如何在LINQ查詢中從字典到新對象獲取值

Question

我有一個數據收集和字典：

• 收集處理學生數據
• 詞典保留學生課程。

我想從字典中獲取課程名稱，並將其作為課程名稱輸入。

private viod GenerateStudentDetails(Student studentData)
{
    var courses = m_courses.GetCoursesDictionary(); // returns Dictionary<Guid,string>()

    var studentDetails= from data in studentData
            select new
            {
                FirstName = data.FirstName, 
                LastName = data.LastName,
                Email = data.Email,
                Mobile = data.Profile.Mobile,
                City = data.Profile.City,
                PostCode = data.Profile.PostCode,
                CourseName = courses[data.Profile.CourseID ?? Guid.Empty]
            };
}

“ LINQ to Entities無法識別方法'System.String get_Item（System.Guid）'，並且該方法無法轉換為商店表達式。”

Answer 1

您可以嘗試以下操作：

private viod GenerateStudentDetails(Student studentData)
{
    var courses = m_courses.GetCoursesDictionary(); // returns Dictionary<Guid,string>()

    var studentDetails= (from data in studentData
                         select new
                         {
                             FirstName = data.FirstName, 
                             LastName = data.LastName,
                             Email = data.Email,
                             Mobile = data.Profile.Mobile,
                             City = data.Profile.City,
                             PostCode = data.Profile.PostCode,
                             CourseID = data.Profile.CourseID
                         }).AsEnumerable()
                           .Select(item=>new
                         {  
                             FirstName = item.FirstName, 
                             LastName = item.LastName,
                             Email = item.Email,
                             Mobile = item.Profile.Mobile,
                             City = item.Profile.City,
                             PostCode = item.Profile.PostCode,
                             CourseName = courses[item.Profile.CourseID ?? Guid.Empty]
                         });
}

有什么問題？

問題是您創建的匿名類型中的最后一個表達式，

 CourseName = courses[data.Profile.CourseID ?? Guid.Empty]

不能在這個地方，因為它無法正確翻譯。 所以你有這個選擇。 您可以從studentData聲明所需數據的序列，然后將您想要的任何調用轉換為我們創建的新匿名類型。

Answer 2

只是想一想

var eCourses = ((IEnumerable<KeyValuePair<Guid, string>>) courses);

var studentDetails = (from data in studentData
                        select new
                        {
                            data.FirstName,
                            data.LastName,
                            data.Email,
                            data.Profile.Mobile,
                            data.Profile.City,
                            data.Profile.PostCode,
                            CourseName = eCourses.FirstOrDefault(s => s.Key == data.Profile.CourseID).Value
                        });