在 C# 中的 LINQ 中按多列分组

Question

我有一个类如下：

public class ActualClass
{
    public string BookName { get; set; }
    public string IssuerName { get; set; }
    public DateTime DateOfIssue { get; set; }
    public bool Status { get; set; }
}

表中有以下数据：

我想将它们按 IssuerName 和 DateOfIssue 分组，用于以下 viewModel 类：

public class ViewModel
{
   public string IssuerName { get; set; }
   public DateTime DateOfIssue { get; set; }
   public List<string> Books { get; set; }
}

并且数据会显示如下：（分组成功后，截图数据会替换为之前的表格数据）

特别注意： 按照我的预期，我的 ViewModel 有什么问题吗？

在遵循一些 stackoverflow 答案后，我尝试了很多，但没有一个对我有用。任何帮助将不胜感激。

我试过的代码：

var viewmodel = from b in db.BookIssues
                group b by new
                {
                    b.IssuerName,
                    b.DateOfIssue
                }
                into g
                select new ViewModel()
                {
                    Name = g.key.IssuerName,
                    DateOfIssue = g.Key.DateOfIssue,
                    Books = g.ToList() //Actually this line of code is not working
                };

Answer 1

如果您需要从组结果中选择书籍列表，则需要Books = v.Select(c=>c.BookName).ToList() 还请注意，如果您有时间在发行日期时间，您可能需要使用EntityFunctions.TruncateTime 函数仅按日期部分分组。如果您只存储日期，则可以忽略此功能。

var viewmodel  = db.BookIssues.GroupBy(x=>new {IssuerName =x.IssuerName, DateOfIssue=EntityFunctions.TruncateTime(x.DateOfIssue) })
.Select(v=>new ViewModel(){IssuerName =v.Key.IssuerName, DateOfIssue = v.Key.DateOfIssue, Books  = v.Select(c=>c.BookName).ToList() })
.ToList();

Answer 2

Books = g.ToList() //其实这行不行

可能是因为 Books 属性的类型是 List<string>，而不是 List<ActualClass>。

你能不能试试这个查询，我添加了b.Select(bn => bn.BookName).ToList()来只提取书名：

var books = new List<ActualClass>
{
   new ActualClass { BookName = "A", DateOfIssue = new DateTime(2015, 10, 10, 10, 10, 0), IssuerName = "1" },
   new ActualClass { BookName = "B", DateOfIssue = new DateTime(2015, 10, 10, 10, 10, 0), IssuerName = "1" },
   new ActualClass { BookName = "C", DateOfIssue = new DateTime(2015, 10, 10, 10, 10, 0), IssuerName = "1" },
   new ActualClass { BookName = "D", DateOfIssue = new DateTime(2015, 10, 10, 10, 10, 0), IssuerName = "2" },
   new ActualClass { BookName = "E", DateOfIssue = new DateTime(2015, 10, 10, 12, 10, 0), IssuerName = "2" },
   new ActualClass { BookName = "F", DateOfIssue = new DateTime(2015, 10, 10, 12, 10, 0), IssuerName = "2" }
};

var result = books.GroupBy(x => new { x.IssuerName, x.DateOfIssue })
                .Select(b => new ViewModel
                {
                    Books = b.Select(bn => bn.BookName).ToList(),
                    // Accessing to DateOfIssue and IssuerName from Key.
                    DateOfIssue = b.Key.DateOfIssue,
                    IssuerName = b.Key.IssuerName
                });

我的分组依据：x.IssuerName, x.DateOfIssue。我通过以下方式在GroupBy() 中传递匿名类型：x => new { x.IssuerName, x.DateOfIssue }。

现在它们处于密钥状态，您可以在 SELECT 语句中从 KEY 访问 IssuerName 和 DateOfIssue，如下所示：b.Key.IssuerName 和 b.Key.DateOfIssue。