Sending mouse click to VB6 windowless controls when is in a background desktop

Question

I have a form VB6 with some windowless controls (without hWnd) in it.

I need simulate the mouse click on than controls when the window is in a background desktop. This form references a C# DLL with interop.

I tried to use SendInput but it not works in background desktops.

I tried to use PostMessage/SendMessage with WM_LBUTTONDOWN/WM_BUTTONUP but with windowless controls the click not raise.

What can I use to do this?

Answer 1

The fact that your controls are windowless, doesn't prevent you from doing that. You just have to ensure that you are sending the messages to the container window (the one that has its hWnd handle).

I tested the following code on VB6 app with windowless button. For me it simulates the click even for minimized window. The trick is to first send WM_ACTIVATE message, then the WM_LBUTTONDOWN/UP pair:

using System.Runtime.InteropServices;

// Win32 API definitions
[DllImport("user32.dll", EntryPoint = "FindWindow")]
static extern IntPtr FindWindowByCaption(IntPtr ZeroOnly, string lpWindowName);

[DllImport("user32.dll")]
static extern int PostMessage(IntPtr hWnd,  uint Msg, int wParam, int lParam);


const uint WM_ACTIVATE = 0x0006;
const uint WM_LBUTTONDOWN = 0x201;
const uint WM_LBUTTONUP = 0x202;

const int MK_LBUTTON = 1;

const int WA_ACTIVE = 1;

int MAKELPARAM(int p, int p_2)
{
    return ((p_2 << 16) | (p & 0xFFFF));
}

// The code
IntPtr hWnd = FindWindowByCaption(IntPtr.Zero, "Form1");

PostMessage(hWnd, WM_ACTIVATE, WA_ACTIVE, 0);

int lParam = MAKELPARAM(62, 68); // Button coordinates relative to window "client area"
PostMessage(hWnd, WM_LBUTTONDOWN, MK_LBUTTON, lParam);
PostMessage(hWnd, WM_LBUTTONUP, 0, lParam);

See this repo for working code example -

1. Run WLTest.exe (Set Win XP compatibility mode in the .exe "Properties") and minimize it's window.
2. Then run the C# code and ensure the counter indeed increments in the VB6 window.