You can check the password using Regular Expressions.
In your code you can check the password with the Regex.IsMatch() function. You insert the entered value and a pattern. The pattern holds information about how 'strong' your password must be.
Check
this link[
^] for a couple of examples of patterns.
The pattern
@"(?=^.{6,255}$)((?=.*\d)(?=.*[A-Z])(?=.*[a-z])|(?=.*\d)(?=.*[^A-Za-z0-9])(?=.*[a-z])|(?=.*[^A-Za-z0-9])(?=.*[A-Z])(?=.*[a-z])|(?=.*\d)(?=.*[A-Z])(?=.*[^A-Za-z0-9]))^.*"
comes close to what you want.