C++ SendMessage trying to receive string - c++

Question

Working with C++, I've spent way to many hours already trying to figure this out. This code is from a working program, I'm rewriting it in C#, but there is some things happening I do not understand.
The below code is exactly what runs when I press "Step Into". How is it going from ::SendMessage(...) to ::OnCopyData(..) with pCopyDataStruct containing data now?
main.cpp
void COTP::main()
{
//string will be returned using WM_COPYDATA message
::SendMessage(hWnd, 33508, (WPARAM)GetSafeHwnd(), 11);
// WPARAM is a typedef for UINT_PTR which is an unsigned int.
}
afxwin2.inl
_AFXWIN_INLINE HWND CWnd::GetSafeHwnd() const
{ return this == NULL ? NULL : m_hWnd; }
main.cpp
BOOL COTP::OnCopyData(CWnd* pWnd, COPYDATASTRUCT* pCopyDataStruct)
{
CString str, str2;
switch (pCopyDataStruct->dwData)
{
case JRC_COPYDATA_ID_TRACK_FILENAME:
str = (LPCTSTR)pCopyDataStruct->lpData;
break;
}
}
Any help super appreciated, I have looked thru all the msdn documents that I can tonight, it just seems like I'm missing something simple somewhere. Thought it might be a callback, but that doesn't seem right.

Answer 1

The code is sending a message (33508) to a specific window. Apparently the receiving window processes that message by sending a WM_COPYDATA message to the HWND you passed in the WPARAM in the original SendMessage() call. It's essentially implementing a callback mechanism.
The original message number (33508) is not one the standard Win32 messages (at least not that I recognize), so it's likely a custom message. Also, the fact that it's using WM_COPYDATA to respond would suggest that the receiving window is in a different process (i.e., not owned by your application).

Answer 2

Alright got it figured out now. With the help of this page and here. Below is what I'm currently using for C#. This is by no means finished code, but this does work and is good for learning purposes.
Click calculate button,
[DllImport("user32.dll", CharSet = CharSet.Auto, ExactSpelling = true)]
public static extern IntPtr GetActiveWindow();
private void btnCalculate_Click(object sender, EventArgs e)
{
int hwnd = 0;
hwnd = Win32.FindWindow("The App pulling from", "Window");
int s = (int)GetActiveWindow();
int s3 = Win32.SendMessage(hwnd, 33508, s, 11);
Then what happens is a callback occurs and activates WndProc(ref Message m),
protected override void WndProc(ref Message m)
{
// Prevents error creating window handle message.
base.WndProc(ref m);
// WM_COPYDATA
// m.Msg = 0x4a
//msg=0x4a (WM_COPYDATA) hwnd=0x251e62 wparam=0x69063e lparam=0x1c42cca0 result=0x0
if (m.Msg == 0x4a)
{
Console.WriteLine(m);
WndProc(m.HWnd, m.Msg, m.WParam, m.LParam);
}
}
public struct CopyDataStruct : IDisposable
{
public IntPtr dwData;
public int cbData;
public IntPtr lpData;
public void Dispose()
{
if (this.lpData != IntPtr.Zero)
{
LocalFree(this.lpData);
this.lpData = IntPtr.Zero;
}
}
}
[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
public unsafe struct DataStruct
{
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 300)]
public string s;
public double d;
public char c;
};
protected void WndProc(IntPtr hwnd, int msg, IntPtr wParam, IntPtr lParam)
{
CopyDataStruct cps = (CopyDataStruct)Marshal.PtrToStructure(lParam, typeof(CopyDataStruct));
DataStruct data = (DataStruct)Marshal.PtrToStructure(cps.lpData, typeof(DataStruct));
// data.s is what we needed.
Console.WriteLine(data.s);
}
Thank you very much to everyone that replied! I am confident I would have given up if not for your help. :)