隐式转换类型

我在WPF中做这件事，并且我使用实体框架。

这是我的CRUD类文件中的查询代码：

 public class QuestionHint
    {
        public int? QuestionNo { get; set; } //change the type accordingly 
        public int? ActivityID { get; set; } //change the type accordingly 
        public int? TaskID { get; set; } //change the type accordingly 
        public string Answer { get; set; }   //change the type accordingly 
        public string QuestionContent { get; set; }   //change the type accordingly 
        public string joined { get; set; }   //change the type accordingly 
        public string joinOption { get; set; }   //change the type accordingly 
    }
        public IList<QuestionHint> GetListKeys(int listTask, int listActivity)
    {
        IList<QuestionHint> lstRecords = context.questionhints.GroupBy(x => new { x.QuestionNo, x.ActivityID, x.TaskID }).ToList().Select(g => new QuestionHint()
        {
            QuestionNo = g.Key.QuestionNo,
            ActivityID = g.Key.ActivityID,
            TaskID = g.Key.TaskID,
            joined = String.Join(" ",
                g.OrderBy(q => q.questionhintID)
                 .Select(i => i.QuestionContent + "[" + i.Answer + "]")),
            joinOption = String.Join(" ",
                   g.OrderBy(q => q.questionhintID)
                    .Select(a => "[" + a.Option1 + "," + a.Option2 + "]"))

        }).Where(x => x.TaskID == listTask && x.ActivityID == listActivity)
            //.Take(50)
    .ToList();
        return lstRecords;
    }

我在代码后面称之为：

 private DAO.DAOQuestionHint qh = new DAO.DAOQuestionHint();
    public MainWindow2()
    {
        InitializeComponent();
        PopulateQuestion(1, 5);
    }
    private void PopulateQuestion(int activityID, int taskID)
    {

        IList<QuestionHint> lstQuestionHints = qh.GetListKeys(taskID, activityID); // ERROR
        //codes here...
        }

我在xaml.cs:的代码后面得到了这个错误

无法隐式转换类型'System.Collections.Generic.IList'到"System.Collections.Generic.IList"显式转换存在（是否缺少强制转换？）

iStellar是该项目的名称。DAOQuestionHint是CRUD类文件的名称。

CRUD类文件中没有错误，我使用相同的查询来检索其他项目中的记录，它运行良好，不知道为什么它在这里不工作。