interactively validating user input in the .net textbox

Question

how does one interactively prevent the user from inputting invalid characters in input controls of vb .net..... it seems the the keypress event of the .net input controls is read only and therefore cannot be assigned a zero(0) or null("") value unlike in vb6 where one can set the value of the keyascii parameter of the keypress event to 0 to prevent characters typed from being shown is there some other way to do the above in vb .net