PostMessage unable to pass a string C#

Question

Here is my prototype:

    [DllImport("user32.dll", CharSet = CharSet.Auto)]
    public static extern bool PostMessage(int hhwnd, uint msg, IntPtr wparam, IntPtr lparam);

And here is how I'm using it:

PostMessage(HWND_BROADCAST, msg, Marshal.StringToHGlobalAuto("bob"), IntPtr.Zero);

In a different thread I can intercept this message, but when I try to get bob back using:

string str = Marshal.PtrToStringAuto(m.WParam); // where m = the Message object

I don't get bob in str.

I'm thinking this has got to be due to the fact that I referenced the "bob" string on one thread's stack and that reference has absolutely no meaning in a different thread's stack. But if that's the case, are these wparam and lparam pointers only really used for messages being passed in the same thread?

Edit* Correction: By thread I mean Process. This is a problem of passing a string between processes, not threads.

Answer 1

HGLOBALs arn't, in any way, global anymore. Not since win16. And HWND_BROADCAST looks like you are sending the message to a different process, never mind just a different thread.

So, unless you either use one of the standard messages that the OS knows how to marshal, you need to place your string, "bob" in a shared memory area that different processes can access.