group by date in linq -

I have a table in which the datetime null field in the SQL server is to group results based on the year of that region I am trying. But, when I wrote a Linux query, I did not get intensive for the year for that area. It only shows HasValue and value because I think it changes the faucet type, how do I group in this case?

You must group the group's value on year Nullable & lt; DateTime & gt; such as

  obj.DateTimeField.Value group obj In G