我希望左键单击增加，右键单击减少我尝试过代码中显示的内容

private：System :: Void button4_MouseDown（System :: Object ^ sender，System :: Windows :: Forms :: MouseEventArgs ^ e）{

switch (MouseButtons)
{
case MouseButtons.Left:

    c = Convert::ToInt32(bc->Text);
    c++;
    bc->Text = c.ToString();

    break;

case MouseButtons.right:

    c = Convert::ToInt32(bc->Text);
    c--;
    bc->Text = c.ToString();

    break;

}

}

但是我收到了这个错误
错误（活动）'this'不能用于常量表达式

visual-c++