Using nullable types in Linq expressions

Question

var quantSubset =
                 from userAns in userAnalysis.AllUserAnswers
                 join ques in userAnalysis.AllSeenQuestions on userAns.QID equals ques.QID
                 where (ques.QuestionType == "QT")
                 select new { QuestionLevel = ques.LevelID, TimeTaken = userAns.TimeTaken, Points = userAns.Points, UsedWeapon = (userAns.UsedBy2 && userAns.UsedHint), WasCorrect = userAns.WasCorrect.HasValue ? userAns.WasCorrect.Value : null };

In my select expression I want to select a nullable type WasCorrect (last part of the expression) but apparently I cannot do it the way I am currently trying.

How can I get WasCorrect as nullable type

I tried ?WasCorrect but that also doesnt gives error in Visual Studio.

Answer 1

You need to cast the null value to the nullable type explicitly:

WasCorrect = userAns.WasCorrect.HasValue ?
    userAns.WasCorrect.Value : (TheTypeName?)null

Otherwise C# won’t know which type the conditional expression should be.

Apart from that, the code is completely redundant. You can simply write:

WasCorrect = userAns.WasCorrect

Answer 2

You absolutely must be able to write

select new { WasCorrect = userAns.WasCorrect }

if userAns.WasCorrect is Nullable<bool>.

This code executes without a problem:

class Test {
    public bool? NullableBool { get; set;}
}

class MainClass
{
    public static void Main ()
    {
        Test t1 = new Test { NullableBool = true };
        var a1 = new { NB = t1.NullableBool };

        Test t2 = new Test { NullableBool = null };
        var a2 = new { NB = t2.NullableBool };
    }
}