Question

What would be the best way to make sure a user does not enter (or copies) Japanese characters into a C# WinForms TextBox?

The problem is that our software will be used by German, English and Japanese users. All kinds of characters and symbols are handled well, but Japanese characters seem to cause some trouble in the underlying persistence framework.

Answer 1

Could you use a whitelist? For example:

bool IsValidInput(String input)
{
    return input != null && input.All(c => IsValidLetter(c));
}

bool IsValidLetter(char c)
{
     return Char.IsNumber(c) || (c >= 'A' && c <= 'Z') || (c >= 'a' && c <= 'z');
} 

Although i must agree that the best approach would be to fix your problems with japanese characters if you want japanese customers.