By default, when the user TABs to an edit control in a dialog box, the entire contents of the edit control are autoselected. This occurs because the edit control responds with the DLGC_HASSETSEL flag in response to the WM_GETDLGCODE message. To prevent it from happening, remove that flag.

LRESULT CALLBACK RemoveHasSetSelSubclassProc
    (HWND hwnd, UINT uiMsg, WPARAM wParam, LPARAM lParam,
     UINT_PTR uIdSubclass, DWORD_PTR dwRefData)
{
    switch (uiMsg) {
    case WM_NCDESTROY:
        RemoveWindowSubclass(hwnd, RemoveHasSetSelSubclassProc,
                             uIdSubclass);
        break;
    case WM_GETDLGCODE:
        return DefSubclassProc(hwnd, uiMsg, wParam, lParam)
                             & ~DLGC_HASSETSEL;
    }
    return DefSubclassProc(hwnd, uiMsg, wParam, lParam);
}

All this subclass procedure does is remove the DLGC_HASSETSEL flag from the return value of the WM_GETDLGCODE message.

INT_PTR CALLBACK DlgProc(HWND hdlg, UINT uiMsg,
                         WPARAM wParam, LPARAM lParam)
{
    switch (uiMsg) {
    case WM_INITDIALOG:
        SetWindowSubclass(GetDlgItem(hdlg, 100),
                          RemoveHasSetSelSubclassProc, 0, 0);
        break;
    case WM_COMMAND:
        switch (GET_WM_COMMAND_ID(wParam, lParam)) {
        case IDCANCEL:
            EndDialog(hdlg, 1);
            break;
        }
    }
    return FALSE;
}

The subclass procedure is installed when the dialog box is initialized.

int WINAPI WinMain(HINSTANCE hinst, HINSTANCE hinstPrev,
                   LPSTR lpCmdLine, int nShowCmd)
{
    DialogBox(hinst, MAKEINTRESOURCE(1), NULL, DlgProc);
    return 0;
}

1 DIALOGEX DISCARDABLE  0, 0, 200,200
STYLE DS_SHELLFONT | WS_POPUP | WS_VISIBLE | WS_CAPTION | WS_SYSMENU
CAPTION "sample"
FONT 8, "MS Shell Dlg"
BEGIN
  CONTROL         "Blah blah",100,"Edit",WS_TABSTOP,7,4,100,10
    DEFPUSHBUTTON "&Bye", IDCANCEL, 7,24,50,14, WS_TABSTOP
END

And here is the dialog box that we display.

There really isn't much to it, but I figured a complete sample program might help somebody out. Plus it lets me show off the SetWindowSubclass function.