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22LINQ查询运算符返回IEnumerable<T>实例汇总

2014-04-01 16:18 423 查看

本篇体验LINQ的各种查询运算符。

先创建一个泛型方法，用来显示查询结果：

        private static void DisplayQuery<T>(IEnumerable<T> query)


[code]{

foreach (T item in query)

//Console.WriteLine(item.ToString());

Console.Write(item.ToString() + " ");

[/code]

.csharpcode, .csharpcode pre
{
font-size: small;
color: black;
font-family: consolas, "Courier New", courier, monospace;
background-color: #ffffff;
/*white-space: pre;*/
}
.csharpcode pre { margin: 0em; }
.csharpcode .rem { color: #008000; }
.csharpcode .kwrd { color: #0000ff; }
.csharpcode .str { color: #006080; }
.csharpcode .op { color: #0000c0; }
.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
.csharpcode .alt
{
background-color: #f4f4f4;
width: 100%;
margin: 0em;
}
.csharpcode .lnum { color: #606060; }

对一个集合进行操作

□ Where()对一个序列操作

● 使用Where<TSource>(this IEnumerable<TSource> , Func<TSource, bool>):

            List<int> list = new List<int>()


[code]{

1,2,3,4,5,6,7,8

};

IEnumerable<int> query = list.Where(x => x > 5);

DisplayQuery<int>(query);

Console.ReadKey();

[/code]

.csharpcode, .csharpcode pre
{
font-size: small;
color: black;
font-family: consolas, "Courier New", courier, monospace;
background-color: #ffffff;
/*white-space: pre;*/
}
.csharpcode pre { margin: 0em; }
.csharpcode .rem { color: #008000; }
.csharpcode .kwrd { color: #0000ff; }
.csharpcode .str { color: #006080; }
.csharpcode .op { color: #0000c0; }
.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
.csharpcode .alt
{
background-color: #f4f4f4;
width: 100%;
margin: 0em;
}
.csharpcode .lnum { color: #606060; }

结果：6 7 8

● 使用Where<TSource>(IEnumerable<TSource>,Func<TSource, int, bool>)，根据数据源和元素索引返回值：

            List<int> list = new List<int>()


[code]{

1,2,3,4,5,6,7,8

};

IEnumerable<int> query = list.Where((x,index) => index>=1 && x<=3);

DisplayQuery<int>(query);

Console.ReadKey();

[/code]

.csharpcode, .csharpcode pre
{
font-size: small;
color: black;
font-family: consolas, "Courier New", courier, monospace;
background-color: #ffffff;
/*white-space: pre;*/
}
.csharpcode pre { margin: 0em; }
.csharpcode .rem { color: #008000; }
.csharpcode .kwrd { color: #0000ff; }
.csharpcode .str { color: #006080; }
.csharpcode .op { color: #0000c0; }
.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
.csharpcode .alt
{
background-color: #f4f4f4;
width: 100%;
margin: 0em;
}
.csharpcode .lnum { color: #606060; }

结果：2 3

□ OfType()对一个序列操作

● OfType()是对非泛型IEnumerable的扩展，所以先确定集合类型后，才可以用该方法。

● OfType()的返回类型是IEnumerable<T>泛型，所以在使用OfType()之后，可以继续链式使用其它操作符。

            ArrayList list = new ArrayList();


[code]            list.Add(1);

list.Add(DateTime.Now);

list.Add("spriing is coming");

list.Add("enjoy your day");

var query = list.OfType<string>().Where(x => x.StartsWith("enjoy"));

DisplayQuery<string>(query);

Console.ReadKey();

[/code]

.csharpcode, .csharpcode pre
{
font-size: small;
color: black;
font-family: consolas, "Courier New", courier, monospace;
background-color: #ffffff;
/*white-space: pre;*/
}
.csharpcode pre { margin: 0em; }
.csharpcode .rem { color: #008000; }
.csharpcode .kwrd { color: #0000ff; }
.csharpcode .str { color: #006080; }
.csharpcode .op { color: #0000c0; }
.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
.csharpcode .alt
{
background-color: #f4f4f4;
width: 100%;
margin: 0em;
}
.csharpcode .lnum { color: #606060; }

结果： enjoy your day

□ Cast()将非泛型序列转换成泛型序列

● 对于一个非泛型集合，就不能使用LINQ操作符：

● 使用Cast()可以将非泛型集合转换成泛型集合：

            ArrayList list = new ArrayList();


[code]            list.Add("super star");

list.Add("good day");

var query = list.Cast<string>().Where(x => x.StartsWith("good"));

DisplayQuery<string>(query);

Console.ReadKey();

[/code]

.csharpcode, .csharpcode pre
{
font-size: small;
color: black;
font-family: consolas, "Courier New", courier, monospace;
background-color: #ffffff;
/*white-space: pre;*/
}
.csharpcode pre { margin: 0em; }
.csharpcode .rem { color: #008000; }
.csharpcode .kwrd { color: #0000ff; }
.csharpcode .str { color: #006080; }
.csharpcode .op { color: #0000c0; }
.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
.csharpcode .alt
{
background-color: #f4f4f4;
width: 100%;
margin: 0em;
}
.csharpcode .lnum { color: #606060; }

结果：good day

□ OrderBy(), ThenBy(),OrderByDescending(),ThenByDescending()

关于产品和分类：

    public class Category


[code]{

public int Id { get; set; }

public string Name { get; set; }

public class Product

public int Id { get; set; }

public string Name { get; set; }

public Category Category { get; set; }

public DateTime BuyDate { get; set; }

public decimal Price { get; set; }

public override string ToString()

return String.Format("编号：{0}，名称：{1}，类别：{2},购买时间：{3}，价格：{4}", Id, Name, Category.Name, BuyDate, Price);

public static class ProductHelper

public static IEnumerable<Product> GetProducts()

Category category1 = new Category();

category1.Id = 1;

category1.Name = "数码电子";

Category category2 = new Category();

category2.Id = 2;

category2.Name = "服饰类";

return new List<Product>()

new Product(){Id = 1,BuyDate = new DateTime(2012,1,1),Category = category1,Name = "数码相机",Price = 800m},

new Product(){Id = 2,BuyDate = new DateTime(2012,1,3),Category = category1,Name = "录像机",Price = 900m},

new Product(){Id = 3,BuyDate = new DateTime(2012,1,2),Category = category2,Name = "体恤衫",Price = 300m},

new Product(){Id = 4,BuyDate = new DateTime(2012,1,8),Category = category2,Name = "茄克衫",Price = 600m}

};

            var query = ProductHelper.GetProducts().OrderBy(x => x.Id);


[code]DisplayQuery<Product>(query);

Console.ReadKey();

● OrderBy()对属性的计算值排序：

            var query = ProductHelper.GetProducts().OrderBy(x => x.BuyDate.Date);


[code]DisplayQuery<Product>(query);

Console.ReadKey();

● OrderBy()组合ThenBy()链式排序：

            var query = ProductHelper.GetProducts().OrderByDescending(x => x.Category.Id).ThenBy(x => x.Price);


[code]DisplayQuery<Product>(query);

Console.ReadKey();

注意：ThenBy()是针对IOrderedEnumerable<T>的扩展，因此不能直接在IEnumerable<T>泛型集合使用ThenBy()。

● OrderyBy()接收IComparer<T>接口类型。

创建实现IComparer<T>接口的、针对Product的比较类：

   public class ProductComparer : IComparer<Product>, IEqualityComparer<Product>


[code]{

public int Compare(Product x, Product y)

if (x == y)//如果类别名称相同就比较产品价格

return x.Price.CompareTo(y.Price);

else //如果类别名称不同，比较类别的编号

return x.Category.Id.CompareTo(y.Category.Id);

public bool Equals(Product x, Product y)

if (x.Category.Name == y.Category.Name)

return true;

else

return false;

public int GetHashCode(Product obj)

return obj.GetHashCode();

var query = ProductHelper.GetProducts().OrderBy(x => x, new ProductComparer());


[code]DisplayQuery<Product>(query);

Console.ReadKey();

□ Select()投影

            var query = ProductHelper.GetProducts().Select((x, index) => new {Name = x.Name, Index = index});


[code]foreach (var item in query)

Console.WriteLine("名称：{0},索引：{1}",item.Name,item.Index);

Console.ReadKey();

在ASP.NET MVC中，经常需要填充下拉框，可以用Select()投影出IEnumerable<SelectListItem>集合：

list.Select(x => new SelectListItem{Text = x.Name, Value = x.Code});

□ Take()和Skip()

            List<int> list = new List<int>()


[code]{

1,2,3,4,5,6,7,8

};

var query = list.Skip(7).Take(1);

DisplayQuery<int>(query);

Console.ReadKey();

[/code]

.csharpcode, .csharpcode pre
{
font-size: small;
color: black;
font-family: consolas, "Courier New", courier, monospace;
background-color: #ffffff;
/*white-space: pre;*/
}
.csharpcode pre { margin: 0em; }
.csharpcode .rem { color: #008000; }
.csharpcode .kwrd { color: #0000ff; }
.csharpcode .str { color: #006080; }
.csharpcode .op { color: #0000c0; }
.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
.csharpcode .alt
{
background-color: #f4f4f4;
width: 100%;
margin: 0em;
}
.csharpcode .lnum { color: #606060; }

结果：8

□ TakeWhile()和SkipWhile()

● TakeWhile()在遇到不符合条件的元素时，就停止在集合中的遍历，而Where在遇到不符合条件的元素时，会继续遍历直到结束。

            int[] array = {1, 2, 10, 5, 6};


[code]            var query = array.TakeWhile(x => x <= 5);

DisplayQuery<int>(query);

Console.ReadKey();

[/code]

.csharpcode, .csharpcode pre
{
font-size: small;
color: black;
font-family: consolas, "Courier New", courier, monospace;
background-color: #ffffff;
/*white-space: pre;*/
}
.csharpcode pre { margin: 0em; }
.csharpcode .rem { color: #008000; }
.csharpcode .kwrd { color: #0000ff; }
.csharpcode .str { color: #006080; }
.csharpcode .op { color: #0000c0; }
.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
.csharpcode .alt
{
background-color: #f4f4f4;
width: 100%;
margin: 0em;
}
.csharpcode .lnum { color: #606060; }

结果： 1 2

● SkipWhile()会跳过符合条件的元素，直到遇到第一个不符合条件的元素，把该元素以及后面的元素返回。

            int[] array = {1, 2, 10, 5, 6};


[code]            var query = array.SkipWhile(x => x <= 5);

DisplayQuery<int>(query);

Console.ReadKey();

[/code]

.csharpcode, .csharpcode pre
{
font-size: small;
color: black;
font-family: consolas, "Courier New", courier, monospace;
background-color: #ffffff;
/*white-space: pre;*/
}
.csharpcode pre { margin: 0em; }
.csharpcode .rem { color: #008000; }
.csharpcode .kwrd { color: #0000ff; }
.csharpcode .str { color: #006080; }
.csharpcode .op { color: #0000c0; }
.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
.csharpcode .alt
{
background-color: #f4f4f4;
width: 100%;
margin: 0em;
}
.csharpcode .lnum { color: #606060; }

结果： 10 5 6

□ Reverse()反转元素排列

            int[] arr = {1, 2};


[code]            var query = arr.Reverse();

DisplayQuery<int>(query);

Console.ReadKey();

[/code]

.csharpcode, .csharpcode pre
{
font-size: small;
color: black;
font-family: consolas, "Courier New", courier, monospace;
background-color: #ffffff;
/*white-space: pre;*/
}
.csharpcode pre { margin: 0em; }
.csharpcode .rem { color: #008000; }
.csharpcode .kwrd { color: #0000ff; }
.csharpcode .str { color: #006080; }
.csharpcode .op { color: #0000c0; }
.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
.csharpcode .alt
{
background-color: #f4f4f4;
width: 100%;
margin: 0em;
}
.csharpcode .lnum { color: #606060; }

结果： 2 1

□ DefaultIfEmpty()针对集合为空的处理

● 如果是空的整型集合，默认返回0

            int[] array = {};


[code]            var query = array.DefaultIfEmpty();

DisplayQuery<int>(query);

Console.ReadKey();

            int[] array = { };


[code]            var query = array.DefaultIfEmpty(100);

DisplayQuery<int>(query);

Console.ReadKey();

[/code]

.csharpcode, .csharpcode pre
{
font-size: small;
color: black;
font-family: consolas, "Courier New", courier, monospace;
background-color: #ffffff;
/*white-space: pre;*/
}
.csharpcode pre { margin: 0em; }
.csharpcode .rem { color: #008000; }
.csharpcode .kwrd { color: #0000ff; }
.csharpcode .str { color: #006080; }
.csharpcode .op { color: #0000c0; }
.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
.csharpcode .alt
{
background-color: #f4f4f4;
width: 100%;
margin: 0em;
}
.csharpcode .lnum { color: #606060; }

结果： 100

● 如果是空的类类型，默认值是null,不指定默认值会抛异常

            Product[] arraryProducts = {};


[code]            var query = arraryProducts.DefaultIfEmpty();

DisplayQuery<Product>(query);

Console.ReadKey();

● 为空的类类型指定默认值

            Product[] arraryProducts = { };


[code]            Category category = new Category();

category.Id = 1;

category.Name = "酒类";

var query = arraryProducts.DefaultIfEmpty(new Product(){Id = 1,BuyDate = DateTime.Now,Category = category,Name = "贵州茅台",Price = 200m});

DisplayQuery<Product>(query);

Console.ReadKey();

□ Distinct()剔除重复元素

● 剔除重复的整型

            int[] array = {1, 2, 2};


[code]            var query = array.Distinct();

DisplayQuery<int>(query);

Console.ReadKey();

   public class Category


[code]{

public int Id { get; set; }

public string Name { get; set; }

public override string ToString()

return String.Format("编号：{0}，名称：{1}", Id, Name);

public static class CategoryHelper

public static IEnumerable<Category> GetCategories()

return new List<Category>()

new Category(){Id = 1,Name = "烟类"},

new Category(){Id = 2,Name = "烟类"},

new Category(){Id = 3,Name = "酒类"}

};

public class CategoryComparer : IEqualityComparer<Category>

public bool Equals(Category x, Category y)

return x.Name == y.Name;

public int GetHashCode(Category obj)

return obj.Name.GetHashCode();

            IEqualityComparer<Category> customComparer = new CategoryComparer();


[code]            var query = CategoryHelper.GetCategories().Distinct(customComparer);

DisplayQuery<Category>(query);

Console.ReadKey();

注意：

public int GetHashCode(Category obj)方法中，一定要返回所比较属性的哈希值。

● 写一个适合于任何类、任何属性的泛型方法

   public class PropertyComparer<T> : IEqualityComparer<T>


[code]{

//需要比较的属性的PropertyInfo

private PropertyInfo _PropertyInfo;

//通过构造函数把需要比较的属性传进来

public PropertyComparer(string propertyName)

_PropertyInfo = typeof (T).GetProperty(propertyName,

BindingFlags.GetProperty | BindingFlags.Instance | BindingFlags.Public);

if (_PropertyInfo == null)

throw new ArgumentException(string.Format("{0} 不是{1}的属性",propertyName,typeof(T)));

public bool Equals(T x, T y)

object xValue = _PropertyInfo.GetValue(x, null);

object yValue = _PropertyInfo.GetValue(y, null);

//如果xValue的属性值为null,那yValue的属性值也必须是null，才返回true

if (xValue == null)

return yValue == null;

return xValue.Equals(yValue);

public int GetHashCode(T obj)

object propertyValue = _PropertyInfo.GetValue(obj, null);

if (propertyValue == null)

return 0;

else

return propertyValue.GetHashCode();

            IEqualityComparer<Category> customComparer = new PropertyComparer<Category>("Name");


[code]            var query = CategoryHelper.GetCategories().Distinct(customComparer);

DisplayQuery<Category>(query);

Console.ReadKey();

□ GroupBy()

   public class Category


[code]{

public int Id { get; set; }

public string Name { get; set; }

public override string ToString()

return String.Format("编号：{0}，名称：{1}", Id, Name);

public static class CategoryHelper

public static IEnumerable<Category> GetCategories()

return new List<Category>()

new Category(){Id = 1,Name = "烟类"},

new Category(){Id = 2,Name = "烟类"},

new Category(){Id = 3,Name = "酒类"}

};

            IEnumerable<IGrouping<string,Category>> query = CategoryHelper.GetCategories().GroupBy(x => x.Name);


[code]foreach (var item in query)

Console.WriteLine("<{0}>",item.Key);

foreach (var p in item)

Console.WriteLine(p.ToString());

Console.ReadKey();

GroupBy()的返回类型是IGrouping<TKey,TSource>的集合,该接口只提供了一个Key属性,也就是用来分组的属性值。

public interface IGrouping<out TKey,out TElement>


[code]{

TKey Key { get; }

● 投影

            var query = CategoryHelper.GetCategories()


[code]                .GroupBy(x => x.Name, x => new {Text = x.Id + x.Name, Value = x.Name});

foreach (var item in query)

Console.WriteLine("<{0}>", item.Key);

foreach (var p in item)

Console.WriteLine(p.Text.PadRight(20) + p.Value);

Console.ReadKey();

对多个集合进行操作

□ Intersect()获取2个集合的交集

            int[] arr1 = {0, 1, 2,3};


[code]            int[] arr2 = {2, 3, 4};

var query = arr1.Intersect(arr2);

DisplayQuery<int>(query);

Console.ReadKey();

[/code]

.csharpcode, .csharpcode pre
{
font-size: small;
color: black;
font-family: consolas, "Courier New", courier, monospace;
background-color: #ffffff;
/*white-space: pre;*/
}
.csharpcode pre { margin: 0em; }
.csharpcode .rem { color: #008000; }
.csharpcode .kwrd { color: #0000ff; }
.csharpcode .str { color: #006080; }
.csharpcode .op { color: #0000c0; }
.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
.csharpcode .alt
{
background-color: #f4f4f4;
width: 100%;
margin: 0em;
}
.csharpcode .lnum { color: #606060; }

结果： 2 3

□ Except()获取第一个集合中有，而第二个集合中没有的元素

            int[] arr1 = {0, 1, 2,3};


[code]            int[] arr2 = {2, 3, 4};

var query = arr1.Except(arr2);

DisplayQuery<int>(query);

Console.ReadKey();

[/code]

结果：0 1
.csharpcode, .csharpcode pre
{
font-size: small;
color: black;
font-family: consolas, "Courier New", courier, monospace;
background-color: #ffffff;
/*white-space: pre;*/
}
.csharpcode pre { margin: 0em; }
.csharpcode .rem { color: #008000; }
.csharpcode .kwrd { color: #0000ff; }
.csharpcode .str { color: #006080; }
.csharpcode .op { color: #0000c0; }
.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
.csharpcode .alt
{
background-color: #f4f4f4;
width: 100%;
margin: 0em;
}
.csharpcode .lnum { color: #606060; }

□ Concat()将2个集合串联起来，不剔除重复元素

            int[] arr1 = { 1, 2 };


[code]            int[] arr2 = { 2, 3 };

var query = arr1.Concat(arr2);

DisplayQuery<int>(query);

Console.ReadKey();

[/code]

.csharpcode, .csharpcode pre
{
font-size: small;
color: black;
font-family: consolas, "Courier New", courier, monospace;
background-color: #ffffff;
/*white-space: pre;*/
}
.csharpcode pre { margin: 0em; }
.csharpcode .rem { color: #008000; }
.csharpcode .kwrd { color: #0000ff; }
.csharpcode .str { color: #006080; }
.csharpcode .op { color: #0000c0; }
.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
.csharpcode .alt
{
background-color: #f4f4f4;
width: 100%;
margin: 0em;
}
.csharpcode .lnum { color: #606060; }

结果：1 2 2 3

□ Union()将2个集合串联起来，剔除重复元素

            int[] arr1 = { 1, 2 };


[code]            int[] arr2 = { 2, 3 };

var query = arr1.Union(arr2);

DisplayQuery<int>(query);

Console.ReadKey();

[/code]

.csharpcode, .csharpcode pre
{
font-size: small;
color: black;
font-family: consolas, "Courier New", courier, monospace;
background-color: #ffffff;
/*white-space: pre;*/
}
.csharpcode pre { margin: 0em; }
.csharpcode .rem { color: #008000; }
.csharpcode .kwrd { color: #0000ff; }
.csharpcode .str { color: #006080; }
.csharpcode .op { color: #0000c0; }
.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
.csharpcode .alt
{
background-color: #f4f4f4;
width: 100%;
margin: 0em;
}
.csharpcode .lnum { color: #606060; }

结果：1 2 3

□ Zip()合并2个集合中位置相同的元素，将2个元素的操作结果返回一个新的元素。如果两个集合的长度不相等，以长度短的为准。

            int[] arr1 = {1, 2};


[code]            string[] arr2 = {"星期一", "星期二", "星期三"};

var query = arr1.Zip(arr2, (x, y) => String.Format("{0},{1}", x, y));

DisplayQuery<string>(query);

Console.ReadKey();

总结

● 一般性的条件筛选：Where()

● 返回具体的集合类型再进行链式操作：OfType()

● 非泛型集合转换成泛型集合后再使用LINQ操作符：Cast()

● 排序、链式排序：OrderBy(), ThenBy()，实现IComparer<T>接口可以自定义排序规则

● 投影：Select()

● 返回前N个，跳过N个，分页：Take()和Skip()
● 返回符合/不符合条件，但不执行完遍历：TakeWhile()和SkipWhile()

● 反转集合元素：Reverse()

● 空集合处理：DefaultIfEmpty()

● 剔除集合中的重复元素：Distinct()，实现IEqualityComparer<Category>可自定义相等规则，针对某具体类或写一个泛型方法

● 分组以及分组后投影：GroupBy()

● 2个集合的交集：Intersect()

● 2个集合的查集：Except()

● 2个集合的串联：Concat()和Union()

● 2个集合的合并：Zip()

参考资料：

※ 《.NET之美》--张子阳,感谢写了这么好的书！

※ A Generic IEqualityComparer for Linq Distinct()

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