Question

我有一个要查询的SQL表，以便它返回由另一个属性（某些ID）的键映射的特定列的所有值。例如：

class school {
    int SchoolType;
    Guid CountryID;
    string Name;
}

我想得到一个字典，其中的键是CountryID，值是所有学校名称的列表，以便该学校属于某种类型。

List<Guid, List<string>> mapCountryToSchools

Answer 1

您可以通过使用GroupBy语句和ToDictionary调用来实现此目的，这是一些示例代码。

// Simulating Database data
var America = Guid.NewGuid();
var Mexico = Guid.NewGuid();
var schools = new List<School>()
{
    new School(){ SchoolType = 1, Name = "School A", CountryID = America},
    new School(){ SchoolType = 1, Name = "School B", CountryID = America},
    new School(){ SchoolType = 1, Name = "School C", CountryID = America},
    new School(){ SchoolType = 1, Name = "School E", CountryID = America},
    new School(){ SchoolType = 1, Name = "School F", CountryID = Mexico},
    new School(){ SchoolType = 1, Name = "School G", CountryID = Mexico},
    new School(){ SchoolType = 1, Name = "School H", CountryID = Mexico},
    new School(){ SchoolType = 1, Name = "School I", CountryID = Mexico},
};
// Groups schools by their CountryID and then creates a dictionary
var mapCountryToSchools = schools.GroupBy(p => p.CountryID)
                                    .ToDictionary(p => p.Key, p => p.ToList());

用列表查询DbSet到字典

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