orderby在C# LINQ内连接中的应用

Question

我正在从C#试用此msdn页面的LINQ。

我无法在内部连接上申请订单，即使它适用于组连接。

运行查询的数据是：

    class Product
    {
        public string Name { get; set; }
        public int CategoryID { get; set; }
    }

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

    // Specify the first data source.
    static 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.
    static List<Product> products = new List<Product>()
    {
        new Product{Name="Cola",  CategoryID=001},
        new Product{Name="Tea",  CategoryID=001},
        new Product{Name="Mustard", CategoryID=002},
        new Product{Name="Pickles", CategoryID=002},
        new Product{Name="Carrots", CategoryID=003},
        new Product{Name="Bok Choy", CategoryID=003},
        new Product{Name="Peaches", CategoryID=005},
        new Product{Name="Melons", CategoryID=005},
    };

所需的输出是(按括号中的类别列出顺序，然后按产品排序)：

Cola(Beverages)
Tea(Beverages)
Mustard(Condiments)
Pickles(Condiments)
Melons(Fruit)
Peaches(Fruit)
Bok Choy(Vegetables)
Carrots(Vegetables)

我能够使用group-join与orderby和第二个from-select生成这个输出，以变形组层次结构并生成简单的列表，如下所示：

var listGroupJoinOrderBy =
            from category in categories
            join product in products on category.ID equals product.CategoryID into prodGroup
            from prod in prodGroup
            orderby category.Name, prod.Name  //first order by category name then product name
            select
            new
            {
                Category = category.Name,
                Product = prod.Name
            };

但是，如果将orderby应用于内部联接(即没有into子句的group)，则无法产生相同的输出。我尝试了以下变体：

变体#1

var innerJoinOrderBy =
            from category in categories
            orderby category.Name    //orderby category name
            join product in products on category.ID equals product.CategoryID                
            orderby product.Name     //orderby product name
            select
            new
            {
                Category = category.Name,
                Product = product.Name
            };

变体2

var innerJoinOrderBy =
            from category in categories

            join product in products on category.ID equals product.CategoryID                
            orderby category.Name, product.Name     //orderby category first and then by product name
            select
            new
            {
                Category = category.Name,
                Product = product.Name
            };    

但是，这两个变体提供相同的输出，就好像没有使用orderby一样，并产生以下输出：

Cola (Beverages)
Tea (Beverages)
Mustard (Condiments)
Pickles (Condiments)
Carrots (Vegetables)
Bok Choy (Vegetables)
Peaches (Fruit)
Melons (Fruit)

如何使用inner-join 和 orderby**?**生成所需的输出？

无论如何，要打印查询结果，可以使用以下foreach (只需更改查询变量名)：

foreach (var product in simpleInnerJoin)
{
    Console.WriteLine(product.Product + " (" + product.Category + ")");
}

Answer 1

你的第二个选择应该很好。

var innerJoinOrderBy =
            from c in categories
            join p in products on c.ID equals p.CategoryID
            orderby c.Name, p.Name
            select new {
                Category = c.Name,
                Product = p.Name
            };

输出：

[
    { Product="Cola", Category="Beverages" },
    { Product="Tea", Category="Beverages" },
    { Product="Mustard", Category="Condiments" },
    { Product="Pickles", Category="Condiments" },
    { Product="Melons", Category="Fruit" },
    { Product="Peaches", Category="Fruit" },
    { Product="Bok Choy", Category="Vegetables" },
    { Product="Carrots", Category="Vegetables" }
]

从您的输出中，我看到项目按原来的顺序排列。确保您在查询中应用了orderby运算符。

Answer 2

将订单从主Linq中取出并使用

foreach (var product in simpleInnerJoin.Orderby(i=> i.Category.Name).ThenBy(i=>i.Product.Name))
{
    Console.WriteLine(product.Product + " (" + product.Category + ")");
}

我认为这应该给你你所需要的输出。