PostMessage unable to pass a string C#

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开发者_运维技巧, 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.

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To answer your last question. I have tried the same and when I tried to convert lParam to string and backwards in the same window it works very gentled, but not when passing to another window. So I tried to use SendMessage instead and that worked great.

http://boycook.wordpress.com/2008/07/29/c-win32-messaging-with-sendmessage-and-wm_copydata/

I downloaded this class and that worked very well. :)

Use like this:

    public void SendMsg(string msg)
{
    MessageHelper msgHelper = new MessageHelper();
    int hWnd = msgHelper.getWindowId(null, "The title of the form you want to send a message to");
    int result = msgHelper.sendWindowsStringMessage(hWnd, 0, msg);
    //Or for an integer message
    result = msgHelper.sendWindowsMessage(hWnd, MessageHelper.WM_USER, 123, 456);
}

//In your form window where you want to receive the message

protected override void WndProc(ref Message m)
{
    switch (m.Msg)
    {
        case MessageHelper.WM_USER:
            MessageBox.Show("Message recieved: " + m.WParam + " - " + m.LParam);
            break;
        case MessageHelper.WM_COPYDATA:
            MessageHelper.COPYDATASTRUCT mystr = new MessageHelper.COPYDATASTRUCT();
            Type mytype = mystr.GetType();
            mystr = (COPYDATASTRUCT)m.GetLParam(mytype);
            MessageBox.Show(mystr.lpData);
            break;
    }
    base.WndProc(ref m);
}

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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.