Convert TimeSpan from format "hh:mm:ss" to "hh:mm"

You need to convert your data to TimeSpan and then use format:"hh\:mm"

string test ="08:00:00";
TimeSpan ts = TimeSpan.Parse(test);
Console.Write(ts.ToString(@"hh\:mm"));

In your case:

var test = dataRow.Field<TimeSpan>("fstart").ToString(@"hh\:mm"));

Remember to escape the colon :

You may see: Custom TimeSpan Format Strings

There is no need to convert from hh.mm.ss to hh.mm. TimeSpan is stored as a number of ticks (1 tick == 100 nanoseconds) and has no inherent format. What you have to do, is to convert the TimeSpan into a human readable string! This involves formatting. If you do not specify a format explicitly, a default format will be used. In this case hh.mm.ss.

string formatted = timespan.ToString(@"hh\.mm");

Note: This overload of ToString exists since .NET 4.0. It does not support date and time placeholder separator symbols! Therefore you must include them as (escaped) string literals.

The usual way of formatting strings seems not to work for some odd reason (tested with .NET 3.5). (It does not make any difference whether you escape the separator symbol or not):

var timespan = TimeSpan.FromSeconds(1234);
string formatted = String.Format("{0:hh.mm}", timespan); // ==> 00:20:34

However, you can construct the string like this

string formatted =
    String.Format("{0:00}.{1:00}", Math.Floor(timespan.TotalHours), timespan.Minutes);

or starting with VS2015 / C# 6.0, using string interpolation:

string formatted = $"{Math.Floor(timespan.TotalHours):00}.{timespan.Minutes:00}";

You can use TimeSpan methods:

ToString("hh':'mm")
// or
ToString(@"hh\:mm")

Also check all available formats here http://msdn.microsoft.com/en-us/library/8kb3ddd4.aspx