How to show the characters of a text box in which its PasswordCharacter is set to '*'?

[C# Programming]

Hey my friends ...... I have a password text box(i.e PasswordCharacter='*') there is one check box which allows the user to see its password or just the star ....... I tried to change the value of the text boxes passwordCharacter(i.e txtbox1.PasswordCharacter='') but it says "it is not allowed to make empty" ... so how can I do that??

Thank you