101个LINQ例子

101 LINQ Samples

说明：本文摘抄自Microsoft的MSDN网站，因为原文不方便快速阅读和学习，故此调整如下。本文所有权利归Microsoft公司所有。如果您觉得本文侵犯了您的著作权，请来信告知，我将即时改正！多谢！

所有的代码运行结果请点击代码上的链接，到MSDN网站去看。

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Restriction Operators

• Where - Simple 1

public void Linq1() {
    int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };
    var lowNums =
        from n in numbers
        where n < 5
        select n;
    Console.WriteLine("Numbers < 5:");
    foreach (var x in lowNums) {
        Console.WriteLine(x);
    }
}

• Where - Simple 2

public void Linq2() {
    List products = GetProductList();
    var soldOutProducts =
        from p in products
        where p.UnitsInStock == 0
        select p;
    Console.WriteLine("Sold out products:");
    foreach (var product in soldOutProducts) {
        Console.WriteLine("{0} is sold out!", product.ProductName);
    }
}

• Where - Simple 3

public void Linq3() {
    List products = GetProductList();
    var expensiveInStockProducts =
        from p in products
        where p.UnitsInStock > 0 && p.UnitPrice > 3.00M
        select p;
    Console.WriteLine("In-stock products that cost more than 3.00:");
    foreach (var product in expensiveInStockProducts) {
        Console.WriteLine("{0} is in stock and costs more than 3.00.", product.ProductName);
    }
}

• Where - Drilldown

public void Linq4() {
    List customers = GetCustomerList();
    var waCustomers =
        from c in customers
        where c.Region == "WA"
        select c;
    Console.WriteLine("Customers from Washington and their orders:");
    foreach (var customer in waCustomers) {
        Console.WriteLine("Customer {0}: {1}", customer.CustomerID, customer.CompanyName);
        foreach (var order in customer.Orders) {
            Console.WriteLine(" Order {0}: {1}", order.OrderID, order.OrderDate);
        }
    }
}

• Where - Indexed

public void Linq5() {
    string[] digits = { "zero", "one", "two", "three", "four", "five", "six", "seven", "eight", "nine" };
    var shortDigits = digits.Where((digit, index) => digit.Length < index);
    Console.WriteLine("Short digits:");
    foreach (var d in shortDigits) {
        Console.WriteLine("The word {0} is shorter than its value.", d);
    }
}

Projection Operators

• Select - Simple 1

public void Linq6() {
    int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };
    var numsPlusOne =
        from n in numbers
        select n + 1;
    Console.WriteLine("Numbers + 1:");
    foreach (var i in numsPlusOne) {
        Console.WriteLine(i);
    }
}

• Select - Simple 2

public void Linq7() {
    List products = GetProductList();
    var productNames =
        from p in products
        select p.ProductName;
    Console.WriteLine("Product Names:");
    foreach (var productName in productNames) {
        Console.WriteLine(productName);
    }
}

• Select - Transformation

public void Linq8() {
    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[n];
    Console.WriteLine("Number strings:");
    foreach (var s in textNums) {
        Console.WriteLine(s);
    }        
}

• Select - Anonymous Types 1

public void Linq9() {
    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);
    }
}

• Select - Anonymous Types 2

public void Linq10() {
    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[n], Even = (n % 2 == 0)};
    foreach (var d in digitOddEvens) {
        Console.WriteLine("The digit {0} is {1}.", d.Digit, d.Even ? "even" : "odd");
    }
}

• Select - Anonymous Types 3

public void Linq11() {
    List products = GetProductList();
    var productInfos =
        from p in products
        select new {p.ProductName, p.Category, Price = p.UnitPrice};
    Console.WriteLine("Product Info:");
    foreach (var productInfo in productInfos) {
        Console.WriteLine("{0} is in the category {1} and costs {2} per unit.", productInfo.ProductName, productInfo.Category, productInfo.Price);
    }
}

• Select - Indexed

public void Linq12() {
    int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };
    var numsInPlace = numbers.Select((num, index) => new {Num = num, InPlace = (num == index)});
    Console.WriteLine("Number: In-place?");
    foreach (var n in numsInPlace) {
        Console.WriteLine("{0}: {1}", n.Num, n.InPlace);
    }
}

• Select - Filtered

public void Linq13() {
    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[n];
    Console.WriteLine("Numbers < 5:");
    foreach (var num in lowNums) {
        Console.WriteLine(num);
    }    
}

• SelectMany - Compound from 1

public void Linq14() {
    int[] numbersA = { 0, 2, 4, 5, 6, 8, 9 };
    int[] numbersB = { 1, 3, 5, 7, 8 };
    var pairs =
        from a in numbersA,
                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);
    }
}

• SelectMany - Compound from 2

public void Linq15() {
    List customers = GetCustomerList();
    var orders =
        from c in customers,
                o in c.Orders
        where o.Total < 500.00M
        select new {c.CustomerID, o.OrderID, o.Total};
    ObjectDumper.Write(orders);
}

• SelectMany - Compound from 3

public void Linq16() {
    List customers = GetCustomerList();
    var orders =
        from c in customers,
                o in c.Orders
        where o.OrderDate >= new DateTime(1998, 1, 1)
        select new {c.CustomerID, o.OrderID, o.OrderDate};
    ObjectDumper.Write(orders);
}

• SelectMany - from Assignment

public void Linq17() {
    List customers = GetCustomerList();
    var orders =
        from c in customers,
                o in c.Orders,
                total = o.Total
        where total >= 2000.0M
        select new {c.CustomerID, o.OrderID, total};
    ObjectDumper.Write(orders);
}

• SelectMany - Multiple from

public void Linq18() {
    List 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};
    ObjectDumper.Write(orders);
}

• SelectMany - Indexed

public void Linq19() {
    List customers = GetCustomerList();
    var customerOrders =
        customers.SelectMany(
            (cust, custIndex) =>
            cust.Orders.Select(o => "Customer #" + (custIndex + 1) +
                                    " has an order with OrderID " + o.OrderID) );
    ObjectDumper.Write(customerOrders);
}

Partitioning Operators

• Take - Simple

public void Linq20() {
            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);
            }
        }

• Take - Nested

public void Linq21() {
            List<Customer> customers = GetCustomerList();
            var first3WAOrders = (
                from c in customers
                from o in c.Orders
                where c.Region == "WA"
                select new {c.CustomerID, o.OrderID, o.OrderDate} )
                .Take(3);
            Console.WriteLine("First 3 orders in WA:");
            foreach (var order in first3WAOrders) {
                ObjectDumper.Write(order);
            }
        }

• Skip - Simple

public void Linq22() {
            int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };
            var allButFirst4Numbers = numbers.Skip(4);
            Console.WriteLine("All but first 4 numbers:");
            foreach (var n in allButFirst4Numbers) {
                Console.WriteLine(n);
            }
        }

• Skip - Nested

public void Linq23() {
            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);
            Console.WriteLine("All but first 2 orders in WA:");
            foreach (var order in allButFirst2Orders) {
                ObjectDumper.Write(order);
            }
        }

• TakeWhile - Simple

public void Linq24() {
            int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };
            var firstNumbersLessThan6 = numbers.TakeWhile(n => n < 6);
            Console.WriteLine("First numbers less than 6:");
            foreach (var n in firstNumbersLessThan6) {
                Console.WriteLine(n);
            }
        }

• SkipWhile - Simple

public void Linq26() {
            int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };
            var allButFirst3Numbers = numbers.SkipWhile(n => n % 3 != 0);
            Console.WriteLine("All elements starting from first element divisible by 3:");
            foreach (var n in allButFirst3Numbers) {
                Console.WriteLine(n);
            }
        }

• SkipWhile - Indexed

public void Linq27() {
            int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };
            var laterNumbers = numbers.SkipWhile((n, index) => n >= index);
            Console.WriteLine("All elements starting from first element less than its position:");
            foreach (var n in laterNumbers) {
                Console.WriteLine(n);
            }
        }

Ordering Operators

• OrderBy - Simple 1

publicvoid Linq28() {
    string[] words = { "cherry", "apple", "blueberry" };
    
    var sortedWords =
        from w in words
        orderby w
        select w;
    
    Console.WriteLine("The sorted list of words:");
    foreach (var w in sortedWords) {
        Console.WriteLine(w);
    }
}

• OrderBy - Simple 2

public void Linq29() {
    string[] words = { "cherry", "apple", "blueberry" };
    var sortedWords =
        from w in words
        orderby w.Length
        select w;
    Console.WriteLine("The sorted list of words (by length):");
    foreach (var w in sortedWords) {
        Console.WriteLine(w);
    }
}

• OrderBy - Simple 3

public void Linq30() {
    List products = GetProductList();
    var sortedProducts =
        from p in products
        orderby p.ProductName
        select p;
    ObjectDumper.Write(sortedProducts);
}

• OrderBy - Comparer

public class CaseInsensitiveComparer : IComparer<string>
{
    public int Compare(string x, string y)
    {
        return string.Compare(x, y, true);
    }
}
public void Linq31() {
    string[] words = { "aPPLE", "AbAcUs", "bRaNcH", "BlUeBeRrY", "ClOvEr", "cHeRry"};
    var sortedWords = words.OrderBy(a => a, new CaseInsensitiveComparer());
    ObjectDumper.Write(sortedWords);
}

• OrderByDescending - Simple 1

public void Linq32() {
    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);
    }
}

• OrderByDescending - Simple 2

public void Linq33() {
    List products = GetProductList();
    var sortedProducts =
        from p in products
        orderby p.UnitsInStock descending
        select p;
    ObjectDumper.Write(sortedProducts);
}

• OrderByDescending - Comparer

public class CaseInsensitiveComparer : IComparerspan class="qs-keyword">string>
{
    publicint Compare(string x, string y)
    {
        returnstring.Compare(x, y, true);
    }
}

publicvoid Linq34() {
    string[] words = { "aPPLE", "AbAcUs", "bRaNcH", "BlUeBeRrY", "ClOvEr", "cHeRry"};
    
    var sortedWords = words.OrderByDescending(a => a, new CaseInsensitiveComparer());
        
    ObjectDumper.Write(sortedWords);
}

• ThenBy - Simple

publicvoid Linq35() {
    string[] digits = { "zero", "one", "two", "three", "four", "five", "six", "seven", "eight", "nine" };

    var sortedDigits =
        from d in digits 
        orderby d.Length, d
        select d;

    Console.WriteLine("Sorted digits:");
    foreach (var d in sortedDigits) {
        Console.WriteLine(d);
    }
}

• ThenBy - Comparer

public class CaseInsensitiveComparer : IComparerspan class="qs-keyword">string>
{
    publicint Compare(string x, string y)
    {
        returnstring.Compare(x, y, true);
    }
}

publicvoid Linq36() {
    string[] words = { "aPPLE", "AbAcUs", "bRaNcH", "BlUeBeRrY", "ClOvEr", "cHeRry"};
    
    var sortedWords =
        words.OrderBy(a => a.Length)
                .ThenBy(a => a, new CaseInsensitiveComparer());
        
    ObjectDumper.Write(sortedWords);
}

• ThenByDescending - Simple

publicvoid Linq37() {
    List products = GetProductList();var sortedProducts =
        from p in products
        orderby p.Category, p.UnitPrice descendingselect p;

    ObjectDumper.Write(sortedProducts);
}

• ThenByDescending - Comparer

public class CaseInsensitiveComparer : IComparerspan class="qs-keyword">string>
{
    publicint Compare(string x, string y)
    {
        returnstring.Compare(x, y, true);
    }
}

publicvoid Linq38() {
    string[] words = { "aPPLE", "AbAcUs", "bRaNcH", "BlUeBeRrY", "ClOvEr", "cHeRry"};
    
    var sortedWords =
        words.OrderBy(a => a.Length)
                .ThenByDescending(a => a, new CaseInsensitiveComparer());
        
    ObjectDumper.Write(sortedWords);
}

• Reverse

publicvoid Linq39() {
    string[] digits = { "zero", "one", "two", "three", "four", "five", "six", "seven", "eight", "nine" };
    
    var reversedIDigits = (
        from d in digits
        where d[1] == 'i'
        select d)
        .Reverse();
    
    Console.WriteLine("A backwards list of the digits with a second character of 'i':");
    foreach (var d in reversedIDigits) {
        Console.WriteLine(d);
    }             
}

Grouping Operators

• GroupBy - Simple 1

public void Linq40() {
            int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };
            var numberGroups =
                from n in numbers
                group n by n % 5 into g
                select new { Remainder = g.Key, Numbers = g };
            foreach (var g in numberGroups) {
                Console.WriteLine("Numbers with a remainder of {0} when divided by 5:", g.Remainder);
                foreach (var n in g.Numbers) {
                    Console.WriteLine(n);
                }
            }
}

• GroupBy - Simple 2

public void Linq41() {
            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);
                }
            }
        }

• GroupBy - Simple 3

public void Linq42() {
            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);
        }

• GroupBy - Nested

public void Linq43() {
            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 }
                                     }
                        };
            ObjectDumper.Write(customerOrderGroups, 3);
        }

• GroupBy - Comparer

public class AnagramEqualityComparer : IEqualityComparer 
{ 
public bool Equals(string x, string y) { return getCanonicalString(x) == getCanonicalString(y); } 
public int GetHashCode(string obj) { return getCanonicalString(obj).GetHashCode(); } 
private string getCanonicalString(string word) 
{ 
    char[] wordChars = word.ToCharArray(); Array.Sort(wordChars); return new string(wordChars); 
} 
} 

publicvoid Linq44() 
{ 
string[] anagrams = {"from ", " salt", " earn ", " last ", " near ", " form "}; 
var orderGroups = anagrams.GroupBy(w => w.Trim(), new AnagramEqualityComparer()); 
ObjectDumper.Write(orderGroups, 1); 
}

• GroupBy - Comparer, Mapped

public void Linq45() {
            string[] anagrams = {"from ", " salt", " earn ", " last ", " near ", " form "};
            var orderGroups = anagrams.GroupBy(
                        w => w.Trim(),
                        a => a.ToUpper(),
                        new AnagramEqualityComparer()
                        );
            ObjectDumper.Write(orderGroups, 1);
        }
public class AnagramEqualityComparer : IEqualityComparer<string>
        {
            public bool Equals(string x, string y) {
                return getCanonicalString(x) == getCanonicalString(y);
            }
            public int GetHashCode(string obj) {
                return getCanonicalString(obj).GetHashCode();
            }
            private string getCanonicalString(string word) {
                char[] wordChars = word.ToCharArray();
                Array.Sort<char>(wordChars);
                return new string(wordChars);
            }
        }

Set Operators

• Distinct - 1

publicvoid Linq46() {
    int[] factorsOf300 = { 2, 2, 3, 5, 5 };
    
    var uniqueFactors = factorsOf300.Distinct();

    Console.WriteLine("Prime factors of 300:");
    foreach (var f in uniqueFactors) {
        Console.WriteLine(f);
    }
}

• Distinct - 2

public void Linq47() {
    List products = GetProductList();
    var categoryNames = (
        from p in products
        select p.Category)
        .Distinct();
                                            
    Console.WriteLine("Category names:");
    foreach (var n in categoryNames) {
        Console.WriteLine(n);
    }
}

• Union - 1

publicvoid Linq48() {
    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);
    }
}

• Union - 2

publicvoid Linq49() {
    List products = GetProductList();List customers = GetCustomerList();
    
    var productFirstChars =
        from p in products
        select p.ProductName[0];
    var customerFirstChars =
        from c in customers
        select c.CompanyName[0];
    
    var uniqueFirstChars = productFirstChars.Union(customerFirstChars);
    
    Console.WriteLine("Unique first letters from Product names and Customer names:");
    foreach (var ch in uniqueFirstChars) {
        Console.WriteLine(ch);
    }
}

• Intersect - 1

publicvoid Linq50() {
    int[] numbersA = { 0, 2, 4, 5, 6, 8, 9 };
    int[] numbersB = { 1, 3, 5, 7, 8 };
    
    var commonNumbers = numbersA.Intersect(numbersB);
    
    Console.WriteLine("Common numbers shared by both arrays:");
    foreach (var n in commonNumbers) {
        Console.WriteLine(n);
    }
}

• Intersect - 2

publicvoid Linq51() {
    List products = GetProductList();
    List customers = GetCustomerList();
    
    var productFirstChars =
        from p in products
        select p.ProductName[0];
    var customerFirstChars =
        from c in customers
        select c.CompanyName[0];
    
    var commonFirstChars = productFirstChars.Intersect(customerFirstChars);
    
    Console.WriteLine("Common first letters from Product names and Customer names:");
    foreach (var ch in commonFirstChars) {
        Console.WriteLine(ch);
    }
}

• Except - 1

public void Linq52() {
    int[] numbersA = { 0, 2, 4, 5, 6, 8, 9 };
    int[] numbersB = { 1, 3, 5, 7, 8 };
    IEnumerable<int> aOnlyNumbers = numbersA.Except(numbersB);
    Console.WriteLine("Numbers in first array but not second array:");
    foreach (var n in aOnlyNumbers) {
        Console.WriteLine(n);
    }
}

• Except - 2

public void Linq53() {
    List products = GetProductList();
    List customers = GetCustomerList();
    var productFirstChars =
        from p in products
        select p.ProductName[0];
    var customerFirstChars =
        from c in customers
        select c.CompanyName[0];
    var productOnlyFirstChars = productFirstChars.Except(customerFirstChars);
    Console.WriteLine("First letters from Product names, but not from Customer names:");
    foreach (var ch in productOnlyFirstChars) {
        Console.WriteLine(ch);
    }
}

Conversion Operators

• To Array

public void Linq54() {
    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]);
    }
}

• To List

public void Linq55() {
    string[] words = { "cherry", "apple", "blueberry" };
    var sortedWords =
        from w in words
        orderby w
        select w;
    var wordList = sortedWords.ToList();
    Console.WriteLine("The sorted word list:");
    foreach (var w in wordList) {
        Console.WriteLine(w);
    }
}

• To Dictionary

public void Linq56() {
    var scoreRecords = new [] { new {Name = "Alice", Score = 50},
                                new {Name = "Bob" , Score = 40},
                                new {Name = "Cathy", Score = 45}
                              };
    var scoreRecordsDict = scoreRecords.ToDictionary(sr => sr.Name);
    Console.WriteLine("Bob's score: {0}", scoreRecordsDict["Bob"]);
}

• OfType

public void Linq57() {
    object[] numbers = { null, 1.0, "two", 3, 4.0f, 5, "six", 7.0 };
    var doubles = numbers.OfType<double>();
    Console.WriteLine("Numbers stored as doubles:");
    foreach (var d in doubles) {
        Console.WriteLine(d);
    }
}

Element Operators

• First - Simple

public void Linq58() {
    List products = GetProductList();
    Product product12 = (
        from p in products
        where p.ProductID == 12
        select p )
        .First();
    ObjectDumper.Write(product12);
}

• First - Indexed

public void Linq60() {
    int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };
    int evenNum = numbers.First((num, index) => (num % 2 == 0) && (index % 2 == 0));
    Console.WriteLine("{0} is an even number at an even position within the list.", evenNum);
}

• FirstOrDefault - Simple

public void Linq61() {
    int[] numbers = {};
    int firstNumOrDefault = numbers.FirstOrDefault();
    Console.WriteLine(firstNumOrDefault);
}

• FirstOrDefault - Condition

public void Linq62() {
    List products = GetProductList();
    Product product789 = products.FirstOrDefault(p => p.ProductID == 789);
    Console.WriteLine("Product 789 exists: {0}", product789 != null);
}

• FirstOrDefault - Indexed

public void Linq63() {
    double?[] doubles = { 1.7, 2.3, 4.1, 1.9, 2.9 };
    double? num = doubles.FirstOrDefault((n, index) => (n >= index - 0.5 && n <= index + 0.5));
    if (num != null)
        Console.WriteLine("The value {1} is within 0.5 of its index position.", num);
    else
        Console.WriteLine("There is no number within 0.5 of its index position.", num);
}

• ElementAt

public void Linq64() {
    int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };
    int fourthLowNum = (
        from n in numbers
        where n < 5
        select n )
        .ElementAt(3); // 3 because sequences use 0-based indexing
    Console.WriteLine("Fourth number < 5: {0}", fourthLowNum);
}

Generation Operators

• Range

public void Linq65() {
   var numbers =
      from n in Sequence.Range(100, 50)
      selectnew {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

public void Linq66() {
   var numbers = Sequence.Repeat(7, 10);
   foreach (var n in numbers) {
      Console.WriteLine(n);
   }
}

Quantifiers

• Any - Simple

public void Linq67() {
   string[] words = { "believe", "relief", "receipt", "field" };
   bool iAfterE = words.Any(w => w.Contains("ei"));
   Console.WriteLine("There is a word that contains in the list that contains 'ei': {0}", iAfterE);
}

• Any - Indexed

public void Linq68() {
   int[] numbers = { -9, -4, -8, -3, -5, -2, -1, -6, -7 };
   bool negativeMatch = numbers.Any((n, index) => n == -index);
   Console.WriteLine("There is a number that is the negative of its index: {0}", negativeMatch);
}

• Any - Grouped

public void Linq69() {
   List products = GetProductList();
   var productGroups =
      from p in products
      group p by p.Category into g
      where g.Group.Any(p => p.UnitsInStock == 0)
      select new {Category = g.Key, Products = g.Group};
   ObjectDumper.Write(productGroups, 1);
}

• All - Simple

public void Linq70() {
   int[] numbers = { 1, 11, 3, 19, 41, 65, 19 };
   bool onlyOdd = numbers.All(n => n % 2 == 1);
   Console.WriteLine("The list contains only odd numbers: {0}", onlyOdd);
}

• All - Indexed

public void Linq71() {
   int[] lowNumbers = { 1, 11, 3, 19, 41, 65, 19 };
   int[] highNumbers = { 7, 19, 42, 22, 45, 79, 24 };
   bool allLower = lowNumbers.All((num, index) => num < highNumbers[index]);
   Console.WriteLine("Each number in the first list is lower than its counterpart in the second list: {0}", allLower);
}

• All - Grouped

public void Linq72() {
   List products = GetProductList();
   var productGroups =
      from p in products
      group p by p.Category into g
      where g.Group.All(p => p.UnitsInStock > 0)
      select new {Category = g.Key, Products = g.Group};
   ObjectDumper.Write(productGroups, 1);
}

Aggregate Operators

• Count - Simple

public void Linq73() {
   int[] factorsOf300 = { 2, 2, 3, 5, 5 };
   int uniqueFactors = factorsOf300.Distinct().Count();
   Console.WriteLine("There are {0} unique factors of 300.", uniqueFactors);
}

• Count - Conditional

public void Linq74() {
   int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };
   int oddNumbers = numbers.Count(n => n % 2 == 1);
   Console.WriteLine("There are {0} odd numbers in the list.", oddNumbers);
}

• Count - Indexed

public void Linq75() {
   int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };
   int oddEvenMatches = numbers.Count((n, index) => n % 2 == index % 2);
   Console.WriteLine("There are {0} numbers in the list whose odd/even status " +
        "matches that of their position.", oddEvenMatches);
}

• Count - Nested

public void Linq76() {
   List customers = GetCustomerList();
   var orderCounts =
      from c in customers
      select new {c.CustomerID, OrderCount = c.Orders.Count()};
   ObjectDumper.Write(orderCounts);
}

• Count - Grouped

public void Linq77() {
   List products = GetProductList();
   var categoryCounts =
      from p in products
      group p by p.Category into g
      select new {Category = g.Key, ProductCount = g.Group.Count()};
   ObjectDumper.Write(categoryCounts);
}

• Sum - Simple

public void Linq78() {
   int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };
   double numSum = numbers.Sum();
   Console.WriteLine("The sum of the numbers is {0}.", numSum);
}

• Sum - Projection

public void Linq79() {
   string[] words = { "cherry", "apple", "blueberry" };
   double totalChars = words.Sum(w => w.Length);
   Console.WriteLine("There are a total of {0} characters in these words.", totalChars);
}

• Sum - Grouped

public void Linq80() {
   List products = GetProductList();
   var categories =
      from p in products
      group p by p.Category into g
      select new {Category = g.Key, TotalUnitsInStock = g.Group.Sum(p => p.UnitsInStock)};
   ObjectDumper.Write(categories);
}

• Min - Simple

public void Linq81() {
   int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };
   int minNum = numbers.Min();
   Console.WriteLine("The minimum number is {0}.", minNum);
}

• Min - Projection

public void Linq82() {
   string[] words = { "cherry", "apple", "blueberry" };
   int shortestWord = words.Min(w => w.Length);
   Console.WriteLine("The shortest word is {0} characters long.", shortestWord);
}

• Min - Grouped

public void Linq83() {
   List products = GetProductList();
   var categories =
      from p in products
      group p by p.Category into g
      select new {Category = g.Key, CheapestPrice = g.Group.Min(p => p.UnitPrice)};
   ObjectDumper.Write(categories);
}

• Min - Elements

public void Linq84() {
   List products = GetProductList();
   var categories =
      from p in products
      group p by p.Category into g
      from minPrice = g.Group.Min(p => p.UnitPrice)
      select new {Category = g.Key, CheapestProducts = g.Group.Where(p => p.UnitPrice == minPrice)};
   ObjectDumper.Write(categories, 1);
}

• Max - Simple

public void Linq85() {
   int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };
   int maxNum = numbers.Max();
   Console.WriteLine("The maximum number is {0}.", maxNum);
}

• Max - Projection

public void Linq86() {
   string[] words = { "cherry", "apple", "blueberry" };
   int longestLength = words.Max(w => w.Length);
   Console.WriteLine("The longest word is {0} characters long.", longestLength);
}

• Max - Grouped

public void Linq87() {
   List products = GetProductList();
   var categories =
      from p in products
      group p by p.Category into g
      select new {Category = g.Key, MostExpensivePrice = g.Group.Max(p => p.UnitPrice)};
   ObjectDumper.Write(categories);
}

• Max - Elements

public void Linq88() {
   List products = GetProductList();
   var categories =
      from p in products
      group p by p.Category into g
      from maxPrice = g.Group.Max(p => p.UnitPrice)
      select new {Category = g.Key, MostExpensiveProducts = g.Group.Where(p => p.UnitPrice == maxPrice)};
   ObjectDumper.Write(categories, 1);
}

• Average - Simple

public void Linq89() {
   int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };
   double averageNum = numbers.Average();
   Console.WriteLine("The average number is {0}.", averageNum);
}

• Average - Projection

public void Linq90() {
   string[] words = { "cherry", "apple", "blueberry" };
   double averageLength = words.Average(w => w.Length);
   Console.WriteLine("The average word length is {0} characters.", averageLength);
}

• Average - Grouped

public void Linq91() {
   List products = GetProductList();
   var categories =
      from p in products
      group p by p.Category into g
      select new {Category = g.Key, AveragePrice = g.Group.Average(p => p.UnitPrice)};
   ObjectDumper.Write(categories);
}

• Fold - Simple

public void Linq92() {
   double[] doubles = { 1.7, 2.3, 1.9, 4.1, 2.9 };
   double product = doubles.Fold((runningProduct, nextFactor) => runningProduct * nextFactor);
   Console.WriteLine("Total product of all numbers: {0}", product);
}

• Fold - Seed

public void Linq93() {
   double startBalance = 100.0;
   int[] attemptedWithdrawals = { 20, 10, 40, 50, 10, 70, 30 };
   double endBalance =
      attemptedWithdrawals.Fold(startBalance,
         (balance, nextWithdrawal) =>
            ( (nextWithdrawal <= balance) ? (balance - nextWithdrawal) : balance ) );
   Console.WriteLine("Ending balance: {0}", endBalance);
}

Miscellaneous Operators

• Concat - 1

public void Linq94() {
    int[] numbersA = { 0, 2, 4, 5, 6, 8, 9 };
    int[] numbersB = { 1, 3, 5, 7, 8 };
    var allNumbers = numbersA.Concat(numbersB);
    Console.WriteLine("All numbers from both arrays:");
    foreach (var n in allNumbers) {
        Console.WriteLine(n);
    }
}

• Concat - 2

public void Linq95() {
    List customers = GetCustomerList();
    List products = GetProductList();
    var customerNames =
        from c in customers
        select c.CompanyName;
    var productNames =
        from p in products
        select p.ProductName;
    var allNames = customerNames.Concat(productNames);
    Console.WriteLine("Customer and product names:");
    foreach (var n in allNames) {
        Console.WriteLine(n);
    }
}

• EqualAll - 1

public void Linq96() {
    var wordsA = new string[] { "cherry", "apple", "blueberry" };
    var wordsB = new string[] { "cherry", "apple", "blueberry" };
    bool match = wordsA.EqualAll(wordsB);
    Console.WriteLine("The sequences match: {0}", match);
}

• EqualAll - 2

public void Linq97() {
    var wordsA = new string[] { "cherry", "apple", "blueberry" };
    var wordsB = new string[] { "apple", "blueberry", "cherry" };
    bool match = wordsA.EqualAll(wordsB);
    Console.WriteLine("The sequences match: {0}", match);
}

Custom Sequence Operators

• Combine

public static class CustomSequenceOperators
{
    public static IEnumerable Combine(this IEnumerable first, IEnumerable second, Func func) {
        using (IEnumerator e1 = first.GetEnumerator(), e2 = second.GetEnumerator()) {
            while (e1.MoveNext() && e2.MoveNext()) {
                yield return func(e1.Current, e2.Current);
            }
        }
    }
}
public void Linq98() {           
    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);
}

Query Execution

• Deferred

public void Linq99() {
    // Sequence operators form first-class queries that
    // are not executed until you enumerate over them.
    int[] numbers = new int[] { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };
    int i = 0;
    var q =
        from n in numbers
        select ++i;
    // Note, the local variable 'i' is not incremented
    // until each element is evaluated (as a side-effect):
    foreach (var v in q) {
        Console.WriteLine("v = {0}, i = {1}", v, i);        
    }
}

• Immediate

public void Linq100() {
    // Methods like ToList() cause the query to be
    // executed immediately, caching the results.
    int[] numbers = new int[] { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };       
    int i = 0;
    var q = (
        from n in numbers
        select ++i )
        .ToList();
    // The local variable i has already been fully
    // incremented before we iterate the results:
    foreach (var v in q) {
        Console.WriteLine("v = {0}, i = {1}", v, i);
    }
}

• Query Reuse

public void Linq101() {
    // Deferred execution lets us define a query once
    // and then reuse it later after data changes.
    int[] numbers = new int[] { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };
    var lowNumbers =
        from n in numbers
        where n <= 3
        select n;
    Console.WriteLine("First run numbers <= 3:");
    foreach (int n in lowNumbers) {
        Console.WriteLine(n);
    }
    for (int i = 0; i < 10; i++) {
        numbers[i] = -numbers[i];
    }
    // During this second run, the same query object,
    // lowNumbers, will be iterating over the new state
    // of numbers[], producing different results:
    Console.WriteLine("Second run numbers <= 3:");
    foreach (int n in lowNumbers) {
        Console.WriteLine(n);
    }
}