Please refer below code.
C#
string starttime = "10:50:00";
string endTime = "11:00:00";
TimeSpan startTimeSpan = new TimeSpan(Convert.ToInt32(starttime.Split(':')[0]), Convert.ToInt32(starttime.Split(':')[1]), Convert.ToInt32(starttime.Split(':')[2]));
TimeSpan endTimeSpan = new TimeSpan(Convert.ToInt32(endTime.Split(':')[0]), Convert.ToInt32(endTime.Split(':')[1]), Convert.ToInt32(endTime.Split(':')[2]));
TimeSpan diff = endTimeSpan.Subtract(startTimeSpan);
string timeDiff = diff.ToString();
SQL
DECLARE @startTime DATETIME, @endTime DATETIME
SELECT @startTime = '10:50:00',@endTime='11:00:00'
SELECT CONVERT(VARCHAR(5),DATEDIFF(s, @startTime, @endTime)/3600)+':'
+CONVERT(VARCHAR(5),DATEDIFF(s, @startTime, @endTime)%3600/60)+':'
+CONVERT(VARCHAR(5),(DATEDIFF(s, @startTime, @endTime)%60)) AS [hh:mm:ss]
Output
00:10:00