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。
1 public class Category 2 { 3 public int CategoryID { get; set; } 4 public string CategoryName { get; set; } 5 } 6 7 public class Product 8 { 9 public int ProductID { get; set; } 10 public string ProductName { get; set; } 11 public int CategoryID { get; set; } 12 public int Count { get; set; } 13 public string Description { get; set; } 14 }
接着创建一些Category类型和Product类型的测试数组列表。
1 Category[] categories = new Category[]{ 2 new Category{CategoryID=1,CategoryName="TOYOTA"}, 3 new Category{CategoryID=2,CategoryName="BMW"}, 4 new Category{CategoryID=3,CategoryName="BENZ"}, 5 new Category{CategoryID=4,CategoryName="HONDA"}, 6 new Category{CategoryID=5,CategoryName="LEXUS"} 7 }; 8 9 Product[] products = new Product[]{ 10 new Product{ProductID=1,ProductName="Car1",CategoryID=1,Count=5,Description="TOYOTA Car"}, 11 new Product{ProductID=2,ProductName="Car2",CategoryID=1,Count=8,Description="TOYOTA SUV"}, 12 new Product{ProductID=3,ProductName="Car3",CategoryID=2,Count=2,Description="BMW Car"}, 13 new Product{ProductID=4,ProductName="Car4",CategoryID=2,Count=3,Description="BMW SUV"}, 14 new Product{ProductID=5,ProductName="Car5",CategoryID=3,Count=5,Description="BENZ Car"}, 15 new Product{ProductID=6,ProductName="Car6",CategoryID=4,Count=10,Description="HONDA Car"}, 16 new Product{ProductID=7,ProductName="Car7",CategoryID=5,Count=6,Description="LEXUS Car"}, 17 new Product{ProductID=8,ProductName="Car8",CategoryID=5,Count=2,Description="LEXUS SUV"} 18 };
1.查询产品列表中所有记录的ProductName,Count,Description列。(select)
1 // LINQ 2 var productQuery1 = from p in products 3 select new 4 { 5 p.ProductName, 6 p.Count, 7 p.Description 8 }; 9 10 Console.WriteLine("LINQ---"); 11 foreach(var item in productQuery1) 12 { 13 Console.WriteLine("Product Name: {0} , Count: {1} , Description: {2}", item.ProductName, item.Count, item.Description); 14 } 15 16 // LAMBDA 17 var productQuery2 = products.Select(p => new 18 { 19 ProductName = p.ProductName, 20 Count = p.Count, 21 Description = p.Description 22 }); 23 24 Console.WriteLine("LAMBDA---"); 25 foreach (var item in productQuery2) 26 { 27 Console.WriteLine("Product Name: {0} , Count: {1} , Description: {2}", item.ProductName, item.Count, item.Description); 28 }
2.查询产品列表中Count大于5的记录。(where)
1 // LINQ 2 var productQuery1 = from p in products 3 where p.Count > 5 4 select p; 5 6 Console.WriteLine("LINQ---"); 7 foreach (var item in productQuery1) 8 { 9 Console.WriteLine(item.ProductName); 10 } 11 12 // LAMBDA 13 var productQuery2 = products.Where(P => P.Count > 5); 14 15 Console.WriteLine("LAMBDA---"); 16 foreach (var item in productQuery2) 17 { 18 Console.WriteLine(item.ProductName); 19 }
3.查找产品列表中类别为TOYOTA,BENZ,LEXUS的记录。(where / contains)
1 // LINQ 2 var productQuery1 = from p in products 3 where (new int[] { 1,3,5}).Contains(p.CategoryID) 4 select p; 5 6 Console.WriteLine("LINQ---"); 7 foreach (var item in productQuery1) 8 { 9 Console.WriteLine("Product Name: {0} , Category ID: {1} , Description: {2}", item.ProductName, item.CategoryID, item.Description); 10 } 11 12 // LAMBDA 13 var productQuery2 = products.Where(p => new int[] { 1, 3, 5 }.Contains(p.CategoryID)); 14 15 Console.WriteLine("LAMBDA---"); 16 foreach (var item in productQuery2) 17 { 18 Console.WriteLine("Product Name: {0} , Category ID: {1} , Description: {2}", item.ProductName, item.CategoryID, item.Description); 19 }
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)
1 // LINQ 2 var productQuery1 = from p in products 3 where p.CategoryID==1 || p.CategoryID==5 4 orderby p.Count descending 5 select p; 6 7 Console.WriteLine("LINQ---"); 8 foreach (var item in productQuery1) 9 { 10 Console.WriteLine("Product Name: {0} , Category ID: {1} , Count: {2} , Description: {3}", item.ProductName, item.CategoryID, item.Count,item.Description); 11 } 12 13 // LAMBDA 14 var productQuery2 = products.Where(p => (p.CategoryID == 1 || p.CategoryID == 5)).OrderByDescending(p => p.Count); 15 16 Console.WriteLine("LAMBDA---"); 17 foreach (var item in productQuery2) 18 { 19 Console.WriteLine("Product Name: {0} , Category ID: {1} , Count: {2} , Description: {3}", item.ProductName, item.CategoryID, item.Count, item.Description); 20 }
5.产品列表和类别列表联合查询。(join)
1 // LINQ 2 var productJoinCategoryQuery1 = from p in products 3 join c in categories 4 on p.CategoryID equals c.CategoryID 5 select new { p, c }; 6 7 Console.WriteLine("LINQ---"); 8 foreach (var item in productJoinCategoryQuery1) 9 { 10 Console.WriteLine("Product Name: {0} , Category Name: {1} , Count: {2} , Description: {3}", item.p.ProductName, item.c.CategoryName, item.p.Count, item.p.Description); 11 } 12 13 // LAMBDA 14 var productJoinCategoryQuery2 = products.Join(categories, 15 p => p.CategoryID, 16 c => c.CategoryID, 17 (p, c) => new { p, c }); 18 19 Console.WriteLine("LAMBDA---"); 20 foreach (var item in productJoinCategoryQuery2) 21 { 22 Console.WriteLine("Product Name: {0} , Category Name: {1} , Count: {2} , Description: {3}", item.p.ProductName, item.c.CategoryName, item.p.Count, item.p.Description); 23 }
6.产品列表和类别列表分组联合查询。(join & group)
1 // LINQ 2 var productByCategoryResult1 = from c in categories 3 join p in products 4 on c.CategoryID equals p.CategoryID 5 into CategoryProducts 6 select new 7 { 8 c.CategoryName, 9 Products = CategoryProducts 10 }; 11 12 Console.WriteLine("LINQ---"); 13 foreach (var item in productByCategoryResult1) 14 { 15 Console.WriteLine("Category: {0}", item.CategoryName); 16 foreach (var product in item.Products) 17 { 18 Console.WriteLine("{0} ({1})", product.ProductName, product.Description); 19 } 20 Console.WriteLine(); 21 } 22 23 // LAMBDA 24 var productByCategoryResult2 = categories.GroupJoin(products, 25 c => c.CategoryID, 26 p => p.CategoryID, 27 (c, p) => new { c.CategoryName, Products = p }); 28 29 Console.WriteLine("LAMBDA---"); 30 foreach (var item in productByCategoryResult2) 31 { 32 Console.WriteLine("Category: {0}", item.CategoryName); 33 foreach (var product in item.Products) 34 { 35 Console.WriteLine("{0} ({1})", product.ProductName, product.Description); 36 } 37 Console.WriteLine(); 38 }
7.LAMBDA的一些其他用法。
1 // LAMBDA 一些其他常用方法 2 3 // Min 4 var productQueryMin = products.Min(p => p.Count); 5 Console.WriteLine("Count Min: {0}", productQueryMin); 6 7 // Max 8 var productQueryMax = products.Max(p => p.Count); 9 Console.WriteLine("Count Max: {0}", productQueryMax); 10 11 // Average 12 var productQueryAverage = products.Average(p => p.Count); 13 Console.WriteLine("Count Average: {0}", productQueryAverage); 14 15 // Sum 16 var productQuerySum= products.Sum(p => p.Count); 17 Console.WriteLine("Count Sum: {0}", productQuerySum); 18 19 // Skip / Take 20 var productQueryPage = products.Skip(3).Take(3).Select(p => new { p.ProductName, p.Count, p.Description }); 21 foreach (var item in productQueryPage) 22 { 23 Console.WriteLine("Product Name: {0} , Count: {1} , Description: {2}", item.ProductName, item.Count, item.Description); 24 } 25 26 // First (返回第一条记录，如果没有记录，则抛出异常) 27 var firstProduct = products.First(); 28 29 // 此处查询的记录不存在，所以这行代码会抛出异常 30 var firstProductNull = products.Where(p => p.Count > 100).First(); 31 32 // FirstOrDefault (返回第一条记录，如果没有，则返回null) 33 var firstOrDefaultProduct = products.FirstOrDefault(); 34 35 // 此处查询的记录不存在，所以这行代码返回null 36 var firstOrDefaultProductNull = products.Where(p => p.Count > 100).FirstOrDefault(); 37 38 // 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) 39 var singleProduct = products.Single(); 40 41 // 这行代码执行记录小于一条记录，所以也会抛出异常 42 var singleProductNull = products.Where(p => p.Count > 100).SingleOrDefault();
好了，本篇就先到此，希望对你有所帮助，谢谢！
