c# - 如何在 LINQ 查询中从字典获取值到新对象

Question

我有一个数据集和字典:

• 收集处理学生数据
• 词典保留学生类(class)。

我想从字典中获取类(class)名称并将其作为类(class)名称放入。

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 does not recognize the method 'System.String get_Item(System.Guid)' method, and this method cannot be translated into a store expression."

Answer 1

你可以试试下面的方法:

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]
                         });
}

What's the problem?

问题是你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，因为翻译不当。所以你有这个选择。您可以从 studentData 声明您想要的数据序列，然后将任何您想要的调用转换为我们创建的新匿名类型。