通过KeyPress事件的KeyAscii参数和文本框的SelStart属性结合起来实现限制文本框只可输入数字(包括负数、小数点)。实际上这个方法也可以用于其它控件，如PictureBox控件。所以我写了一个子过程，方便大家调用。-By the KeyPress event KeyAscii parameter and text box together SelStart property can only achieve limited text box enter a number (including negative numbers, decimal point). In fact this method can also be used for other controls, such as the PictureBox control. So I wrote a sub-process, to facilitate calls.