C# PostMessage - How to prevent data loss?

Question

I have an application that will form a packet and send the packet data to an external program to send. I have everything working, but my only method I know that doesn't require the window to be the foremost is PostMessage. However, it seems to always lose 0-2 characters at the beginning of the message. Is there a way I can make a check to prevent the loss? I've tried looping GetLastError() and re-sending it if it's 0, but it doesn't help any. Here's the code I've gotten so far:

    public void SendPacket(string packet)
    {
        //Get window name
        IntPtr hWnd = Window.FindWindow(null, "???????????");
        //Get the first edit box handle
        IntPtr edithWnd = Window.FindWindowEx(hWnd, IntPtr.Zero, "TEdit", "");
        //Get the handle for the send button
        IntPtr buttonhWnd = Window.FindWindowEx(hWnd, IntPtr.Zero, "TButton", "SEND");
        //Iterate twice to get the edit box I need
        edithWnd = Window.FindWindowEx(hWnd, edithWnd, "TEdit", "");
        edithWnd = Window.FindWindowEx(hWnd, edithWnd, "TEdit", "");
        foreach (Char c in packet)
        {
            SendCheck(c, edithWnd);
        }
        //Press button
        TextSend.PostMessageA(buttonhWnd, 0x00F5, 0, 0);
        //Clear the edit box
        TextSend.SendMessage(edithWnd, 0x000C, IntPtr.Zero, "");
    }

    public void SendCheck(char c, IntPtr handle)
    {
        //Send the character
        TextSend.PostMessageA(handle, 0x102, c, 1);
        //If error code is 0 (failure), resend that character
        if (TextSend.GetLastError() == 0)
            SendCheck(c, handle);
        return;
    }

And here are the definitions in TextSend class:

        [DllImport("Kernel32.dll")]
        public static extern int GetLastError();
        [return: MarshalAs(UnmanagedType.Bool)]
        [DllImport("user32.dll", SetLastError = true)]
        public static extern bool SendMessage(IntPtr hWnd, uint Msg, IntPtr wParam, string s);  

Answer 1

The fact that you're finding a TEdit and a TButton makes me think that the target application was written in Delphi. If so, depending on the version of Delphi it may or may not be a Unicode application. You're calling PostMessageA instead of PostMessageW which means it's sending a single-byte Ansi char instead of a 16-bit Unicode char from the c# application.

Do you have source to the target application? Stuffing data in an edit box and clicking a button seems a bit fragile. If you can modify the target application there are certainly other options available than to send one character at a time.