LINQ表达式用法整理

收集一些Linq表达式中的一些比较常用的写法，希望能给大家工作当中带来一些便利。

1. Where子句条件过滤结果集型（集合数据使用这种，譬如数组、列表数据，同样适用于Datatable等多列数据集）

类SQL语句的写法，对集合进行运算；

int[] factorsOf300 = { 2, 2, 3, 5, 5 };

var filters = from p in factorsOf300 where p<5 select p;

foreach (var filter in filters)

{

Console.WriteLine("Value: " + Convert.ToString(filter));

}

2. 对数据集进行函数运算型(特别注意select方法里面需要用new关键字来确定记录子集合）

string[] words = { "aPPLE", "BlUeBeRrY", "cHeRry" };

var upperLowerWords = from w in words

select new { Upper = w.ToUpper(), Lower = w.ToLower() };

foreach (var ul in upperLowerWords)

{

Console.WriteLine("Uppercase: {0}, Lowercase: {1}", ul.Upper, ul.Lower);

}

3. 多个数据子集进行复合计算：

int[] numbersA = { 0, 2, 4, 5, 6, 8, 9 };

int[] numbersB = { 1, 3, 5, 7, 8 };

var pairs = from a in numbersA

from b in numbersB

where a < b

select new { a, b };

Console.WriteLine("Pairs where a < b:");

foreach (var pair in pairs)

{

Console.WriteLine("{0} is less than {1}", pair.a, pair.b);

}

4. 子集运算，其中常见的算符有 Take, Skip, TakeWhile, SkipWhile.

int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };

var first3Numbers = numbers.Take(3);

Console.WriteLine("First 3 numbers:");

foreach (var n in first3Numbers)

{

Console.WriteLine(n);

}

5. 排序运算，

double[] doubles = { 1.7, 2.3, 1.9, 4.1, 2.9 };

var sortedDoubles = from d in doubles

orderby d descending select d;

Console.WriteLine("The doubles from highest to lowest:");

foreach (var d in sortedDoubles)

{

Console.WriteLine(d);

}

6. 分组运算

string[] words = { "blueberry", "chimpanzee", "abacus", "banana", "apple", "cheese" };

var wordGroups = from w in words

group w by w[0] into g

select new { FirstLetter = g.Key, Words = g };

foreach (var g in wordGroups)

{

Console.WriteLine("Words that start with the letter '{0}':", g.FirstLetter);

foreach (var w in g.Words)

{

Console.WriteLine(w);

}

}

7. 集合运算，算符包括Distinct, Union, Intersect, Except。

int[] numbersA = { 0, 2, 4, 5, 6, 8, 9 };

int[] numbersB = { 1, 3, 5, 7, 8 };

var uniqueNumbers = numbersA.Union(numbersB);

Console.WriteLine("Unique numbers from both arrays:");

foreach (var n in uniqueNumbers)

{

Console.WriteLine(n);

}

此集合输出为进行整合过后的一个数据集；

8. 转换操作，主要是包含 ToArray, ToList, ToDictionary, OfType这几个操作指令。

double[] doubles = { 1.7, 2.3, 1.9, 4.1, 2.9 };

var sortedDoubles = from d in doubles

orderby d descending

select d;

var doublesArray = sortedDoubles.ToArray();

Console.WriteLine("Every other double from highest to lowest:");

for (int d = 0; d < doublesArray.Length; d += 2)

{

Console.WriteLine(doublesArray[d]);

}

9. 元素操作：

-- First - Simple

-- First - Condition

-- FirstOrDefault - Simple

-- FirstOrDefault - Condition

-- ElementAt

string[] strings = { "zero", "one", "two", "three", "four", "five", "six", "seven", "eight", "nine" };

string startsWithO = strings.First(s => s[0] == 'o');

10. 子序列生成功能：

Range功能:

var numbers = from n in Enumerable.Range(100, 50)

select new { Number = n, OddEven = n % 2 == 1 ? "odd" : "even" };

foreach (var n in numbers)

{

Console.WriteLine("The number {0} is {1}.", n.Number, n.OddEven);

}

Repeat功能:

var numbers = Enumerable.Repeat(7, 10);

foreach (var n in numbers)

{

Console.WriteLine(n);

}

11.集合筛选判断功能：

-- Any - Simple

-- Any - Grouped

-- All - Simple

-- All - Grouped

- Any - Simple

string[] words = { "believe", "relief", "receipt", "field" };

bool iAfterE = words.Any(w => w.Contains("ei"));

-- Any - Grouped功能：

List products = GetProductList();

var productGroups =

from p in products

group p by p.Category into g

where g.Any(p => p.UnitsInStock == 0)

select new { Category = g.Key, Products = g };

12.数据集的聚合功能： Count, Sum, Min, Max, Average, Aggregate函数

int[] factorsOf300 = { 2, 2, 3, 5, 5 };

int uniqueFactors = factorsOf300.Distinct().Count();

13.集合叠加以及顺序比较功能： Concat，EqualAll函数

int[] numbersA = { 0, 2, 4, 5, 6, 8, 9 };

int[] numbersB = { 1, 3, 5, 7, 8 };

var allNumbers = numbersA.Concat(numbersB);

14.自定义函数功能：

int[] vectorA = { 0, 2, 4, 5, 6 };

int[] vectorB = { 1, 3, 5, 7, 8 };

int dotProduct = vectorA.Combine(vectorB, (a, b) => a * b).Sum();

Console.WriteLine("Dot product: {0}", dotProduct);

15. Query的使用：Deferred Execution、Immediate Execution、Query Reuse。

int[] numbers = new int[] { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };

int i = 0;

var q = from n in numbers select ++n;

foreach (var v in q)

{

Console.WriteLine("v = {0}, i = {1}", v, ++i);

}

16. 集合连接运算： Cross Join、Group Join、Cross Join with Group Join、Left Outer Join。

string[] productsPrefix = new string[]{

"B",

"D",

"V",

"D",

"R"

};

string[] categories = new string[]{

"Beverages",

"Condiments",

"Vegetables",

"Dairy Products",

"Seafood" };

var q =

from p in productsPrefix

join c in categories on p equals c.Substring(0, 1)

select new { p, Category = c};

foreach (var v in q)

{

Console.WriteLine(v.p + ": " + v.Category);

}

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