c# - 在 C# 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; }
}

表中有如下数据:

对于以下 viewModel 类，我想按 IssuerName 和 DateOfIssue 对它们进行分组:

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被之前的表格数据替换)

特别注意: 我的ViewModel是否按照我的预期有什么问题？

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

Books = g.ToList() //Actually this line of is not working

可能是因为 Books 属性是 List<string> 的类型, 不是 List<ActualClass> .

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

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
                });

我分组依据:x.IssuerName, x.DateOfIssue .我通过在 GroupBy() 中传递匿名类型来做到这一点通过以下方式:x => new { x.IssuerName, x.DateOfIssue } .

现在它们在 key 中，您可以从 SELECT 语句中的 KEY 访问 IssuerName 和 DateOfIssue，如下所示:b.Key.IssuerName和 b.Key.DateOfIssue .