C# linq left out join 不能使用 DefaultIfEmpty

Question

我是 C# 新手，我有一个关于在 LINQ 中使用左连接的查询。

我有两个数据源 - 一个是类别集合，一个是产品集合。我想将这两个数据源连接在一起，这样我就可以得到这样的结果：

Category Name (Fruit) - Product Name ( peach)- Source ( QLD,NSW).

请注意，我在 products 集合中将 Source 作为 List 集合。

我这里有两个问题：

1. 当我使用 DefaultIfEmpty 时，如果 Source 集合不存在，我无法设置对 null Source 集合的正确引用 - 但是，如果产品名称不存在，我可以引用有意义的字符串说“不存在”。如果左连接找不到产品的来源，任何人都可以帮助展示如何放置一个空字符串？
2. 我找不到输出 Category Name 的方法 - 就像最终结果中的 Fruit ,Vegetables 虽然我可以在 group join 中输出，但知道该怎么做吗？

我在这里附上了我的代码。

class Product
{
    public string Name { get; set; }
    public int CategoryID { get; set; }
    public List<string> Source;
}

class Category
{
    public string Name { get; set; }
    public int ID { get; set; }
}

// Specify the first data source.
List<Category> categories = new List<Category>()
{ 
    new Category(){Name="Beverages", ID=001},
    new Category(){ Name="Condiments", ID=002},
    new Category(){ Name="Vegetables", ID=003},
    new Category() {  Name="Grains", ID=004},
    new Category() {  Name="Fruit", ID=005}            
};

// Specify the second data source.
List<Product> products = new List<Product>()
{
     new Product{Name="Cola",CategoryID=001,  Source = new List<string> {"NSW","VIC","QLD"} },  
     new Product{Name="Mustard", CategoryID=002 ,  Source = new List<string> {"NSW","VIC","SA"} },
     new Product{Name="Pickles", CategoryID=002 ,  Source = new List<string> {"NSW","VIC","NT"} },
     new Product{Name="Carrots", CategoryID=003 ,  Source = new List<string> {"NSW","VIC","TAS"} },
     new Product{Name="Bok Choy", CategoryID=003 ,  Source = new List<string> {"NSW","VIC","ACT"} },
     new Product{Name="Eggplant", CategoryID=003 ,  Source = new List<string> {"QLD","NSW"} },
     new Product{Name="Broccoli", CategoryID=003 ,  Source = new List<string> {"QLD,SA"} },
     new Product{Name="Peaches", CategoryID=005 ,  Source = new List<string> {"NSW","VIC","NZ"} },
     new Product{Name="Melons", CategoryID=005,  Source = new List<string> {"NSW","VIC"} },
};

以下代码不适用于 LINQ 中的 left join：

void LeftOuterJoin()
{
   var leftOuterQuery =
       from category in categories
       join prod in products on category.ID equals prod.CategoryID into prodGroup          
       select prodGroup.DefaultIfEmpty(new Product()
       { Name = "Nothing!", CategoryID = category.ID})
       ;

    // Store the count of total items (for demonstration only). 
    int totalItems = 0;

    Console.WriteLine("Left Outer Join:");

    // A nested foreach statement  is required to access group items 
    foreach (var prodGrouping in leftOuterQuery)
    {
        Console.WriteLine("\nGroup count: {0}", prodGrouping.Count());

        // Intellisense does not show category name  if I use prodGrouping.Name 
         //and want to get categorhy name since left join produce hierarchy data 
         //why category name is not showing up - I am itinerate from 1st loop ??
        foreach (var item in prodGrouping)
        {
            totalItems++;
            Console.WriteLine("  {0,-10}{1}", item.Name, item.CategoryID);

            foreach (var s in item.Source)
            {
                Console.WriteLine("Souce state: {0}", s.ToString());
            }
        }
    }

    Console.WriteLine(System.Environment.NewLine);
}

嗨，Marcin，我更改了原来的代码，但它给了我重复的记录，你知道为什么吗？

 void LeftOuterJoin()
    {
        // Create the query. 
        var leftOuterQuery =
           from category in categories
           join prod in products on category.ID equals prod.CategoryID into prodGroup
           from prodG in prodGroup.DefaultIfEmpty()       
            select new
            {
                Category = category.Name,
                //Products = products.OrderBy(x => x.Name)
                Products = from prod2 in prodGroup
                           orderby prod2.Name
                           select prod2
            };


        Console.WriteLine("Left Outer Join:");

        // A nested foreach statement  is required to access group items 
        foreach (var item in leftOuterQuery)
        {                 

             Console.WriteLine("  {0,-10}", item.Category);
             foreach ( var p in item.Products)
             {
                 Console.WriteLine("ProductName: {0}", p.Name);
                 foreach (var s in p.Source)
                 {
                     Console.WriteLine("SourceName: {0}", s.ToString());

                 }
             }                   


        }

    }

我想得到这样的结果：

  Group Beverages:1
  Cola      1
  Souce state: NSW
  Souce state: VIC
  Souce state: QLD

 Group : Condiments
  Mustard   2
   Souce state: NSW
   Souce state: VIC
   Souce state: SA
 Pickles   2
  Souce state: NSW
  Souce state: VIC
  Souce state: NT

但是我得到了这样的结果：

Beverages
 ProductName: Cola
 SourceName: NSW
 SourceName: VIC
 SourceName: QLD

Condiments
 ProductName: Mustard
 SourceName: NSW
 SourceName: VIC
 SourceName: SA    
 ProductName: Pickle
 SourceName: NSW
 SourceName: VIC
 SourceName: NT

Condiments
 ProductName: Mustard
 SourceName: NSW
 SourceName: VIC
 SourceName: SA
 ProductName: Pickle
 SourceName: NSW
 SourceName: VIC
 SourceName: NT

Answer 1

我仍然不确定您要实现什么，但要使用 LINQ 获得 left outer join，您需要使用 join ... into 和 from ....DefaultIfEmpty()。

您正在使用select ....DefaultIfEmpty()，这并不是您所需要的。

var leftOuterQuery =
    from category in categories
    join prod in products on category.ID equals prod.CategoryID into prodGroup
    from products in prodGroup.DefaultIfEmpty()       
    select new
    {
        Category = category.Name,
        Products = products.OrderBy(x => x.Name)
    }

Answer 2

这里发布了一个使用 linq 的左外连接的简单而整洁的扩展：http://www.codeproject.com/Tips/724333/Left-Outer-Join-extension-for-Entity-Framework 我想这可能会对你有所帮助。