C# Program to remove duplicate characters from a string

Sir please give me this problem solution without taking contain method

Thank you

Simple:
string str = "Something @to ,Write.;';";
str = string.Join("", str.Split('@', ',', '.', ';', '\''));

But string.contains is O(n)
So this algorithm is O(n^2)

Also space complexity should be improved by using string builder otherwise a new string is created on every new character.