Showing Difference between two datetime values in hours

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var hours = (datevalue1 - datevalue2).TotalHours;

I think you're confused because you haven't declared a TimeSpan you've declared a TimeSpan? which is a nullable TimeSpan. Either remove the question mark if you don't need it to be nullable or use variable.Value.TotalHours.

In the sample, we are creating two datetime objects, one with current time and another one with 75 seconds added to the current time. Then we will call the method .Subtract() on the second DateTime object. This will return a TimeSpan object. Once we get the TimeSpan object, we can use the properties of TimeSpan to get the actual Hours, Minutes and Seconds.

DateTime startTime = DateTime.Now;

 DateTime endTime = DateTime.Now.AddSeconds( 75 );

 TimeSpan span = endTime.Subtract ( startTime );
 Console.WriteLine( "Time Difference (seconds): " + span.Seconds );
 Console.WriteLine( "Time Difference (minutes): " + span.Minutes );
 Console.WriteLine( "Time Difference (hours): " + span.Hours );
 Console.WriteLine( "Time Difference (days): " + span.Days );

Result:

Time Difference (seconds): 15
Time Difference (minutes): 1
Time Difference (hours): 0
Time Difference (days): 0

Is there a reason you're using Nullable?

If you want to use Nullable then you can write variable.Value.TotalHours.

Or you can just write: (datevalue1 - datevalue2).TotalHours.