Appendix. The standard query operators

Filtering

Operator name	Description
OfType	Selects values, depending on their ability to be cast to a specified type.
Where	Selects values, depending on a predicate function.

Projection

Operator name	Description
Select	Selects values, depending on a selector function.
SelectMany	Selects values, depending on a selector function, and combines resulting sequences into one sequence. SelectMany performs a one-to-many element projection over a sequence. It differs from Select in that the selector function is expected to return a sequence that is then expanded.

Partitioning

Operator name	Description
Skip	Skips n elements from a sequence.
SkipWhile	Skips elements based on a predicate function until an element does not satisfy the condition.
Take	Takes n elements from a sequence.
TakeWhile	Takes elements based on a predicate function until an element does not satisfy the condition.

Join

Operator name	Description
GroupJoin	Joins two sequences based on key selector functions and groups the resulting matches for each element.
Join	Joins two sequences based on key selector functions and extracts pairs of values.

Concatenation

Operator name	Description
Concat	Concatenates two sequences to form one sequence.

Sorting

Operator name	Description
OrderBy	Sorts values in ascending order.
OrderByDescending	Sorts values in descending order.
ThenBy	Performs a secondary sort in ascending order.
ThenByDescending	Performs a secondary sort in descending order.
Reverse	Reverses the order of the elements in a sequence.

Grouping

Operator name	Description
GroupBy	Groups elements that share a common attribute. Each group is represented by an IGrouping<TKey, TElement> object.
ToLookup	Inserts elements into a Lookup<TKey, TElement> (a one-to-many dictionary) based on a key selector function.

Set

Operator name	Description
Distinct	Removes duplicate values from a collection.
Except	Returns the set difference, which means the elements of one sequence that do not appear in a second sequence.
Intersect	Returns the set intersection, which means elements that appear in each of two sequences.
Union	Returns the set union, which means unique elements that appear in either of two sequences.

Conversion

Operator name	Description
AsEnumerable	Returns the input typed as IEnumerable<T>.
AsQueryable	Converts a (generic) IEnumerable to a (generic) IQueryable.
Cast	Casts the elements of a sequence to a specified type.
OfType	Selects values, depending on their ability to be cast to a specified type.
ToArray	Converts a collection to an array. This method forces query execution.
ToDictionary	Puts elements into a (one-to-one) Dictionary<TKey, TValue> based on a key selector function.
ToList	Converts a collection to a List<T>.
ToLookup	Inserts elements into a Lookup<TKey, TElement> (a one-to-many dictionary) based on a key selector function.

 

Note

By convention, the “ToXXX” operators cause the queries to execute. The “AsXXX” operators do not. This applies to the conversion operators, but should be respected for clarity for other operators as well, including custom ones.

 

Equality

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

Element

Operator name	Description
ElementAt	Returns the element at a specified index in a sequence.
ElementAtOrDefault	Returns the element at a specified index in a sequence or default(T) if the index is out of range.
First	Returns the first element of a sequence, or the first element that satisfies a condition.
FirstOrDefault	Returns the first element of a sequence, or the first element that satisfies a condition. Returns default(T) if no such element exists.
Last	Returns the last element of a sequence, or the last element that satisfies a condition.
LastOrDefault	Returns the last element of a sequence, or the last element that satisfies a condition. Returns default(T) if no such element exists.
Single	Returns the only element of a sequence, or the only element that satisfies a condition. Raises an InvalidOperationException if the sequence does not contain exactly one element.
SingleOrDefault	Returns the only element of a sequence, or the only element that satisfies a condition. Returns default(T) if no such element exists. Raises an InvalidOperationException if the sequence contains more than one element.

Generation

Operator name	Description
DefaultIfEmpty	Replaces an empty sequence with a default valued singleton sequence.
Empty	Returns an empty sequence.
Range	Generates a sequence of integral numbers within a specified range.
Repeat	Generates a sequence that contains one repeated value.

Quantifiers

Operator name	Description
All	Determines whether all the elements in a sequence satisfy a condition.
Any	Determines whether any elements in a sequence satisfy a condition.
Contains	Determines whether a sequence contains a specified element.

Aggregation

Operator name	Description
Aggregate	Performs a custom aggregation operation on the values of a sequence.
Average	Calculates the average value of a sequence of values.
Count	Counts the elements in a sequence, optionally only those elements that satisfy a predicate function.
LongCount	Counts the elements in a large sequence, optionally only those elements that satisfy a predicate function.
Max	Determines the maximum value in a sequence.
Min	Determines the minimum value in a sequence.
Sum	Calculates the sum of the values in a sequence.

 

Note

In general, operators that return something other than an IEnumerable<T> will cause immediate query execution.