C# – Calling SendMessage (P/Invoke) keeps crashing

c++pinvoke

I am having to write an application that communicates with a third-party program (AOL, I'm sorry. :()

Doing a lot of research I found some ways to do this with P/Invoke, and for the most part it works okay, but it crashes upon subsequent trials, specifically with SendMessage. I'm outlining the crashing code below.

All of this was ported to .NET from old, old Visual Basic files. It's archaic as it can be, and I understand if it's not doable – I was just hoping there was a better way than Visual Basic 4.0 to get this done.

[DllImport("kernel32.dll", SetLastError = true)]
[return: MarshalAs(UnmanagedType.Bool)]
public static extern bool CloseHandle(IntPtr hObject);

[DllImport("user32.dll", SetLastError = true)]
public static extern IntPtr FindWindow(string lpClassName, string lpWindowName);

[DllImport("user32.dll", SetLastError = true)]
public static extern IntPtr FindWindowEx(IntPtr hwndParent,
                                         IntPtr hwndChildAfter,
                                         string lpszClass,
                                         string lpszWindow);

[DllImport("user32.dll", SetLastError = true)]
public static extern IntPtr FindWindowEx(IntPtr parentHandle,
                                         IntPtr childAfter,
                                         string className,
                                         IntPtr windowTitle);

[DllImport("user32.dll", CharSet = CharSet.Auto)]
public static extern IntPtr SendMessage(HandleRef hWnd,
                                        UInt32 Msg,
                                        IntPtr wParam,
                                        IntPtr lParam);

[DllImport("user32.dll", EntryPoint="SendMessageW")]
public static extern IntPtr SendMessageByString(HandleRef hWnd,
                                                UInt32 Msg,
                                                IntPtr wParam,
                                                StringBuilder lParam);

[DllImport("user32.dll", CharSet = CharSet.Unicode , EntryPoint = "SendMessageW")]
public static extern IntPtr SendMessageByString(HandleRef hWnd,
                                                UInt32 Msg,
                                                IntPtr wParam,
                                                String lParam);

public IntPtr FindClientWindow()
{
    IntPtr aol = IntPtr.Zero;
    IntPtr mdi = IntPtr.Zero;
    IntPtr child = IntPtr.Zero;
    IntPtr rich = IntPtr.Zero;
    IntPtr aollist = IntPtr.Zero;
    IntPtr aolicon = IntPtr.Zero;
    IntPtr aolstatic = IntPtr.Zero;

    aol = Invoke.FindWindow("AOL Frame25", null);
    mdi = Invoke.FindWindowEx(aol, IntPtr.Zero, "MDIClient", null);
    child = Invoke.FindWindowEx(mdi, IntPtr.Zero, "AOL Child", null);
    rich = Invoke.FindWindowEx(child, IntPtr.Zero, "RICHCNTL", null);
    aollist = Invoke.FindWindowEx(child, IntPtr.Zero, "_AOL_Listbox", null);
    aolicon = Invoke.FindWindowEx(child, IntPtr.Zero, "_AOL_Icon", null);
    aolstatic = Invoke.FindWindowEx(child, IntPtr.Zero, "_AOL_Static", null);

    if (rich != IntPtr.Zero &&
        aollist != IntPtr.Zero &&
        aolicon != IntPtr.Zero &&
        aolstatic != IntPtr.Zero)

        return child;
    do
    {
        child = Invoke.FindWindowEx(mdi, child, "AOL Child", null);
        rich = Invoke.FindWindowEx(child, IntPtr.Zero, "RICHCNTL", null);
        aollist = Invoke.FindWindowEx(child, IntPtr.Zero, "_AOL_Listbox", null);
        aolicon = Invoke.FindWindowEx(child, IntPtr.Zero, "_AOL_Icon", null);
        aolstatic = Invoke.FindWindowEx(child, IntPtr.Zero, "_AOL_Static", null);

        if (rich != IntPtr.Zero &&
            aollist != IntPtr.Zero &&
            aolicon != IntPtr.Zero &&
            aolstatic != IntPtr.Zero)

            return child;
    }
    while (child != IntPtr.Zero)
        ;

    return child;
}

IntPtr room = IntPtr.Zero;
IntPtr child = IntPtr.Zero;
IntPtr length = IntPtr.Zero;
IntPtr roomHandle = IntPtr.Zero;

child = FindClientWindow();
room = FindChildByClass(child, "RICHCNTLREADONLY");

HandleRef n = new HandleRef(IntPtr.Zero, room);

length = SendMessage(n, 0x000E, IntPtr.Zero, IntPtr.Zero);

// This is the line that keeps crashing on me.
SendMessageByString(n, 0x000D, new IntPtr( length.ToInt32() + 1 ), str);

public IntPtr FindChildByClass(IntPtr parent, string child)
{
    return Invoke.FindWindowEx(parent, IntPtr.Zero, child, null);
}

Best Solution

You are using the Wide byte SendMessage..ie for Wide Characters, have you tried the normal Sendmessage.. public static extern int SendMessage(IntPtr hWnd, UInt32 Msg, Int32 wParam, Int32 lParam);

I also notice it's like as if you are trying to change the value based on the handle of the richtextbox's control hence the looking around for the AOL's client window in another process...is that correct?

That could be the source of the problem, directly modifying a control that belongs to a window that is not yours (your program is managed, modifying a unmanaged process's window)...that could explain why it crashed. Can you clarify what is the hex constants for?

Edit: When you use the WM_GETTEXTLENGTH and WM_GETTEXT, they are part of the Windows Messages to retrieve the text length and the actual text from the control. If you look here and see what pinvoke.net has to say about them..When you issue a 'SendMessage', with WM_GETTEXTLENGTH and WM_GETTEXT, you are telling Windows - 'Hey, get me the length of the text in that associated handle which I've given you in the parameter n. Just occurred to me, worth trying out...I would get rid of those SendMessage pinvokes and use just this one..

[DllImport("user32.dll", CharSet = CharSet.Auto)]
static extern IntPtr SendMessage(IntPtr hWnd, UInt32 Msg, IntPtr wParam, StringBuilder lParam);
//If you use '[Out] StringBuilder', initialize the string builder with proper length first.

child = FindClientWindow();
room = FindChildByClass(child, "RICHCNTLREADONLY");

length = SendMessage(n, 0x000E, IntPtr.Zero, IntPtr.Zero);

StringBuilder sbBuf = new StringBuilder(length);

SendMessageByString(room, 0x000D, new IntPtr( length.ToInt32() + 1 ), out sbBuf); // this is the line that keeps crashing on me.

Try that and get back here ... :)

Hope this helps, Best regards, Tom.

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