How do you return a default value if a LINQ to entities query returns no values

Question

In a LINQ to entities expression like this:

var vote = (from vote in db.Vote where
    vote.Voter.Id == user.Id
    select v).FirstOrDefault();

How do you add a DefaultIfEmpty value so that when there's no vote I'd get a default value?

Answer 1

Another approach, if Vote is a reference type and thus uses null as its default value, would be to use the null coalescing operator:

var vote = (db.Vote
   .Where(v => v.Voter.Id == user.Id)
   .FirstOrDefault()) ?? defaultVote;

Answer 2

Add your own extension method. For instance:

public static class Extension
{
    public static T FirstOrDefault(this IEnumerable<T> sequence, T defaultValue)
    { 
        return sequence.Any() ? sequence.First() : defaultValue;
    }
}

With that class in scope, you can say:

var vote = (from vote in db.Vote where
    vote.Voter.Id == user.Id
    select v).FirstOrDefault(yourDefaultValue);

Of course, your method can also have an overload that returns default(T), if that was what you where looking for. There is already defined a DefaultIfEmpty extension method in the built-in Extension class, so I named the method in the example "FirstOrDefault", which seems like a better fit.