使用它..
Int32.Parse [

use this..
Int32.Parse[^]

(1)存在Int32.TryParse [

(1) There''s the Int32.TryParse[^] method for that (please read the documentation).
(2) Once TryParse returns true you have the integer and you may test if it is less than zero.

尝试一下，

Try this,

int checkNumber;
if (int.TryParse(TextBox2.Text, out checkNumber) == false)
{
    //Entered number is not interger;
}
else
{
    if (Convert.ToInt16(TextBox2.Text) < 0)
    {
        //Entered no. is negative
    }
    else
    {
        //Entered no. is positive
    }
}