LINQ系列：Linq to Object聚合操作符

妖狐艹你老母 2022-01-09 23:45 316阅读 0赞

聚合函数在一系列的值上执行特定的运算，并返回单个值，如在给定元素的值上执行求和或计数运算。  
　　LINQ共有7种聚合操作符：Aggregate、Average、Count、LongCount、Max、Min和Sum。

**1. Aggregate**

Aggregate操作符从某一特定序列或集合中收集值。当聚合完成时，它将序列中返回的值进行累积并返回结果。

1>. 原型定义

public static TSource Aggregate<TSource>(this IEnumerable<TSource> source, Func<TSource, TSource, TSource> func);

2>. 示例

var expr = context.Categories
        .Select(c => c.CategoryName)
        .ToArray()
        .Aggregate((current, next) => String.Format("{0}-{1}", current, next));
    
    Console.WriteLine(expr);

**2. Average**

Average操作符计算一个数值序列的平均值。

1>.原型定义

public static double Average(this IEnumerable<double> source);

public static double Average<TSource>(this IEnumerable<TSource> source, Func<TSource, int> selector);

2>.示例

int[] fibonacci = new int[] { 1, 1, 2, 3, 5, 8, 13, 21 };
    Console.WriteLine(fibonacci.Average());

var expr = context.Products
        .Average(p => p.UnitPrice);
    
    Console.WriteLine(expr);

var expr = context.Products
        .Select(p=>p.UnitPrice)
        .Average();

var expr = from p in context.Products
               select p;
    
    expr.Average(p => p.UnitPrice)

var expr = from p in context.Products
               select p.UnitPrice;
    expr.Average();

**3. Count**

Count操作符计算一个特定集合中元素的个数。

1>. 原型定义

public static int Count<TSource>(this IQueryable<TSource> source);

public static int Count<TSource>(this IQueryable<TSource> source, Expression<Func<TSource, bool>> predicate);

2>. 示例

int[] fibonacci = new int[] { 1, 1, 2, 3, 5, 8, 13, 21 };
    Console.WriteLine(fibonacci.Count());

var expr = context.Products.Count();

var expr = context.Products
        .Where(p => p.UnitPrice > 10m)
        .Count();

var expr = context.Products
        .Count(p => p.UnitPrice > 10m);

int[] fibonacci = new int[] { 1, 1, 2, 3, 5, 8, 13, 21 };
    var expr = from f in fibonacci
                where f > 5
                select f;
    expr.Count();

var expr = from c in context.Categories
               select new
               {
                   c.CategoryName,
                   ProductsCount = c.Products.Count()
               };

**4. LongCount**

LongCount操作符会返回一个Int64类型的值，用来计算元素数大于Int32.MaxValue的集合的元素数。LongCount的使用方法与Count相同。

1>. 原型定义

public static long LongCount<TSource>(this IQueryable<TSource> source);

public static long LongCount<TSource>(this IQueryable<TSource> source, Expression<Func<TSource, bool>> predicate);

2>. 示例

var expr = context.Products
        .LongCount(p => p.UnitPrice > 10m);

**5. Max**

Max操作符返回一个序列中的最大值。

1>. 原型定义

public static TSource Max<TSource>(this IQueryable<TSource> source);

public static TResult Max<TSource, TResult>(this IQueryable<TSource> source, Expression<Func<TSource, TResult>> selector);

2>. 示例

int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };
    int maxNumber = numbers.Max();

var expr = from p in context.Products
                select p;
    expr.Max(p => p.UnitPrice > 10m);

var expr = from p in context.Products
                select p.UnitPrice;
    expr.Max();

var expr = context.Products
        .Max(p => p.UnitPrice > 10m);

var expr = context.Products
        .Select(p => p.UnitPrice)
        .Max();

IEnumerable<Product> query = from p in context.Products
                                 where p.UnitPrice == context.Products.Max(product => product.UnitPrice)
                                 select p;

IEnumerable<Product> query = from p in context.Products
                                 where p.UnitPrice == context.Products.OrderByDescending(product => product.UnitPrice).First().UnitPrice
                                 select p;

**6. Min**

Min操作符返回一个序列中的最小值。

1>. 原型定义

public static TSource Min<TSource>(this IQueryable<TSource> source);

public static TResult Min<TSource, TResult>(this IQueryable<TSource> source, Expression<Func<TSource, TResult>> selector);

2>. 示例

string[] words = { "cherry", "apple", "blueberry" };
    int shortestWord = words.Min(w => w.Length);

int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };
    int minNumber = numbers.Min();

var expr = context.Products
        .Select(p => p.UnitPrice)
        .Min();

**7. Sum**

Sum操作符计算集合中选定值的总和。

1>. 原型定义

public static int Sum(this IQueryable<int> source);

public static int Sum<TSource>(this IQueryable<TSource> source, Expression<Func<TSource, int>> selector);

2>. 示例

int[] fibonacci = new int[] { 1, 1, 2, 3, 5, 8, 13, 21 };
    var expr = from f in fibonacci
                select f;
    expr.Sum();

int[] fibonacci = new int[] { 1, 1, 2, 3, 5, 8, 13, 21 };
    var expr = from f in fibonacci
                where f > 5
                select f;
    expr.Sum();

var expr = from p in context.Products
               select p;
    expr.Sum(p => p.UnitPrice);

var expr = from p in context.Products
               select p.UnitPrice;
    expr.Sum();

转载于:https://www.cnblogs.com/libingql/p/4042570.html