How can you remove duplicate characters in a string?

Question

I have to implements a function that takes a string as an input and finds the non-duplicate character from this string.

So an an example is if I pass string str = \"

Answer 1

// Remove both upper-lower duplicates

public static string RemoveDuplicates(string key)
    {
        string Result = string.Empty;
        foreach (char a in key)
        {
            if (Result.Contains(a.ToString().ToUpper()) || Result.Contains(a.ToString().ToLower()))
                continue;
            Result += a.ToString();
        }
        return Result;
    }