Remove duplicate characters in a string in C#

        Func<string, string> RemoveDuplicate = delegate(string s)
{
    BitArray _arr = new BitArray(256);
    StringBuilder _sb = new StringBuilder();
    s = s.ToLower();
    for (int j = 0; j < s.Length; j++)
    {
        if (_arr[(int)s[j]])
        {
            continue;
        }
        else
        {
            _arr[(int)s[j]] = true;
            _sb.Append(s[j]);
        }
    }
    return _sb.ToString();
};