private void button1_MouseDown(object sender, MouseEventArgs e)
{
    foreach (Control t in this.Controls)
    {
        if (t is TextBox)
        {
            if (t.Focused)
            {
                MessageBox.Show(t.Name);
            }
        }
    }
}

private Control _focusedControl;

private void TextBox_GotFocus(object sender, EventArgs e)
{
    _focusedControl = (Control)sender;
}

private void button1_Click(object sender, EventArgs e)
{
    if (_focusedControl != null)
    {
        //Change the color of the previously-focused textbox
        _focusedControl.BackColor = Color.Red;
    }
}

private void BtnKeyboard_Click(object sender, EventArgs e)
    {
        if (MasterKeyboard.Visible)
        {
            btnKeyboard.ButtonImage = Properties.Resources._001_22;
            MasterKeyboard.Visible = false;
            _lastFocusedControl.Focus();
        }
        else
        {
            btnKeyboard.ButtonImage = Properties.Resources._001_24;
            MasterKeyboard.Visible = true;
            MasterKeyboard.BringToFront();
            _lastFocusedControl.Focus();
        }
    }

    private Control _lastFocusedControl;
    private void BtnKeyboard_MouseHover(object sender, EventArgs e)
    {
        if (ActiveControl!=btnKeyboard)
            _lastFocusedControl = ActiveControl;
    }