LINQ 使用方法

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

var lowNums =

from n in numbers

where n < 5

select n;

List<Product> products = GetProductList();

var soldOutProducts =

from p in products

where p.UnitsInStock == 0

select p;

List<Product> products = GetProductList();

var expensiveInStockProducts =

from p in products

where p.UnitsInStock > 0 && p.UnitPrice > 3.00M

select p;

List<Customer> customers = GetCustomerList();

var waCustomers =

from c in customers

where c.Region == "WA"

select c;

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

var shortDigits = digits.Where((digit, index) => digit.Length < index);

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

var numsPlusOne =

from n in numbers

select n + 1;

List<Product> products = GetProductList();

var productNames =

from p in products

select p.ProductName;

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

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

var textNums =

from n in numbers

select strings
;

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

var upperLowerWords =

from w in words

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

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

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

var digitOddEvens =

from n in numbers

select new { Digit = strings
, Even = (n % 2 == 0) };

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

var numsInPlace = numbers.Select((num, index) => new { Num = num, InPlace = (num == index) });

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

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

var lowNums =

from n in numbers

where n < 5

select digits
;

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 };

List<Customer> customers = GetCustomerList();

var orders =

from c in customers

from o in c.Orders

where o.Total < 500.00M

select new { c.CustomerID, o.OrderID, o.Total };

List<Customer> customers = GetCustomerList();

var orders =

from c in customers

from o in c.Orders

where o.OrderDate >= new DateTime(1998, 1, 1)

select new { c.CustomerID, o.OrderID, o.OrderDate };

List<Customer> customers = GetCustomerList();

var orders =

from c in customers

from o in c.Orders

where o.Total >= 2000.0M

select new { c.CustomerID, o.OrderID, o.Total };

List<Customer> customers = GetCustomerList();

DateTime cutoffDate = new DateTime(1997, 1, 1);

var orders =

from c in customers

where c.Region == "WA"

from o in c.Orders

where o.OrderDate >= cutoffDate

select new { c.CustomerID, o.OrderID };

List<Customer> customers = GetCustomerList();

var customerOrders =

customers.SelectMany(

(cust, custIndex) =>

cust.Orders.Select(o => "Customer #" + (custIndex + 1) +

" has an order with OrderID " + o.OrderID));

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

var first3Numbers = numbers.Take(3);

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

var allButFirst4Numbers = numbers.Skip(4);

All but first
4 numbers:

9

8

6

7

2

0

List<Customer> customers = GetCustomerList();

var waOrders =

from c in customers

from o in c.Orders

where c.Region == "WA"

select new { c.CustomerID, o.OrderID, o.OrderDate };

var allButFirst2Orders = waOrders.Skip(2);

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

var firstNumbersLessThan6 = numbers.TakeWhile(n => n < 6);

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

var firstSmallNumbers = numbers.TakeWhile((n, index) => n >= index);

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

var allButFirst3Numbers = numbers.SkipWhile(n => n % 3 != 0);

3

9

8

6

7

2

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

var laterNumbers = numbers.SkipWhile((n, index) => n >= index);

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);

}

}

List<Product> products = GetProductList();

var orderGroups =

from p in products

group p by p.Category into g

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

ObjectDumper.Write(orderGroups, 1);

List<Customer> customers = GetCustomerList();

var customerOrderGroups =

from c in customers

select

new

{

c.CompanyName,

YearGroups =

from o in c.Orders

group o by o.OrderDate.Year into yg

select

new

{

Year = yg.Key,

MonthGroups =

from o in yg

group o by o.OrderDate.Month into mg

select new { Month = mg.Key, Orders = mg }

}

};

GroupBy - Comparer
This sample uses GroupBy to partition trimmed
elements of an array using a custom comparer that matches words that are
anagrams of each other.

[code]1.      public void Linq44()

2.      {

3.          string[] anagrams = { "from   ", " salt", " earn ", "  last   ", " near ", " form  " };

4.

5.          var orderGroups = anagrams.GroupBy(w => w.Trim(), new AnagramEqualityComparer());

6.

7.          ObjectDumper.Write(orderGroups, 1);

8.      }

9.

10.  public class AnagramEqualityComparer : IEqualityComparer<string>

11.  {

12.      public bool Equals(string x, string y)

13.   {

14.   return getCanonicalString(x) == getCanonicalString(y);

15.      }

16.

17.      public int GetHashCode(string obj)

18.      {

19.   return getCanonicalString(obj).GetHashCode();

20.   }

21.

22.      private string getCanonicalString(string word)

23.      {

24.   char[] wordChars = word.ToCharArray();

25.       Array.Sort<char>(wordChars);

26.   return new string(wordChars);

27.      }

28.  }

Result

...

from

form

...

salt

last

...

earn

near

GroupBy - Comparer, Mapped

This sample uses GroupBy to partition trimmed
elements of an array using a custom comparer that matches words that are
anagrams of each other, and then converts the results to uppercase.

1.      public void Linq45()

2.      {

3.          string[] anagrams = { "from   ", " salt", " earn ", "  last   ", " near ", " form  " };

4.

5.          var orderGroups = anagrams.GroupBy(

6.               w => w.Trim(),

7.                      a => a.ToUpper(),

8.                      new AnagramEqualityComparer()

9.               );

10.

11.      ObjectDumper.Write(orderGroups, 1);

12.  }

13.

14.  public class AnagramEqualityComparer : IEqualityComparer<string>

15.  {

16.   public bool Equals(string x, string y)

17.      {

18.          return getCanonicalString(x) == getCanonicalString(y);

19.      }

20.

21.   public int GetHashCode(string obj)

22.      {

23.          return getCanonicalString(obj).GetHashCode();

24.      }

25.

26.      private string getCanonicalString(string word)

27.      {

28.          char[] wordChars = word.ToCharArray();

29.   Array.Sort<char>(wordChars);

30.          return new string(wordChars);

31.      }

32.  }

Result

...

FROM

FORM

...

SALT

LAST

...

EARN

NEAR

[/code]