无法将类型 'System.Collections.Generic.List<System.Linq.IGrouping<int, XXX>>' 隐式转换为 'System.Collections.Generic.List<XXX>>答案

【问题标题】：Cannot implicitly convert type 'System.Collections.Generic.List<System.Linq.IGrouping<int, XXX>>' to 'System.Collections.Generic.List<XXX>>无法将类型 'System.Collections.Generic.List<System.Linq.IGrouping<int, XXX>>' 隐式转换为 'System.Collections.Generic.List<XXX>>
【发布时间】：2019-12-12 21:03:03
【问题描述】：

我试图在getAllQuestions() 方法中返回List<QuestionsAndChoices>，但我收到了这个错误

Cannot implicitly convert type 'System.Collections.Generic.List<System.Linq.IGrouping<int, HowWellDoYouKnowMeAPI.Models.models.QuestionsAndChoices>>' to 'System.Collections.Generic.List<HowWellDoYouKnowMeAPI.Models.models.QuestionsAndChoices>

获取所有问题

public List<QuestionsAndChoices> getAllQuestions()
{
    List<QuestionsAndChoices> questionsAndChoices1 = context.quizQuestionAndChoices.
        Join(context.quizQuestions, QandC => QandC.QuizQuestions.QuizQuestionsId, qustions => qustions.QuizQuestionsId, (QandC, qustions) =>
            new
            {
                QandC,
                qustions
            }).
            Join(context.quizChoices, QandC1 => QandC1.QandC.QuizChoicesId, choices => choices.QuizChoicesId, (QandC1, choices) =>
            new QuestionsAndChoices
            {
                QuestionId = QandC1.qustions.QuizQuestionsId,
                Question = QandC1.qustions.Questions,
                Choices = choices,
             }).GroupBy(e => e.QuestionId).ToList();

    return questionsAndChoices1;
}

QuestionsAndChoices类

public class QuestionsAndChoices
    {
        public int QuestionId { get; set; }
        public string Question { get; set; }
         public List<QuizChoices> Choices { get; set; }
    }

QuizQuestionAndChoices类

public class QuizQuestionAndChoices
{
    public int QuizQuestionAndChoicesID { get; set; }
    public int QuizChoicesId { get; set; }
    public int QuizQuestionsId { get; set; }
    public QuizChoices quizChoices { get; set; }
    public QuizQuestions QuizQuestions { get; set; }
}

quizQuestions类

 public class QuizQuestions
 {
     public int QuizQuestionsId { get; set; }
     public string Questions { get; set; }
     public List<QuizzerQCdetails> quizzerQCdetails { get; set; }
     public List<QuizQuestionAndChoices> QuizQuestionAndChoices { get; set; }
 }

QuizChoices类

public class QuizChoices
{
    public int QuizChoicesId { get; set; }
    public string Choice { get; set; }
    public List<QuizzerQCdetails> quizzerQCdetails { get; set; }
    public List<QuizQuestionAndChoices> quizQuestionAndChoices { get; set; 
}

我希望这样的输出

{
    "questionId":1,
    "question":"q1",
    "choices":{
        {
        "quizChoicesId":1,
        "choice":"1c1",
        "quizzerQCdetails":null,
        "quizQuestionAndChoices":null
        },
        {
        "quizChoicesId":2,
        "choice":"1c2",
        "quizzerQCdetails":null,
        "quizQuestionAndChoices":null
        }
    }
}

但是我得到了这个输出 @IbraHimM.Nada我已经通过了那个代码，但是我得到了这样的输出

{
"questionId":1,
"question":"q1",
"choices":{
        {
        "quizChoicesId":1,
        "choice":"1c1",
        "quizzerQCdetails":null,
        "quizQuestionAndChoices":null
        }
    }
}
,
{
"questionId":1,
"question":"q1",
"choices":{
        {
        "quizChoicesId":2,
        "choice":"1c2",
        "quizzerQCdetails":null,
        "quizQuestionAndChoices":null
        }
    }
}

【问题讨论】：

请检查这个问题我认为它会有所帮助stackoverflow.com/questions/8521025/…
@IbraHimM.Nada 谢谢，我浏览了那篇文章，但得到了像上面这样的输出（更新了问题），这是不正确的
您的预期输出错误。不应该是“选择”：“2c1”，应该是“选择”：“1c2”，
@jdweng 抱歉，我更新了我的代码。谢谢;)
你为什么有.GroupBy(e => e.QuestionId)？是不是因为您希望许多问题具有相同的 ID，然后您想将所有共享 ID 的问题分组到“组”中，然后返回这些组的列表？因为这就是你正在做的事情。然后你有一个List<> groupings of QuestionsAndChoices。那不是简单的QuestionsAndChoices 列表。也许您只是想删除.GroupBy(e => e.QuestionId) 的东西？你的意思是.OrderBy 吗？

标签： c# asp.net-mvc entity-framework linq

【解决方案1】：

尝试以下：

List<QuestionsAndChoices> questionsAndChoices1 = (
    from qandc in context.quizQuestionAndChoices
    join quest in context.quizQuestions on qandc.QuizQuestionsId equals quest.QuizQuestionsId
    join answers in context.quizChoices on qandc.QuizChoicesId equals answers.QuizChoicesId
    select new { question = quest.Questions, answers = answers })
  .GroupBy(x => x.question)
  .Select(x => new QuestionsAndChoices()
    {
      Question = x.Key,
      Choices = x.Select(y => y.answers).ToList()
    }).ToList();

【讨论】：

感谢@jdweng 非常感谢，我们如何使用 Lambda 表达式编写这个查询。
把它分成两个查询。第一个括号中的所有内容。