LINQ系列：Linq to Object投影操作符

　　投影是指在将序列中的元素转换为一个自定义形式的操作。投影操作符Select和SelectMany用于选择出赋予了适当功能的值。SelectMany操作符可以处理多个集合。

　　LINQ表达式语法：

     

1. Select

　　Select操作符对单个序列或集合中的值进行投影。

1.1 原型定义

public static IEnumerable<TResult> Select<TSource, TResult>(this IEnumerable<TSource> source, Func<TSource, TResult> selector);

public static IEnumerable<TResult> Select<TSource, TResult>(this IEnumerable<TSource> source, Func<TSource, int, TResult> selector);

1.2 查询表达式

int[] fibonacci = new int[] { 1, 1, 2, 3, 5, 8, 13, 21 };

var result = from f in fibonacci

             select f;



foreach (var item in result)

{

    Console.WriteLine(item);

}

IEnumerable<int>

    source = new int[] { 1, 1, 2, 3, 5, 8, 13, 21 },

    filtered = source.Where(f => f > 5),

    sorted = filtered.OrderBy(f => f),

    query = sorted.Select(f => f * 2);

List<int> list = new List<int>() { 1, 2, 3, 4, 5 };

var query = from number in list

            where number < 3

            select number;

3>. 扩展方法

int[] fibonacci = new int[] { 1, 1, 2, 3, 5, 8, 13, 21 };

var result = fibonacci.Select(f => f);



foreach (var item in result)

{

    Console.WriteLine(item);

}

Object[] arr = new Object[] { "String", 12, true, 10.0m, 'a' };

var types = arr.Select(item => item.GetType().Name);



foreach (var type in types)

{

    Console.WriteLine(type);

}

4>. 查询返回列

　　Select可以从序列中查询返回全部列、单独列及指定的多列。

　　返回单列：

using (NorthwindContext context = new NorthwindContext())

{

    var products = from p in context.Products

                   select p.ProductName;

}

var products = context.Products

    .Select(p => p.ProductName);

// using System.IO;

DirectoryInfo di = new DirectoryInfo(@"D:\");

var query = from dir in di.GetDirectories()

            orderby dir.Name

            select dir.Name;

　　返回多列：

var products = from p in context.Products

               select new { p.ProductID, p.ProductName };

var products = context.Products

    .Select(p => new { p.ProductID, p.ProductName });

int[] fibonacci = new int[] { 1, 1, 2, 3, 5, 8, 13 };

var expr = fibonacci.Select((f, i) => new { Index = i, Number = f });

foreach (var item in expr)

{

    Console.WriteLine(item.Index + "-" + item.Number);

}

var expr = context.Products

    .OrderBy(p => p.ProductID)

    .Select(p => new ProductViewModel

    {

        ProductID = p.ProductID,

        ProductName = p.ProductName,

        UnitPrice = p.UnitPrice

    });

var expr = context.Products

    .Select(p => new

    {

        ProductID = p.ProductID,

        ProductName = p.ProductName,

        Available = p.UnitsInStock - p.UnitsOnOrder > 0 ? "In Stock" : "Out Of Stock"

    });

var expr = context.Products

    .Select(p => new

    {

        ProductID = p.ProductID,

        ProductName = p.ProductName,

        Units = new

        {

            p.UnitsInStock,

            p.UnitsOnOrder,

            Available = p.UnitsInStock - p.UnitsOnOrder > 0 ? "In Stock" : "Out Of Stock"

        }

    });

var query = context.Products

    .Select((p, index) => new

    {

        Position = index,

        p.ProductName,

        p.UnitPrice

    });

5>. Select方法实现

public static IEnumerable<TResult> Select<TSource, TResult>(this IEnumerable<TSource> source, Func<TSource, TResult> selector)

{

    foreach (TSource element in source)

    {

        yield return selector(element);

    }

}

2. SelectMany

　　SelectMany操作符可以将多个from字句组合起来，将每个对象的结果合并成一个序列。

1>. 原型定义

public static IEnumerable<TResult> SelectMany<TSource, TCollection, TResult>(this IEnumerable<TSource> source, Func<TSource, IEnumerable<TCollection>> collectionSelector, Func<TSource, TCollection, TResult> resultSelector);

public static IEnumerable<TResult> SelectMany<TSource, TCollection, TResult>(this IEnumerable<TSource> source, Func<TSource, int, IEnumerable<TCollection>> collectionSelector, Func<TSource, TCollection, TResult> resultSelector);

2>. 返回全部列

var expr = from c in context.Customers

           from o in c.Orders

           select o;

var expr = context.Customers

    .SelectMany(c => c.Orders);

3>. 返回指定列

var expr = from c in context.Customers

           from o in c.Orders

           select new { o.OrderID, o.OrderDate };

var expr = context.Customers

    .SelectMany(c => c.Orders

    .Select(o => new { o.OrderID, o.OrderDate }));

var query = context.Categories

    .Where(c => c.CategoryID == 1)

    .SelectMany(c => c.Products

        .Where(p => p.CategoryID == c.CategoryID)

        .Select(p => new

        {

            c.CategoryName,

            p.ProductName,

            p.UnitPrice

        })

    );

var expr = from c in context.Categories

           from p in c.Products

           where c.CategoryName == "LINQ to Object"

           select p;

var query = from c in context.Categories

            select new

            {

                CategoryID = c.CategoryID,

                CategoryName = c.CategoryName,

                ProductIDs = from p in c.Products

                            select p.ProductID

            };

var expr = from c in context.Categories

           from p in c.Products

           select new

           {

               c.CategoryName,

               p.ProductID,

               p.ProductName

           };



var expr = from c in

               context.Categories

                   .SelectMany(c => from p in c.Products

                                       select new

                                       {

                                           c.CategoryName,

                                           p.ProductID,

                                           p.ProductName

                                       })

           select c;



var expr = context.Categories

    .SelectMany(c => c.Products

        .Select(p => new

        {

            c.CategoryName,

            p.ProductID,

            p.ProductName

        }));

3. Let

var expr = from p in context.Products

           let productInfo = p.UnitPrice + "," + p.UnitsInStock

           select new { p.ProductName, productInfo };

var expr = context.Products

    .Select(p => new { p, ProductInfo = p.UnitPrice + "," + p.UnitsInStock })

    .Select(t => new { t.p.ProductName, t.ProductInfo });



foreach (var item in expr)

{

    Console.WriteLine("{0}-{1}", item.ProductName, item.ProductInfo);

}