1. Where

　　限制操作符Where用于过滤序列，按照提供的逻辑对序列中的数据进行过滤。Where可以出现多次。

1.1 原型定义

public static IEnumerable
     
       Where
      
       (this IEnumerable
       
         source, Func
        
          predicate);
        
       
      
     

public static IEnumerable
     
       Where
      
       (this IEnumerable
       
         source, Func
        
          predicate);
        
       
      
     

1.2 单个限制条件

var products = from p in context.Products               where p.UnitPrice > 10m               select p;

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

Func
     
       filter = delegate(Product p) { return p.UnitPrice > 10m; };var query = context.Products    .Where(filter)    .Select(p => new { p.ProductID, p.ProductName });
     

// using System.Linq;int[] fibonacci = new int[] { 1, 1, 2, 3, 5, 8, 13, 21 };IEnumerable
     
       query = Enumerable.Where(fibonacci, f => f > 5);query.ToList().ForEach(f =>{    Console.WriteLine(f);});
     

1.3 多个过滤条件

var products = from p in context.Products               where p.UnitPrice > 10m && p.ProductName.StartsWith("LINQ")               select p;

var products = context.Products    .Where(p => p.UnitPrice > 10m && p.ProductName.StartsWith("LINQ"));

var expr = context.Products    .Where(p => p.UnitPrice > 10m)    .Where(p => p.ProductName.StartsWith("LINQ"));

1.4 Lambda多参数表达式

int[] fibonacci = new int[] { 1, 1, 2, 3, 5, 8, 13, 21 };var expr = fibonacci.Where((f, index) => f > 1 && index > 3);foreach (var item in expr){    Console.WriteLine(item);}

1.5 自定义实现

　　LINQ是在C#3.0出现的，在C#2.0及之前没有LINQ的支持，接下来为LINQ Where操作符的自定义实现。

　　C#2.0实现方式：

public static IEnumerable
     
       Where
      
       (this IEnumerable
       
         source, Func
        
          predicate);
        
       
      
     

using System;using System.Collections.Generic;using System.Text;namespace Northwind.CustomLINQExtension{    public static class CustomLINQExtension    {        public delegate TResult Func
     
      (T arg);        public static IEnumerable
      
        Where
       
        (IEnumerable
        
          source, Func
         
           predicate)        {            foreach (TSource element in source)            {                if (predicate(element))                {                    yield return element;                }            }        }    }}
         
        
       
      
     

using System;using System.Collections.Generic;using System.Text;namespace Northwind.CustomLINQExtension{    class Program    {        static void Main(string[] args)        {            int[] fibonacci = new int[] { 1, 1, 2, 3, 5, 8, 13, 21 };            IEnumerable
     
       expr = CustomLINQExtension.Where(fibonacci, (delegate(int i) { return i > 3; }));            foreach (int item in expr)            {                Console.WriteLine(item);            }        }    }}
     

　　由于在C#2.0中没有扩展方法，调用实现的自定义扩展类需要使用类名。

　　在C#3.0中增加了扩展方法，在C#3.0自定义LINQ Where限制条件，不使用系统LINQ自带。

using System;using System.Collections.Generic;using System.Text;namespace Northwind.CustomLINQExtension{    public static class CustomLINQExtension    {        public delegate TResult Func
     
      (T arg);        public static IEnumerable
      
        Where
       
        (this IEnumerable
        
          source, Func
         
           predicate)        {            foreach (TSource element in source)            {                if (predicate(element))                {                    yield return element;                }            }        }    }}
         
        
       
      
     

using System;using System.Collections.Generic;using System.Text;namespace Northwind.CustomLINQExtension{    class Program    {        static void Main(string[] args)        {            int[] fibonacci = new int[] { 1, 1, 2, 3, 5, 8, 13, 21 };            IEnumerable
     
       expr = fibonacci.Where(delegate(int i) { return i > 3; });            foreach (int item in expr)            {                Console.WriteLine(item);            }        }    }}