Standard Query Operators

Remarks#

Linq queries are written using the Standard Query Operators (which are a set of extension methods that operates mainly on objects of type IEnumerable<T> and IQueryable<T>) or using Query Expressions (which at compile time, are converted to Standard Query Operator method calls).

Query operators provide query capabilities including filtering, projection, aggregation, sorting and more.

Concatenation Operations

Concatenation refers to the operation of appending one sequence to another.

Concat

Concatenates two sequences to form one sequence.

Method Syntax

// Concat

var numbers1 = new int[] { 1, 2, 3 };
var numbers2 = new int[] { 4, 5, 6 };

var numbers = numbers1.Concat(numbers2);

// numbers = { 1, 2, 3, 4, 5, 6 }

Query Syntax

// Not applicable.

Filtering Operations

Filtering refers to the operations of restricting the result set to contain only those elements that satisfy a specified condition.

Where

Selects values that are based on a predicate function.

Method Syntax

// Where

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

var evens = numbers.Where(n => n % 2 == 0);

// evens = { 2, 4, 6, 8 }

Query Syntax

// where

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

var odds = from n in numbers
           where n % 2 != 0
           select n;

// odds = { 1, 3, 5, 7 }

OfType

Selects values, depending on their ability to be cast to a specified type.

Method Syntax

// OfType

var numbers = new object[] { 1, "one", 2, "two", 3, "three" };

var strings = numbers.OfType<string>();

// strings = { "one", "two", "three" }

Query Syntax

// Not applicable.

Join Operations

A join of two data sources is the association of objects in one data source with objects that share a common attribute in another data source.

Join

Joins two sequences based on key selector functions and extracts pairs of values.

Method Syntax

// Join

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

class Order
{
    public string Description { get; set; }
    public int CustomerId { get; set; }
}
...

var customers = new Customer[] 
{
    new Customer { Id = 1, Name = "C1" },
    new Customer { Id = 2, Name = "C2" },
    new Customer { Id = 3, Name = "C3" }
};

var orders = new Order[]
{
    new Order { Description = "O1", CustomerId = 1 },
    new Order { Description = "O2", CustomerId = 1 },
    new Order { Description = "O3", CustomerId = 2 },
    new Order { Description = "O4", CustomerId = 3 },
};

var join = customers.Join(orders, c => c.Id, o => o.CustomerId, (c, o) => c.Name + "-" + o.Description);

// join = { "C1-O1", "C1-O2", "C2-O3", "C3-O4" }

Query Syntax

// join … in … on … equals …

var join = from c in customers
           join o in orders
           on c.Id equals o.CustomerId
           select o.Description + "-" + c.Name;

// join = { "O1-C1", "O2-C1", "O3-C2", "O4-C3" }

GroupJoin

Joins two sequences based on key selector functions and groups the resulting matches for each element.

Method Syntax

// GroupJoin

var groupJoin = customers.GroupJoin(orders,
                                    c => c.Id, 
                                    o => o.CustomerId, 
                                    (c, ors) => c.Name + "-" + string.Join(",", ors.Select(o => o.Description)));

// groupJoin = { "C1-O1,O2", "C2-O3", "C3-O4" }

Query Syntax

// join … in … on … equals … into …

var groupJoin = from c in customers
                join o in orders               
                on c.Id equals o.CustomerId
                into customerOrders
                select string.Join(",", customerOrders.Select(o => o.Description)) + "-" + c.Name;

// groupJoin = { "O1,O2-C1", "O3-C2", "O4-C3" }

Zip

Applies a specified function to the corresponding elements of two sequences, producing a sequence of the results.

var numbers = new [] { 1, 2, 3, 4, 5, 6 };
var words = new [] { "one", "two", "three" };

var numbersWithWords =
    numbers
    .Zip(
        words,
        (number, word) => new { number, word });

// Results

//| number | word   |
//| ------ | ------ |
//| 1      |  one   |
//| 2      |  two   |
//| 3      |  three |

Projection Operations

Projection refers to the operations of transforming an object into a new form.

Select

Projects values that are based on a transform function.

Method Syntax

// Select

var numbers = new int[] { 1, 2, 3, 4, 5 };

var strings = numbers.Select(n => n.ToString());

// strings = { "1", "2", "3", "4", "5" }

Query Syntax

// select

var numbers = new int[] { 1, 2, 3, 4, 5 };

var strings = from n in numbers
              select n.ToString();

// strings = { "1", "2", "3", "4", "5" }

SelectMany

Projects sequences of values that are based on a transform function and then flattens them into one sequence.

Method Syntax

// SelectMany

class Customer
{
    public Order[] Orders { get; set; }
}

class Order
{
    public Order(string desc) { Description = desc; }
    public string Description { get; set; }
}
...

var customers = new Customer[] 
{
    new Customer { Orders = new Order[] { new Order("O1"), new Order("O2") } },
    new Customer { Orders = new Order[] { new Order("O3") } },
    new Customer { Orders = new Order[] { new Order("O4") } },
};

var orders = customers.SelectMany(c => c.Orders);

// orders = { Order("O1"), Order("O3"), Order("O3"), Order("O4") }

Query Syntax

// multiples from

var orders = from c in customers
             from o in c.Orders
             select o;

// orders = { Order("O1"), Order("O3"), Order("O3"), Order("O4") }

Sorting Operations

A sorting operation orders the elements of a sequence based on one or more attributes.

OrderBy

Sorts values in ascending order.

Method Syntax

// OrderBy

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

var ordered = numbers.OrderBy(n => n);

// ordered = { 1, 2, 3, 4, 5, 6, 7, 8, 9 }

Query Syntax

// orderby

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

var ordered = from n in numbers
              orderby n
              select n;

// ordered = { 1, 2, 3, 4, 5, 6, 7, 8, 9 }

OrderByDescending

Sorts values in descending order.

Method Syntax

// OrderByDescending

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

var ordered = numbers.OrderByDescending(n => n);

// ordered = { 9, 8, 7, 6, 5, 4, 3, 2, 1 }

Query Syntax

// orderby

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

var ordered = from n in numbers
              orderby n descending
              select n;

// ordered = { 9, 8, 7, 6, 5, 4, 3, 2, 1 }

ThenBy

Performs a secondary sort in ascending order.

Method Syntax

// ThenBy

string[] words = { "the", "quick", "brown", "fox", "jumps" };

var ordered = words.OrderBy(w => w.Length).ThenBy(w => w[0]);

// ordered = { "fox", "the", "brown", "jumps", "quick" }

Query Syntax

// orderby …, …

string[] words = { "the", "quick", "brown", "fox", "jumps" };

var ordered = from w in words
              orderby w.Length, w[0]
              select w;

// ordered = { "fox", "the", "brown", "jumps", "quick" }

ThenByDescending

Performs a secondary sort in descending order.

Method Syntax

// ThenByDescending

string[] words = { "the", "quick", "brown", "fox", "jumps" };

var ordered = words.OrderBy(w => w[0]).ThenByDescending(w => w.Length);

// ordered = { "brown", "fox", "jumps", "quick", "the" }

Query Syntax

// orderby …, … descending

string[] words = { "the", "quick", "brown", "fox", "jumps" };

var ordered = from w in words
              orderby w.Length, w[0] descending
              select w;

// ordered = { "the", "fox", "quick", "jumps", "brown" }

Reverse

Reverses the order of the elements in a collection.

Method Syntax

// Reverse

var numbers = new int[] { 1, 2, 3, 4, 5 };

var reversed = numbers.Reverse();

// reversed = { 5, 4, 3, 2, 1 }

Query Syntax

// Not applicable.

Conversion Operations

Conversion operations change the type of input objects.

AsEnumerable

Returns the input typed as IEnumerable.

Method Syntax

// AsEnumerable

int[] numbers = { 1, 2, 3, 4, 5 };

var nums = numbers.AsEnumerable();

// nums: static type is IEnumerable<int>

Query Syntax

// Not applicable.

AsQueryable

Converts a IEnumerable to a IQueryable.

Method Syntax

// AsQueryable

int[] numbers = { 1, 2, 3, 4, 5 };

var nums = numbers.AsQueryable();

// nums: static type is IQueryable<int>

Query Syntax

// Not applicable.

Cast

Casts the elements of a collection to a specified type.

Method Syntax

// Cast

var numbers = new object[] { 1, 2, 3, 4, 5 };

var nums = numbers.Cast<int>();

// nums: static type is IEnumerable<int>

Query Syntax

// Use an explicitly typed range variable.

var numbers = new object[] { 1, 2, 3, 4, 5 };

var nums = from int n in numbers select n;

// nums: static type is IEnumerable<int>

OfType

Filters values, depending on their ability to be cast to a specified type.

Method Syntax

// OfType

var objects = new object[] { 1, "one", 2, "two", 3, "three" };

var numbers = objects.OfType<int>();

// nums = { 1, 2, 3 }

Query Syntax

// Not applicable.

ToArray

Converts a collection to an array.

Method Syntax

// ToArray

var numbers = Enumerable.Range(1, 5);

int[] array = numbers.ToArray();

// array = { 1, 2, 3, 4, 5 }

Query Syntax

// Not applicable.

ToList

Converts a collection to a list.

Method Syntax

// ToList

var numbers = Enumerable.Range(1, 5);

List<int> list = numbers.ToList();

// list = { 1, 2, 3, 4, 5 }

Query Syntax

// Not applicable.

ToDictionary

Puts elements into a dictionary based on a key selector function.

Method Syntax

// ToDictionary

var numbers = new int[] { 1, 2, 3 };

var dict = numbers.ToDictionary(n => n.ToString());

// dict = { "1" => 1, "2" => 2, "3" => 3 }

Query Syntax

// Not applicable.

Aggregation Operations

Aggregation operations computes a single value from a collection of values.

Aggregate

Performs a custom aggregation operation on the values of a collection.

Method Syntax

// Aggregate

var numbers = new int[] { 1, 2, 3, 4, 5 };

var product = numbers.Aggregate(1, (acc, n) => acc * n);

// product = 120

Query Syntax

// Not applicable.

Average

Calculates the average value of a collection of values.

Method Syntax

// Average

var numbers = new int[] { 1, 2, 3, 4, 5 };

var average = numbers.Average();

// average = 3

Query Syntax

// Not applicable.

Count

Counts the elements in a collection, optionally only those elements that satisfy a predicate function.

Method Syntax

// Count

var numbers = new int[] { 1, 2, 3, 4, 5 };

int count = numbers.Count(n => n % 2 == 0);

// count = 2

Query Syntax

// Not applicable.

LongCount

Counts the elements in a large collection, optionally only those elements that satisfy a predicate function.

Method Syntax

// LongCount

var numbers = new int[] { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };

long count = numbers.LongCount();

// count = 10

Query Syntax

// Not applicable.

Max

Determines the maximum value in a collection. Throws exception if collection is empty.

Method Syntax

// Max

var numbers = new int[] { 1, 2, 3, 4, 5 };

var max = numbers.Max();

// max = 5

Query Syntax

// Not applicable.

Min

Determines the minimum value in a collection. Throws exception if collection is empty.

Method Syntax

// Min

var numbers = new int[] { 1, 2, 3, 4, 5 };

var min = numbers.Min();

// min = 1

Query Syntax

// Not applicable.

Min-/MaxOrDefault

Unlike other LinQ extensions Min() and Max() do not have an overload without exceptions. Therefor the IEnumerable must be checked for Any() before calling Min() or Max()

// Max

var numbers = new int[] { };

var max = numbers.Any() ? numbers.Max() : 0;

// max = 0

Sum

Calculates the sum of the values in a collection.

Method Syntax

// Sum

var numbers = new int[] { 1, 2, 3, 4, 5 };

var sum = numbers.Sum();

// sum = 15

Query Syntax

// Not applicable.

Quantifier Operations

Quantifier operations return a Boolean value that indicates whether some or all of the elements in a sequence satisfy a condition.

All

Determines whether all the elements in a sequence satisfy a condition.

Method Syntax

// All

var numbers = new int[] { 1, 2, 3, 4, 5 };

bool areLessThan10 = numbers.All(n => n < 10);

// areLessThan10 = true

Query Syntax

// Not applicable.

Any

Determines whether any elements in a sequence satisfy a condition.

Method Syntax

// Any

var numbers = new int[] { 1, 2, 3, 4, 5 };

bool anyOneIsEven = numbers.Any(n => n % 2 == 0);

// anyOneIsEven = true

Query Syntax

// Not applicable.

Contains

Determines whether a sequence contains a specified element.

Method Syntax

// Contains

var numbers = new int[] { 1, 2, 3, 4, 5 };

bool appears = numbers.Contains(10);

// appears = false

Query Syntax

// Not applicable.

Grouping Operations

Grouping refers to the operations of putting data into groups so that the elements in each group share a common attribute.

GroupBy

Groups elements that share a common attribute.

Method Syntax

// GroupBy

class Order
{
    public string Customer { get; set; }
    public string Description { get; set; }
}
...

var orders = new Order[] 
{
    new Order { Customer = "C1", Description = "O1" },
    new Order { Customer = "C2", Description = "O2" },
    new Order { Customer = "C3", Description = "O3" },
    new Order { Customer = "C1", Description = "O4" },
    new Order { Customer = "C1", Description = "O5" },
    new Order { Customer = "C3", Description = "O6" },
};

var groups = orders.GroupBy(o => o.Customer);

// groups: { (Key="C1", Values="O1","O4","O5"), (Key="C2", Values="O2"), (Key="C3", Values="O3","O6") }

Query Syntax

// group … by

var groups = from o in orders
             group o by o.Customer;

// groups: { (Key="C1", Values="O1","O4","O5"), (Key="C2", Values="O2"), (Key="C3", Values="O3","O6") }

ToLookup

Inserts elements into a one-to-many dictionary based on a key selector function.

Method Syntax

// ToLookUp

var ordersByCustomer = orders.ToLookup(o => o.Customer);

// ordersByCustomer = ILookUp<string, Order>
// {
//     "C1" => { Order("01"), Order("04"), Order("05") },
//     "C2" => { Order("02") },
//     "C3" => { Order("03"), Order("06") }
// }

Query Syntax

// Not applicable.

Partition Operations

Partitioning refers to the operations of dividing an input sequence into two sections, without rearranging the elements, and then returning one of the sections.

Skip

Skips elements up to a specified position in a sequence.

Method Syntax

// Skip

var numbers = new int[] { 1, 2, 3, 4, 5 };

var skipped = numbers.Skip(3);

// skipped = { 4, 5 }

Query Syntax

// Not applicable.

SkipWhile

Skips elements based on a predicate function until an element does not satisfy the condition.

Method Syntax

// Skip

var numbers = new int[] { 1, 3, 5, 2, 1, 3, 5 };

var skipLeadingOdds = numbers.SkipWhile(n => n % 2 != 0);

// skipLeadingOdds = { 2, 1, 3, 5 }

Query Syntax

// Not applicable.

Take

Takes elements up to a specified position in a sequence.

Method Syntax

// Take

var numbers = new int[] { 1, 2, 3, 4, 5 };

var taken = numbers.Take(3);

// taken = { 1, 2, 3 }

Query Syntax

// Not applicable.

TakeWhile

Takes elements based on a predicate function until an element does not satisfy the condition.

Method Syntax

// TakeWhile

var numbers = new int[] { 1, 3, 5, 2, 1, 3, 5 };

var takeLeadingOdds = numbers.TakeWhile(n => n % 2 != 0);

// takeLeadingOdds = { 1, 3, 5 }

Query Syntax

// Not applicable.

Generation Operations

Generation refers to creating a new sequence of values.

DefaultIfEmpty

Replaces an empty collection with a default valued singleton collection.

Method Syntax

// DefaultIfEmpty

var nums = new int[0];

var numbers = nums.DefaultIfEmpty();

// numbers = { 0 }

Query Syntax

// Not applicable.

Empty

Returns an empty collection.

Method Syntax

// Empty

var empty = Enumerable.Empty<string>();

// empty = IEnumerable<string> { }

Query Syntax

// Not applicable.

Range

Generates a collection that contains a sequence of numbers.

Method Syntax

// Range

var range = Enumerable.Range(1, 5);

// range = { 1, 2, 3, 4, 5 }

Query Syntax

// Not applicable.

Repeat

Generates a collection that contains one repeated value.

Method Syntax

// Repeat

var repeats = Enumerable.Repeat("s", 3);

// repeats = { "s", "s", "s" }

Query Syntax

// Not applicable.

Set Operations

Set operations refer to query operations that produce a result set that is based on the presence or absence of equivalent elements within the same or separate collections (or sets).

Distinct

Removes duplicate values from a collection.

Method Syntax

// Distinct

var numbers = new int[] { 1, 2, 3, 1, 2, 3 };

var distinct = numbers.Distinct();

// distinct = { 1, 2, 3 }

Query Syntax

// Not applicable.

Except

Returns the set difference, which means the elements of one collection that do not appear in a second collection.

Method Syntax

// Except

var numbers1 = new int[] { 1, 2, 3, 4, 5 };
var numbers2 = new int[] { 4, 5, 6, 7, 8 };

var except = numbers1.Except(numbers2);

// except = { 1, 2, 3 }

Query Syntax

// Not applicable.

Intersect

Returns the set intersection, which means elements that appear in each of two collections.

Method Syntax

// Intersect

var numbers1 = new int[] { 1, 2, 3, 4, 5 };
var numbers2 = new int[] { 4, 5, 6, 7, 8 };

var intersect = numbers1.Intersect(numbers2);

// intersect = { 4, 5 }

Query Syntax

// Not applicable.

Union

Returns the set union, which means unique elements that appear in either of two collections.

Method Syntax

// Union

var numbers1 = new int[] { 1, 2, 3, 4, 5 };
var numbers2 = new int[] { 4, 5, 6, 7, 8 };

var union = numbers1.Union(numbers2);

// union = { 1, 2, 3, 4, 5, 6, 7, 8 }

Query Syntax

// Not applicable.

Equality Operations

Two sequences whose corresponding elements are equal and which have the same number of elements are considered equal.

SequenceEqual

Determines whether two sequences are equal by comparing elements in a pair-wise manner.

Method Syntax

// SequenceEqual

var numbers1 = new int[] { 1, 2, 3, 4, 5 };
var numbers2 = new int[] { 1, 2, 3, 4, 5 };

var equals = numbers1.SequenceEqual(numbers2);

// equals = true

Query Syntax

// Not Applicable.

Element Operations

Element operations return a single, specific element from a sequence.

ElementAt

Returns the element at a specified index in a collection.

Method Syntax

// ElementAt

var strings = new string[] { "zero", "one", "two", "three" };

var str = strings.ElementAt(2);

// str = "two"

Query Syntax

// Not Applicable.

ElementAtOrDefault

Returns the element at a specified index in a collection or a default value if the index is out of range.

Method Syntax

// ElementAtOrDefault

var strings = new string[] { "zero", "one", "two", "three" };

var str = strings.ElementAtOrDefault(10);

// str = null

Query Syntax

// Not Applicable.

First

Returns the first element of a collection, or the first element that satisfies a condition.

Method Syntax

// First

var numbers = new int[] { 1, 2, 3, 4, 5 };

var first = strings.First();

// first = 1

Query Syntax

// Not Applicable.

FirstOrDefault

Returns the first element of a collection, or the first element that satisfies a condition. Returns a default value if no such element exists.

Method Syntax

// FirstOrDefault

var numbers = new int[] { 1, 2, 3, 4, 5 };

var firstGreaterThanTen = strings.FirstOrDefault(n => n > 10);

// firstGreaterThanTen = 0

Query Syntax

// Not Applicable.

Last

Returns the last element of a collection, or the last element that satisfies a condition.

Method Syntax

// Last

var numbers = new int[] { 1, 2, 3, 4, 5 };

var last = strings.Last();

// last = 5

Query Syntax

// Not Applicable.

LastOrDefault

Returns the last element of a collection, or the last element that satisfies a condition. Returns a default value if no such element exists.

Method Syntax

// LastOrDefault

var numbers = new int[] { 1, 2, 3, 4, 5 };

var lastGreaterThanTen = strings.LastOrDefault(n => n > 10);

// lastGreaterThanTen = 0

Query Syntax

// Not Applicable.

Single

Returns the only element of a collection, or the only element that satisfies a condition.

Method Syntax

// Single

var numbers = new int[] { 1 };

var single = strings.Single();

// single = 1

Query Syntax

// Not Applicable.

SingleOrDefault

Returns the only element of a collection, or the only element that satisfies a condition. Returns a default value if no such element exists or the collection does not contain exactly one element.

Method Syntax

// SingleOrDefault

var numbers = new int[] { 1, 2, 3, 4, 5 };

var singleGreaterThanFour = strings.SingleOrDefault(n => n > 4);

// singleGreaterThanFour = 5

Query Syntax

// Not Applicable.