如何在LINQ查询中从字典到新对象获取值

Question

我有一个数据收集和字典：

• 收集处理学生数据
• 词典保留学生课程。

我想从字典中获取课程名称，并将其作为课程名称输入。

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无法识别方法'System.String get_Item（System.Guid）'，并且该方法无法转换为商店表达式。”

Answer 1

您可以尝试以下操作：

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]
                         });
}

有什么问题？

问题是您创建的匿名类型中的最后一个表达式，

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

不能在这个地方，因为它无法正确翻译。 所以你有这个选择。 您可以从studentData声明所需数据的序列，然后将您想要的任何调用转换为我们创建的新匿名类型。

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
                        });