在LINQ表达式使用可空类型 [英] Using nullable types in Linq expressions

问题描述

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
    };

在我的选择表达我要选择一个可空类型WasCorrect（表达式的最后一部分），但显然我不能做到这一点现在我想要的方式。

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.

我怎样才能得到WasCorrect为可空类型

How can I get WasCorrect as nullable type

我试过吗？WasCorrect但也犯规给了错误的Visual工作室。

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

推荐答案

您需要将空值显式转换为可空类型

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

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

否则C＃不知道哪种类型的条件式应

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

除此之外，该代码是完全是多余的。你可以简单地写：

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

WasCorrect = userAns.WasCorrect

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