Question

I have a LINQ to SQL query function combinding to tables, but the function gives an error on “return res.ToList();”.

• imo_no is int
• position_cordinates is String
• vessel_id is int
• equipment is String.

My function:

List<T> GetAllInitialize()
{
    PositionDataClassesDataContext context = new PositionDataClassesDataContext();

    var res = from positions in context.it_positions
              join vessels in context.it_vessels on positions.imo_no equals vessels.imo_no
              select new { positions.imo_no, positions.position_cordinates, vessels.vessel_id, vessels.equipment };

    return res.ToList();
}

Answer 1

Although you can use anonymous types or collections of objects of anonymous type locally with static typing, there is no way to return them from a function without losing the information about their static type.

If you cannot or for some reason prefer not to create a named class to hold the return data, you need to either return List<dynamic>, or List<object>. Both these approaches have their drawbacks: dynamic will be slower than a comparable statically-typed object, while System.Object would not let you do much with the data that you get back.

The best solution would be creating a named return type:

public class PositionData {
     public int ImoNo {get;set;}
     public string PositionCordinates {get;set;}
     public int VesselId {get;set;}
     public string Equipment  {get;set;}
}

Now your query will look like this:

select new PositionData {
    ImoNo = positions.imo_no
,   PositionCordinates = positions.position_cordinates
,   VesselId = vessels.vessel_id
,   Equipment = vessels.equipment
};

The return type of your method would change to List<PositionData>.