I have two instances of my application running and the other sends a
message to the another:

IntPtr ip = Marshal.StringToHGlobalAnsi("blah blah blah");
SendMessage(hWnd, 0x1234, IntPtr.Zero, ip);

SendMessage is imported as:

[DllImport("user32.dll", CharSet=CharSet.Auto)]
public static extern int SendMessage(IntPtr hwnd,
[MarshalAs(UnmanagedType.U4)] int Msg, IntPtr wParam, IntPtr lParam);

The another instance is trying to read the string this way:

protected override  void WndProc(ref Message message)
{
   base.WndProc(ref message);
      switch (message.Msg) {
         case 0x1234:
            string str = Marshal.PtrToStringAnsi(message.LParam);
            break;
   }

The resulting string is full of junk. What's the right way to
implement this?

Hi,

The pointer you are passing is not valid in the target process. In the
days of 16bit windows the HGlobal was OS global but no longer is
this the case.

You could use the standard message WM_COPYDATA,
this message takes a struct as the LPARAM and the data is
serialized with the message to the other process.

The other way is to use some form of interprocess communication
such as Memory Mapped file or Pipes. If I am not mistaken the
WM_COPYDATA goes the Memory Mapped file route.

Cheers

Chris