new {Population = 1000}, new {Population = 2000}, new {Population = 4000}

};

var averagePopulation = cities.Average(c => c.Population); Console.WriteLine(averagePopulation);

// 2333,33

This method calculates the average of enumerable using delegated function.

Section 8.38: GroupBy

var persons = new[] {

new { Name="Fizz", Job="Developer"}, new { Name="Buzz", Job="Developer"}, new { Name="Foo", Job="Astronaut"}, new { Name="Bar", Job="Astronaut"},

};

var groupedByJob = persons.GroupBy(p => p.Job);

foreach(var theGroup in groupedByJob)

{

Console.WriteLine( "{0} are {1}s",

string.Join(",", theGroup.Select(g => g.Name).ToArray()), theGroup.Key);

}

//Fizz,Buzz are Developers //Foo,Bar are Astronauts

Group invoices by country, generating a new object with the number of record, total paid, and average paid

var a = db.Invoices.GroupBy(i => i.Country)

.Select(g => new { Country = g.Key, Count = g.Count(),

Total = g.Sum(i => i.Paid),

Average = g.Average(i => i.Paid) });

If we want only the totals, no group

var a = db.Invoices.GroupBy(i => 1)

.Select(g => new { Count = g.Count(),

Total = g.Sum(i => i.Paid),

Average = g.Average(i => i.Paid) });

If we need several counts

var a = db.Invoices.GroupBy(g => 1)

.Select(g => new { High = g.Count(i => i.Paid >= 1000), Low = g.Count(i => i.Paid < 1000), Sum = g.Sum(i => i.Paid) });

Section 8.39: ToDictionary

Returns a new dictionary from the source IEnumerable using the provided keySelector function to determine keys.

Section 8.41: ToArray

var numbers = new[] {1,2,3,4,5,6,7,8,9,10}; var someNumbers = numbers.Where(n => n < 6);

Console.WriteLine(someNumbers.GetType().Name);

//WhereArrayIterator`1

var someNumbersArray = someNumbers.ToArray();

Console.WriteLine(someNumbersArray.GetType().Name);

//Int32[]

Section 8.42: ToList

var numbers = new[] {1,2,3,4,5,6,7,8,9,10}; var someNumbers = numbers.Where(n => n < 6);

Console.WriteLine(someNumbers.GetType().Name);

//WhereArrayIterator`1

var someNumbersList = someNumbers.ToList();

Console.WriteLine( someNumbersList.GetType().Name + " - " +

someNumbersList.GetType().GetGenericArguments()[0].Name);

//List`1 - Int32

Section 8.43: ElementAt

var names = new[] {"Foo","Bar","Fizz","Buzz"};

var thirdName = names.ElementAt(2); Console.WriteLine(thirdName); //Fizz

//The following throws ArgumentOutOfRangeException

var minusOnethName = names.ElementAt(-1); var fifthName = names.ElementAt(4);

Section 8.44: ElementAtOrDefault

var names = new[] {"Foo","Bar","Fizz","Buzz"};

var thirdName = names.ElementAtOrDefault(2); Console.WriteLine(thirdName); //Fizz

var minusOnethName = names.ElementAtOrDefault(-1); Console.WriteLine(minusOnethName); //null

var fifthName = names.ElementAtOrDefault(4); Console.WriteLine(fifthName); //null

Section 8.45: SkipWhile

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

var oddNumbers = numbers.SkipWhile(n => (n & 1) == 0);

Console.WriteLine(string.Join(",", oddNumbers.ToArray()));

//1,3,5,7

Section 8.46: TakeWhile

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

var evenNumbers = numbers.TakeWhile(n => (n & 1) == 0);

Console.WriteLine(string.Join(",", evenNumbers.ToArray()));

//2,4,6

Section 8.47: DefaultIfEmpty

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

var numbersOrDefault = numbers.DefaultIfEmpty(); Console.WriteLine(numbers.SequenceEqual(numbersOrDefault)); //True

var noNumbers = new int[0];

var noNumbersOrDefault = noNumbers.DefaultIfEmpty(); Console.WriteLine(noNumbersOrDefault.Count()); //1 Console.WriteLine(noNumbersOrDefault.Single()); //0

var noNumbersOrExplicitDefault = noNumbers.DefaultIfEmpty(34); Console.WriteLine(noNumbersOrExplicitDefault.Count()); //1 Console.WriteLine(noNumbersOrExplicitDefault.Single()); //34

Section 8.48: Join

class Developer

{

public int Id { get; set; } public string Name { get; set; }

}

class Project

{

public int DeveloperId { get; set; } public string Name { get; set; }

}

var developers = new[] { new Developer {

Id = 1,

Name = "Foobuzz"

},

new Developer { Id = 2,

Name = "Barfizz"

}

};

var projects = new[] {