Send text from VB to Delphi apps, using SendMessage - vb.net

Question

) I am trying to send a short text from a VB app to Delphi app.. here is the
VB Code: Sender Program "Sender"
Public Class SendData
Const WM_COPYDATA = &H4A
Public Structure CopyDataStruct
Public dwData As Integer
Public cbData As Integer
Public lpData As String
End Structure
Declare Function FindWindow Lib "user32" Alias "FindWindowA" _
(ByVal lpClassName As String, ByVal lpWindowName As String) As Long
Declare Function SendMessage Lib "user32" Alias "SendMessageA" _
(ByVal hWnd As Long, ByVal wMsg As Long, ByVal wParam As Long, ByVal lParam As _
CopyDataStruct) As Long
Private Sub SendData(ByVal cds)
Dim iHwnd As Long
Dim SS As String = "Test String less than 30 Char"
Dim cds As CopyDataStruct
cds.dwData = 0
cds.cbData = Len(SS)
cds.lpData = SS
iHwnd = FindWindow(0&, "Receive")
SendMessage(iHwnd, &H4A, Me.Handle, cds)
End Sub
here is the Delphi Code: Receiver program "Receive"
procedure TForm1.HandleCopyDataString(copyDataStruct: PCopyDataStruct);
var
s : string;
begin
s := PChar(CopyDataStruct.lpData);
cdMemo.Lines.Add(Format('Received data "%s" at %s',[s, TimeToStr(Now)]));
end;
procedure TForm1.WMCopyData(var Msg: TWMCopyData) ;
var
s : string;
sText: array[0..255] of Char;
copyDataType : TCopyDataType;
begin
copyDataType := TCopyDataType(Msg.CopyDataStruct.dwData);
s := PChar(Msg.CopyDataStruct.dwData);
Form1.cdMemo.Lines.Add(Format('Data from: %d',[msg.From]));
HandleCopyDataString(Msg.CopyDataStruct);
case Msg.CopyDataStruct.dwData of 0: //we are being sent a string
begin
StrLCopy(sText, Msg.CopyDataStruct.lpData, Msg.CopyDataStruct.cbData);
Form1.Label1.Caption := sText;
end;
end;
end;
What am I doing wrong here? It is possible to send strings from VB to Delphi programs using WM_COPYDATA command, and SendMessage function?
please help me :-)
F

Answer 1

There are a few things wrong with your Delphi code.
The dwData field holds an integer, but you type-cast it to PChar, a pointer, and then assign it to your string. That's not the field where you stored your string data. That's lpData.
The string you pass is not null-terminated. The OS only promises to copy exactly as many bytes as you specify in the cbData field. That's not necessarily a problem, but you need to be aware of it when you read the string later. To assign s to hold the string copied from the other process, use SetString like this:
SetString(s, PAnsiChar(Msg.CopyDataStruct.lpData), Msg.CopyDataStruct.cbData);
You haven't shown what TCopyDataType is, but if it's anything other than an integer or integer-subrange type, you're using it wrong. The dwData field is already a DWord, so you can use it wherever a numeric value is expected.
You're calling StrLCopy wrong. The third parameter should be the size of the destination buffer, not the source. It's meant to prevent buffer overflows by not copying more characters than will fit in the destination. The function expects to be able to detect the size of the source buffer by finding the terminating null character (but we already established that that won't be available). You could fix it like this:
StrLCopy(sText, Msg.CopyDataStruct.lpData,
Min(Length(sText), Msg.CopyDataStruct.cbData));
(Min is in the Math unit.)