LINQ DefaultIfEmpty(), generating inferred argument error

Question

I am trying to get the below linq query to return -1 if there isn't any current value. I was looking at this article on MSDN, here, and it seemed that DefaultIfEmpty() was what I wanted.

Unfortunately, I am getting a The type arguments cannot be inferred from the usage. Try specifying the type arguments explicitly. error.

I guess I am not sure what that means or what it is telling me to do. Can someone explain, please.

 public static int CheckForDRIID(int personID)
    {
        using (var context = ConnectDataContext.Create())
        {
            var masterIndex =
                (from applicationAssociation in context.tblApplicationAssociations
                 where applicationAssociation.ApplicationID == 1 && applicationAssociation.PersonID == personID
                 select applicationAssociation.PersonApplicationID).DefaultIfEmpty(-1).Single();


            return Convert.ToInt32(masterIndex);
        }
    }

Answer 1

-1 is an int and applicationAssociation.PersionApplicationID isn't, so it doesn't know what to return. You could replace -1 with the same type as applicationAssociation.PersionApplicationID, "-1". Or specify the type like DefaultIfEmpty<string>. And a third options is to do the Convert.ToInt32 in the select.

Answer 2

That code works fine for me:

public static int CheckForDRIID(int personID)
{
    var someAssociaction = new { ApplicationID = 1, PersonID = 1, PersonApplicationID = 1 };
    var associactions = (new[] { someAssociaction }).ToList();
    associactions.Add(new { ApplicationID = 2, PersonID = 2, PersonApplicationID = 2 });

    int masterIndex =
        (from applicationAssociation in associactions
         where applicationAssociation.ApplicationID == 1 && applicationAssociation.PersonID == personID
         select applicationAssociation.PersonApplicationID).DefaultIfEmpty(-1).Single();

    return masterIndex;
}

I just created a sample set of associactions, because I dont have your datacontext. By the way: you can just declare masterIndex an int, because you know the type the expression will return, so you dont need a var.

So the cause for your problem lies somewhere else: the PersonApplicationID field is not an int, therefore the compiler does not know which type it should infer for the generic function DefaultIfEmpty.

Update: Just tried the above example with setting the PersonApplicationID property to:

• new Nullable<int>(1): Throws "Cannot implicitly convert type 'int?' to 'int'"
• 1f: Throws "Cannot implicitly convert type 'float' to 'int'"
• "": Throws "The type arguments for method DefaultIfEmpty<TSource> cannot be inferred from the usage. Try specifying the type arguments explicitly."

So, I assume you are storing strings in your database's PersonApplicationID field. If so, either change your database, or parse the string to an int in the Linq query:

public static int CheckForDRIID(int personID)
{
    using (var context = ConnectDataContext.Create())
    {
        int masterIndex =
            (from applicationAssociation in context.tblApplicationAssociations
             where applicationAssociation.ApplicationID == 1 && applicationAssociation.PersonID == personID
             select int.Parse(applicationAssociation.PersonApplicationID)).DefaultIfEmpty(-1).Single();

        return masterIndex;
    }
}