, will return false;
public static bool IsMainThread
{
    get { return System.Threading.Thread.CurrentThread.ManagedThreadId == mainThreadId; }
}

EDIT I realized you could do it with reflection too, here is a snippet for that:

public static void CheckForMainThread()
{
    if (Thread.CurrentThread.GetApartmentState() == ApartmentState.STA &&
        !Thread.CurrentThread.IsBackground && !Thread.CurrentThread.IsThreadPoolThread && Thread.CurrentThread.IsAlive)
    {
        MethodInfo correctEntryMethod = Assembly.GetEntryAssembly().EntryPoint;
        StackTrace trace = new StackTrace();
        StackFrame[] frames = trace.GetFrames();
        for (int i = frames.Length - 1; i >= 0; i--)
        {
            MethodBase method = frames[i].GetMethod();
            if (correctEntryMethod == method)
            {
                return;
            }
        }
    }

    // throw exception, the current thread is not the main thread...
}

If you're using Windows Forms or WPF, you can check to see if SynchronizationContext.Current is not null.

The main thread will get a valid SynchronizationContext set to the current context upon startup in Windows Forms and WPF.

Here is another option:

if (App.Current.Dispatcher.Thread == System.Threading.Thread.CurrentThread)
{
    //we're on the main thread
}

Works for me.

EDIT : Forgot to mention that this works only in WPF. I was searching SO for the WPF case, and I didn't notice that this question is general .NET. Another option for Windows Forms could be

if (Application.OpenForms[0].InvokeRequired)
{
    //we're on the main thread
}

Of course, you should first make sure that there is at least one Form in the application.