C# 時間相減

            TimeSpan timekey = DateTime.Parse(06:00).Subtract(DateTime.Parse(10:15)); //時間相減
            decimal a = decimal.Parse(timekey.ToString().Substring(0, 2));
            decimal b = decimal.Parse(timekey.ToString().Substring(3, 2));
            decimal C = a + (b / 60);

計算結果為
a = 4
b = 15
c = 4.25