C# – SetText of textbox in external app. Win32 API


Using Winspector I've found out the ID of the child textbox I want to change is 114. Why isn't this code changing the text of the TextBox?

    static extern IntPtr GetDlgItem(IntPtr hDlg, int nIDDlgItem);
    public static extern int SendMessage(IntPtr hWnd, int msg, int Param, string s);

    const int WM_SETTEXT = 0x000c;

    private void SetTextt(IntPtr hWnd, string text)
        IntPtr boxHwnd = GetDlgItem(hWnd, 114);
        SendMessage(boxHwnd, WM_SETTEXT, 0, text);

Best Solution

The following is what I've used successfully for that purpose w/ my GetLastError error checking removed/disabled:

[DllImport("user32.dll", SetLastError = false)]
public static extern IntPtr GetDlgItem(IntPtr hDlg, int nIDDlgItem);
[DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = false)]
public static extern IntPtr SendMessage(HandleRef hWnd, uint Msg, IntPtr wParam, string lParam);
public const uint WM_SETTEXT = 0x000C;

private void InteropSetText(IntPtr iptrHWndDialog, int iControlID, string strTextToSet)
    IntPtr iptrHWndControl = GetDlgItem(iptrHWndDialog, iControlID);
    HandleRef hrefHWndTarget = new HandleRef(null, iptrHWndControl);
    SendMessage(hrefHWndTarget, WM_SETTEXT, IntPtr.Zero, strTextToSet);

I've tested this code and it works, so if it fails for you, you need to be sure that you are using the right window handle (the handle of the Dialog box itself) and the right control ID. Also try something simple like editing the Find dialog in Notepad.

I can't comment yet in the post regarding using (char *) but it's not necessary. See the second C# overload in p/Invoke SendMessage. You can pass String or StringBuilder directly into SendMessage.

I additionally note that you say that your control ID is 114. Are you certain WinSpector gave you that value in base 10? Because you are feeding it to GetDlgItem as a base 10 number. I use Spy++ for this and it returns control IDs in base 16. In that case you would use:

IntPtr boxHwnd = GetDlgItem(hWnd, 0x0114);
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