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);
}
posted @ 2014-10-21 02:06 libingql 阅读(...) 评论(...) 编辑 收藏