Removing hidden characters from within strings

Question

My problem:

I have a .NET application that sends out newsletters via email. When the newsletters are viewed in outlook, outlook displays a question mark in place

Answer 1

new string(input.Where(c => !char.IsControl(c)).ToArray());

IsControl misses some control characters like left-to-right mark (LRM) (the char which commonly hides in a string while doing copy paste). If you are sure that your string has only digits and numbers then you can use IsLetterOrDigit

new string(input.Where(c => char.IsLetterOrDigit(c)).ToArray())

If your string has special characters, then

new string(input.Where(c => c < 128).ToArray())