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[] { , , , , , , ,  };
var result = from f in fibonacci
             select f;

foreach (var item in result)
{
    Console.WriteLine(item);
}

IEnumerable<int>
    source = new int[] { , , , , , , ,  },
    filtered = source.Where(f => f > ),
    sorted = filtered.OrderBy(f => f),
    query = sorted.Select(f => f * );

List<int> list = new List<int>() { , , , ,  };
var query = from number in list
            where number <
            select number;

3>. 扩展方法

int[] fibonacci = new int[] { , , , , , , ,  };
var result = fibonacci.Select(f => f);

foreach (var item in result)
{
    Console.WriteLine(item);
}

Object[] arr = new Object[] { "String", , 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[] { , , , , , ,  };
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 >  ? "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 >  ? "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 == )
    .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);
}