how to restrict certain characters in textbox in C#?

In KeyPress, reject the event (set e.Handled = true) if the character is in your list of disallowed ones.

That doesn't protect you from pasted text, alt-0nnn entry, drag and drop of text to a text field and any other ways your OS might dream up of changing the text in an edit field. So you also need to implement a TextChanged handler which checks to see if any invalid characters are present, and if so, removes them. Don't assign Text inside TextChanged unless you need to make a modification, though, because it resets the caret position and that's really annoying if it happens every keystroke.

Try this.

e.Handled = (e.KeyChar == (char)Keys.Space) || (e.KeyChar == '\'');

IF you are making a web application then you can do it in javascript like this

function check(e) {
var keynum
var keychar
var numcheck
// For Internet Explorer
if (window.event)
{
keynum = e.keyCode
}
// For Netscape/Firefox/Opera
else if (e.which)
{
keynum = e.which
}
keychar = String.fromCharCode(keynum)
//List of special characters you want to restrict
if (keychar == "'" || keychar == "`")
{

return false;
}
else {
return true;
}
}