LINQ查询表达式和LAMBDA点标记方法基础

在上一篇文章中，我们介绍了LINQ的一些基本用法，这一篇我们来看下另一种更简洁更优雅的表达式，Lambda表达式，也可以叫做点标记方法。

相信大家在实际工作中都用到过这两种方式，下面我们还是用实例来看下两种表达式的一些基本用法。

首先还是创建一个C#控制台程序，准备测试数据，创建两个类，Product产品类和Category类别类，每个产品分配一个类别。

     public class Category
     {
         public int CategoryID { get; set; }
         public string CategoryName { get; set; }
     }

     public class Product
     {
         public int ProductID { get; set; }
         public string ProductName { get; set; }
         public int CategoryID { get; set; }
         public int Count { get; set; }
         public string Description { get; set; }
     }

接着创建一些Category类型和Product类型的测试数组列表。

             Category[] categories = new Category[]{
                 new Category{CategoryID=,CategoryName="TOYOTA"},
                 new Category{CategoryID=,CategoryName="BMW"},
                 new Category{CategoryID=,CategoryName="BENZ"},
                 new Category{CategoryID=,CategoryName="HONDA"},
                 new Category{CategoryID=,CategoryName="LEXUS"}
             };

             Product[] products = new Product[]{
                 new Product{ProductID=,ProductName="Car1",CategoryID=,Count=,Description="TOYOTA Car"},
                 new Product{ProductID=,ProductName="Car2",CategoryID=,Count=,Description="TOYOTA SUV"},
                 new Product{ProductID=,ProductName="Car3",CategoryID=,Count=,Description="BMW Car"},
                 new Product{ProductID=,ProductName="Car4",CategoryID=,Count=,Description="BMW SUV"},
                 new Product{ProductID=,ProductName="Car5",CategoryID=,Count=,Description="BENZ Car"},
                 new Product{ProductID=,ProductName="Car6",CategoryID=,Count=,Description="HONDA Car"},
                 new Product{ProductID=,ProductName="Car7",CategoryID=,Count=,Description="LEXUS Car"},
                 new Product{ProductID=,ProductName="Car8",CategoryID=,Count=,Description="LEXUS SUV"}
             };

1.查询产品列表中所有记录的ProductName,Count,Description列。(select)

             // LINQ
             var productQuery1 = from p in products
                                select new
                                {
                                    p.ProductName,
                                    p.Count,
                                    p.Description
                                };

             Console.WriteLine("LINQ---");
             foreach(var item in productQuery1)
             {
                 Console.WriteLine("Product Name: {0} , Count: {1} , Description: {2}", item.ProductName, item.Count, item.Description);
             }

             // LAMBDA
             var productQuery2 = products.Select(p => new
             {
                 ProductName = p.ProductName,
                 Count = p.Count,
                 Description = p.Description
             });

             Console.WriteLine("LAMBDA---");
             foreach (var item in productQuery2)
             {
                 Console.WriteLine("Product Name: {0} , Count: {1} , Description: {2}", item.ProductName, item.Count, item.Description);
             }

2.查询产品列表中Count大于5的记录。(where)

             // LINQ
             var productQuery1 = from p in products
                                 where p.Count >
                                 select p;

             Console.WriteLine("LINQ---");
             foreach (var item in productQuery1)
             {
                 Console.WriteLine(item.ProductName);
             }

             // LAMBDA
             var productQuery2 = products.Where(P => P.Count > );

             Console.WriteLine("LAMBDA---");
             foreach (var item in productQuery2)
             {
                 Console.WriteLine(item.ProductName);
             }

3.查找产品列表中类别为TOYOTA,BENZ,LEXUS的记录。(where / contains)

             // LINQ
             var productQuery1 = from p in products
                                 where (new int[] { ,,}).Contains(p.CategoryID)
                                 select p;

             Console.WriteLine("LINQ---");
             foreach (var item in productQuery1)
             {
                 Console.WriteLine("Product Name: {0} , Category ID: {1} , Description: {2}", item.ProductName, item.CategoryID, item.Description);
             }

             // LAMBDA
             var productQuery2 = products.Where(p => new int[] { , ,  }.Contains(p.CategoryID));

             Console.WriteLine("LAMBDA---");
             foreach (var item in productQuery2)
             {
                 Console.WriteLine("Product Name: {0} , Category ID: {1} , Description: {2}", item.ProductName, item.CategoryID, item.Description);
             }

4.查找产品列表中类别为TOYOTA,LEXUS的记录，并且结果按产品数量Count降序排列。(where / orderby)

             // LINQ
             var productQuery1 = from p in products
                                 where p.CategoryID== || p.CategoryID==
                                 orderby p.Count descending
                                 select p;

             Console.WriteLine("LINQ---");
             foreach (var item in productQuery1)
             {
                 Console.WriteLine("Product Name: {0} , Category ID: {1} , Count: {2} , Description: {3}", item.ProductName, item.CategoryID, item.Count,item.Description);
             }

             // LAMBDA
             var productQuery2 = products.Where(p => (p.CategoryID ==  || p.CategoryID == )).OrderByDescending(p => p.Count);

             Console.WriteLine("LAMBDA---");
             foreach (var item in productQuery2)
             {
                 Console.WriteLine("Product Name: {0} , Category ID: {1} , Count: {2} , Description: {3}", item.ProductName, item.CategoryID, item.Count, item.Description);
             }

5.产品列表和类别列表联合查询。(join)

             // LINQ
             var productJoinCategoryQuery1 = from p in products
                                 join c in categories
                                 on p.CategoryID equals c.CategoryID
                                 select new { p, c };

             Console.WriteLine("LINQ---");
             foreach (var item in productJoinCategoryQuery1)
             {
                 Console.WriteLine("Product Name: {0} , Category Name: {1} , Count: {2} , Description: {3}", item.p.ProductName, item.c.CategoryName, item.p.Count, item.p.Description);
             }

             // LAMBDA
             var productJoinCategoryQuery2 = products.Join(categories,
                 p => p.CategoryID,
                 c => c.CategoryID,
                 (p, c) => new { p, c });

             Console.WriteLine("LAMBDA---");
             foreach (var item in productJoinCategoryQuery2)
             {
                 Console.WriteLine("Product Name: {0} , Category Name: {1} , Count: {2} , Description: {3}", item.p.ProductName, item.c.CategoryName, item.p.Count, item.p.Description);
             }

6.产品列表和类别列表分组联合查询。(join & group)

             // LINQ
             var productByCategoryResult1 = from c in categories
                                           join p in products
                                           on c.CategoryID equals p.CategoryID
                                           into CategoryProducts
                                           select new
                                           {
                                               c.CategoryName,
                                               Products = CategoryProducts
                                           };

             Console.WriteLine("LINQ---");
             foreach (var item in productByCategoryResult1)
             {
                 Console.WriteLine("Category: {0}", item.CategoryName);
                 foreach (var product in item.Products)
                 {
                     Console.WriteLine("{0} ({1})", product.ProductName, product.Description);
                 }
                 Console.WriteLine();
             }

             // LAMBDA
             var productByCategoryResult2 = categories.GroupJoin(products,
                 c => c.CategoryID,
                 p => p.CategoryID,
                 (c, p) => new { c.CategoryName, Products = p });

             Console.WriteLine("LAMBDA---");
             foreach (var item in productByCategoryResult2)
             {
                 Console.WriteLine("Category: {0}", item.CategoryName);
                 foreach (var product in item.Products)
                 {
                     Console.WriteLine("{0} ({1})", product.ProductName, product.Description);
                 }
                 Console.WriteLine();
             }

7.LAMBDA的一些其他用法。

             // LAMBDA 一些其他常用方法

             // Min
             var productQueryMin = products.Min(p => p.Count);
             Console.WriteLine("Count Min: {0}", productQueryMin);

             // Max
             var productQueryMax = products.Max(p => p.Count);
             Console.WriteLine("Count Max: {0}", productQueryMax);

             // Average
             var productQueryAverage = products.Average(p => p.Count);
             Console.WriteLine("Count Average: {0}", productQueryAverage);

             // Sum
             var productQuerySum= products.Sum(p => p.Count);
             Console.WriteLine("Count Sum: {0}", productQuerySum);

             // Skip / Take
             var productQueryPage = products.Skip().Take().Select(p => new { p.ProductName, p.Count, p.Description });
             foreach (var item in productQueryPage)
             {
                 Console.WriteLine("Product Name: {0} , Count: {1} , Description: {2}", item.ProductName, item.Count, item.Description);
             }

             // First (返回第一条记录，如果没有记录，则抛出异常)
             var firstProduct = products.First();

             // 此处查询的记录不存在，所以这行代码会抛出异常
             var firstProductNull = products.Where(p => p.Count > ).First();

             // FirstOrDefault (返回第一条记录，如果没有，则返回null)
             var firstOrDefaultProduct = products.FirstOrDefault();

             // 此处查询的记录不存在，所以这行代码返回null
             var firstOrDefaultProductNull = products.Where(p => p.Count > ).FirstOrDefault();

             // Single (返回集合中唯一的记录，如果大于或小于一条记录，则抛出异常，这行代码执行记录大于一条，所以会抛出异常)
             var singleProduct = products.Single();

             // 这行代码执行记录小于一条记录，所以也会抛出异常
             var singleProductNull = products.Where(p => p.Count > ).SingleOrDefault();

好了，本篇就先到此，希望对你有所帮助，谢谢！