Sending a String using SendMessage

Mark Salsbery

On the C# side, something like this maybe:

[DllImport("User32", SetLastError = true)]
public static extern IntPtr SendMessage(IntPtr hWnd, uint Msg, IntPtr wParam, IntPtr lParam);
...

string ManagedString = "A string!";
IntPtr PtrToUnmanagedString = Marshal.StringToHGlobalAnsi(ManagedString);
SendMessage(hwnd, WM_SOMEMESSAGE, IntPtr.Zero, PtrToUnmanagedString);
Marshal.FreeHGlobal(PtrToUnmanagedString);

On the C++ side, handle messages in the app's message loop as usual (kind of a big topic for here). Mark

Mark Salsbery Microsoft MVP - Visual C++ :java:

eyalbi007

Yes, both Windows applications. One's GUI is written on .NET 2 C# and the second is C++ application with GUI designed using FLTK. Thank you.

eyalbi007

On the C++ side I have lparam of type int. How should I refer to it in order to get my string? Thanks!

Mark Salsbery

For the ANSI string sample I showed, something like this:

const char *pANSIstring = (const char *)lParam;

For Unicode, you could change the StringToHGlobalAnsi to StringToHGlobalUni on the C# side, and on the C++ side, something like:

const wchar_t *pUnicodestring = (const wchar_t *)lParam;

Mark

Mark Salsbery Microsoft MVP - Visual C++ :java:

eyalbi007

Hi Mark, The two options you suggested doesn't work: At both cases I get 'Bad Ptr' at the C++ side (I get junk and not string). Maybe the reason is that each one of the applications (C# and C++) uses different memory space, so passing pointers from one to another is meaningless. Or am I missing something? Thanks a lot, Eyal.

oobimoo

You must use the WM_COPYDATA msg in the c# proj declare the following

[StructLayout(LayoutKind.Sequential)]
struct CopyDataStruct
{
public IntPtr dwData;
public int dataSize;
public IntPtr data;
}

enum WinMessage
{
    WM\_COPYDATA = 0x4A
}

class NativeAPI
{
    \[DllImport("User32.dll")\]
    public static extern int SendMessage(IntPtr hWnd, WinMessage msg, IntPtr wParam, IntPtr lParam);
    \[DllImport("User32.dll", CharSet = CharSet.Auto)\]
    public static extern IntPtr FindWindow(string className, string windowName);
}

and when you press a button for example

private void button1_Click(object sender, EventArgs e)
{
string s = "This is a message from sharp"; // string to send to the native application
// native window handle
IntPtr hCWindow = NativeAPI.FindWindow("RECIEVE_DATA", "Recieve_Data");
// check and handle the not found (null) case here...

        // GetBytes returns the bytes of the string but not the terminating null char
        byte\[\] tmpStringData = System.Text.Encoding.Unicode.GetBytes(s);
        // so we copy to a new byte array and adding 2 zeros (for unicode)
        byte\[\] stringData = new byte\[tmpStringData.Length + 2\];
        Array.Copy(tmpStringData, stringData, tmpStringData.Length);
        stringData\[stringData.Length - 2\] = stringData\[stringData.Length - 1\] = 0;
        
        CopyDataStruct cds = new CopyDataStruct();
        cds.dwData = IntPtr.Zero;  // no need for this example
        cds.dataSize = stringData.Length; 
        // pinning the managed array and getting the address
        GCHandle gch\_data = GCHandle.Alloc(stringData, GCHandleType.Pinned);
        cds.data = gch\_data.AddrOfPinnedObject();
        // pinning the struct also
        GCHandle gch\_cds = GCHandle.Alloc(cds, GCHandleType.Pinned); 
        // when sending a WM\_COPYDATA msg wParam is the hWnd of the 
        // sender and lParam a pointer to a COPYDATASTRUCT                   
        NativeAPI.SendMessage(hCWindow, WinMessage.WM\_COPYDATA, this.Handle, gch\_cds.AddrOfPinnedObject());
        // free the handles
        gch\_data.Free();
        gch\_cds.Free();
    }

and in the windowproc of the native app

case WM\_COPYDATA:
	{
	PCOPYDATASTRUCT pcds = (PCOPYDATASTRUCT)lParam;
	MessageBox(0, (LPC

eyalbi007

Hi oobimoo, Thanks for you reply. The C# part works fine, however, at the C++ side I can retrieve only the first character of the string ((LPCTSTR)pcds->lpData equals to "T"). I tried some other castings (string and char*) but I only get the first character. Any ideas? Thanks! Eyal.

oobimoo

Your encoding at the c++ side is ascii. As i mentioned you must use the System.Text.Encoding.ASCII.GetBytes(s) instead of the unicode that i use in my example.

Mark Salsbery

eyalbi007 wrote:

Maybe the reason is that each one of the applications (C# and C++) uses different memory space

You're absolutely right! I'm an idiot :) Sorry about that - I totally spaced on the inter-process requirement. Looks like oobimoo showed you a solution. Mark

Mark Salsbery Microsoft MVP - Visual C++ :java:

eyalbi007

Dear oobimoo, Indeed, now it works. However, I'm still concerned about something: if I'm getting this correctly, the calls gch_data.Free() and gch_cds.Free() are responsible for freeing the pinned memory. However, what if the receiving side didn't get the information before this release? Isn't it better to (somehow) release this pinned memory at the receiving side? Consider the following chain of events: 1. Sending the message at C# application. 2. Receiving the message at the C++ application, but right before extracting the sent string: 3. Context switch at the C++ application. 4. The C# application releases the pinned memory. 5. Context switch back to the C++ application. The pinned memory is released and we can't extract the string message. Or maybe these can't happen since SendMessage is blocking? Thanks!

oobimoo

Your are welcome From the msdn (WM_COPYDATA) : "The receiving application should consider the data read-only. The lParam parameter is valid only during the processing of the message. The receiving application should not free the memory referenced by lParam. If the receiving application must access the data after SendMessage returns, it must copy the data into a local buffer. " From the msdn (SendMessage) : "The SendMessage function sends the specified message to a window or windows. It calls the window procedure for the specified window and does not return until the window procedure has processed the message. "

modified on Monday, August 25, 2008 1:08 PM

Alexandro Maceiras

Yeah! Tks a lot!