Maxlength property does not work always?Why?

There is propert Maxlength related to textbox. Suppose Maxlength is 5. It means we cannot enter more than 5 characters in the text box. Suppose we enter abcdefgh (8 characters) through a button click in text box (e.g. in calculator application). Now maxlength property does not work. How can we code so that Maxlength of text box is applicable in every case whether we directly enter characters in text box or through any other method.