VB .Net - Send String to Another Application - vb.net

Question

From VB .Net, I'm trying to send a string to a textbox of another application but I cannot make it work. I'm able to get the handle and even set the focus to the textbox but my SendMessage function doesn't seem to be correct as I get the error message "SendMessage' has unbalanced the stack. This is likely because the managed PInvoke signature does not match the unmanaged target signature. Check that the calling convention and parameters of the PInvoke signature match the target unmanaged signature."
Here is my code:
Module Module1
Private Const WM_SETTEXT As Int32 = &HC
Private Declare Function SetForegroundWindow Lib "user32" (ByVal hwnd As IntPtr) As Long
Private Declare Auto Function FindWindow Lib "user32" (ByVal lpClassName As String, ByVal lpWindowName As String) As IntPtr
Private Declare Auto Function FindWindowEx Lib "user32" (ByVal hwndParent As IntPtr, ByVal hwndChildAfter As IntPtr, ByVal lpszClass As String, ByVal lpszWindow As String) As IntPtr
Private Declare Auto Function SendMessage Lib "user32" (ByVal hwnd As IntPtr, ByVal wMsg As Long, ByVal wParam As Long, ByVal lParam As String) As IntPtr
Sub Main()
Dim AppHwnd As IntPtr = FindWindow(vbNullString, "Test Application"
Dim WinHwnd1 As IntPtr = FindWindowEx(AppHwnd, 0&, "SWT_Window0", vbNullString)
Dim WinHwnd2 As IntPtr = FindWindowEx(WinHwnd1, 0&, "SWT_Window0", vbNullString)
Dim WinHwnd3 As IntPtr = FindWindowEx(WinHwnd2, 0&, "SWT_Window0", vbNullString)
Dim TextBoxHwnd1 As IntPtr = FindWindowEx(WinHwnd3, 0&, "Edit", vbNullString)
Dim TextBoxHwnd2 As IntPtr = FindWindowEx(WinHwnd3, TextBoxHwnd1, "Edit", vbNullString)
MsgBox(TextBoxHwnd2)
SetForegroundWindow(TextBoxHwnd2)
SendMessage(TextBoxHwnd2, WM_SETTEXT, 0&, "text")
End Sub
End Module
The line "MsgBox(TextBoxHwnd2)" returns the handle number I found using Window Detective so I'm assuming the code is correct up to this point. Also, I tested "SetForegroundWindow(TextBoxHwnd1)" and the cursor is on the first textbox whereas "SetForegroundWindow(TextBoxHwnd1) sets the cursor on the second textbox.

Answer 1

Your declaration looks like it might have been migrated from legacy VB code, where Long was a 32-bit integer and Integer was only 16-bit (dating to legacy VB's origination in 16-bit Windows). The length of the data types changed in .NET which was created after the shift to universal 32-bit Windows, so Integer is 32-bit and Long is 64-bit.
Moreover, the Windows API itself needed to be updated for 64-bit Windows. SendMessage is one of the functions that has some parameters that are longer in 64-bit because they are expected to hold pointers.
I checked on the native header file declaration of SendMessage and found that:
The parameter Msg is declared as UINT which would correspond to Integer in VB. This needs to be changed regardless of whether you have a 32-bit or 64-bit build.
The parameter wParam is declared as UINT_PTR so it should in fact be Long for a 64-bit build, but Integer for a 32-bit build; if it's possible to declare it as IntPtr and then pass 0 to it, that would probably be best, because it should automatically adjust the length depending on whether you're building for 32-bit or 64-bit.