C#全角轉換成半角函數

string QjToBj(string str)
        {
            string QJstr = str;
            char[] c = QJstr.ToCharArray();
            for (int i = 0; i < c.Length; i++)
            {
                byte[] b = System.Text.Encoding.Unicode.GetBytes(c, i, 1);
                if (b.Length == 2)
                {
                    if (b[1] == 255)
                    {
                        b[0] = (byte)(b[0] + 32);
                        b[1] = 0;
                        c[i] = System.Text.Encoding.Unicode.GetChars(b)[0];
                    }
                }
            }
            string strNew = new string(c);
            return strNew;
        }