# Queryable Class

**.NET Framework 4.5**

Provides a set of static (Shared in Visual Basic) methods for querying data structures that implement IQueryable<T>.

**Namespace:**System.Linq

**Assemblies:**System.Linq.Queryable (in System.Linq.Queryable.dll)

System.Core (in System.Core.dll)

The Queryable type exposes the following members.

Name | Description | |
---|---|---|

Aggregate<TSource>(IQueryable<TSource>, Expression<Func<TSource, TSource, TSource>>) | Applies an accumulator function over a sequence. | |

Aggregate<TSource, TAccumulate>(IQueryable<TSource>, TAccumulate, Expression<Func<TAccumulate, TSource, TAccumulate>>) | Applies an accumulator function over a sequence. The specified seed value is used as the initial accumulator value. | |

Aggregate<TSource, TAccumulate, TResult>(IQueryable<TSource>, TAccumulate, Expression<Func<TAccumulate, TSource, TAccumulate>>, Expression<Func<TAccumulate, TResult>>) | Applies an accumulator function over a sequence. The specified seed value is used as the initial accumulator value, and the specified function is used to select the result value. | |

All<TSource> | Determines whether all the elements of a sequence satisfy a condition. | |

Any<TSource>(IQueryable<TSource>) | Determines whether a sequence contains any elements. | |

Any<TSource>(IQueryable<TSource>, Expression<Func<TSource, Boolean>>) | Determines whether any element of a sequence satisfies a condition. | |

AsQueryable(IEnumerable) | Converts an IEnumerable to an IQueryable. | |

AsQueryable<TElement>(IEnumerable<TElement>) | Converts a generic IEnumerable<T> to a generic IQueryable<T>. | |

Average(IQueryable<Decimal>) | Computes the average of a sequence of Decimal values. | |

Average(IQueryable<Nullable<Decimal>>) | Computes the average of a sequence of nullable Decimal values. | |

Average(IQueryable<Double>) | Computes the average of a sequence of Double values. | |

Average(IQueryable<Nullable<Double>>) | Computes the average of a sequence of nullable Double values. | |

Average(IQueryable<Int32>) | Computes the average of a sequence of Int32 values. | |

Average(IQueryable<Nullable<Int32>>) | Computes the average of a sequence of nullable Int32 values. | |

Average(IQueryable<Int64>) | Computes the average of a sequence of Int64 values. | |

Average(IQueryable<Nullable<Int64>>) | Computes the average of a sequence of nullable Int64 values. | |

Average(IQueryable<Nullable<Single>>) | Computes the average of a sequence of nullable Single values. | |

Average(IQueryable<Single>) | Computes the average of a sequence of Single values. | |

Average<TSource>(IQueryable<TSource>, Expression<Func<TSource, Decimal>>) | Computes the average of a sequence of Decimal values that is obtained by invoking a projection function on each element of the input sequence. | |

Average<TSource>(IQueryable<TSource>, Expression<Func<TSource, Double>>) | Computes the average of a sequence of Double values that is obtained by invoking a projection function on each element of the input sequence. | |

Average<TSource>(IQueryable<TSource>, Expression<Func<TSource, Int32>>) | Computes the average of a sequence of Int32 values that is obtained by invoking a projection function on each element of the input sequence. | |

Average<TSource>(IQueryable<TSource>, Expression<Func<TSource, Int64>>) | Computes the average of a sequence of Int64 values that is obtained by invoking a projection function on each element of the input sequence. | |

Average<TSource>(IQueryable<TSource>, Expression<Func<TSource, Nullable<Decimal>>>) | Computes the average of a sequence of nullable Decimal values that is obtained by invoking a projection function on each element of the input sequence. | |

Average<TSource>(IQueryable<TSource>, Expression<Func<TSource, Nullable<Double>>>) | Computes the average of a sequence of nullable Double values that is obtained by invoking a projection function on each element of the input sequence. | |

Average<TSource>(IQueryable<TSource>, Expression<Func<TSource, Nullable<Int32>>>) | Computes the average of a sequence of nullable Int32 values that is obtained by invoking a projection function on each element of the input sequence. | |

Average<TSource>(IQueryable<TSource>, Expression<Func<TSource, Nullable<Int64>>>) | Computes the average of a sequence of nullable Int64 values that is obtained by invoking a projection function on each element of the input sequence. | |

Average<TSource>(IQueryable<TSource>, Expression<Func<TSource, Nullable<Single>>>) | Computes the average of a sequence of nullable Single values that is obtained by invoking a projection function on each element of the input sequence. | |

Average<TSource>(IQueryable<TSource>, Expression<Func<TSource, Single>>) | Computes the average of a sequence of Single values that is obtained by invoking a projection function on each element of the input sequence. | |

Cast<TResult> | Converts the elements of an IQueryable to the specified type. | |

Concat<TSource> | Concatenates two sequences. | |

Contains<TSource>(IQueryable<TSource>, TSource) | Determines whether a sequence contains a specified element by using the default equality comparer. | |

Contains<TSource>(IQueryable<TSource>, TSource, IEqualityComparer<TSource>) | Determines whether a sequence contains a specified element by using a specified IEqualityComparer<T>. | |

Count<TSource>(IQueryable<TSource>) | Returns the number of elements in a sequence. | |

Count<TSource>(IQueryable<TSource>, Expression<Func<TSource, Boolean>>) | Returns the number of elements in the specified sequence that satisfies a condition. | |

DefaultIfEmpty<TSource>(IQueryable<TSource>) | Returns the elements of the specified sequence or the type parameter's default value in a singleton collection if the sequence is empty. | |

DefaultIfEmpty<TSource>(IQueryable<TSource>, TSource) | Returns the elements of the specified sequence or the specified value in a singleton collection if the sequence is empty. | |

Distinct<TSource>(IQueryable<TSource>) | Returns distinct elements from a sequence by using the default equality comparer to compare values. | |

Distinct<TSource>(IQueryable<TSource>, IEqualityComparer<TSource>) | Returns distinct elements from a sequence by using a specified IEqualityComparer<T> to compare values. | |

ElementAt<TSource> | Returns the element at a specified index in a sequence. | |

ElementAtOrDefault<TSource> | Returns the element at a specified index in a sequence or a default value if the index is out of range. | |

Except<TSource>(IQueryable<TSource>, IEnumerable<TSource>) | Produces the set difference of two sequences by using the default equality comparer to compare values. | |

Except<TSource>(IQueryable<TSource>, IEnumerable<TSource>, IEqualityComparer<TSource>) | Produces the set difference of two sequences by using the specified IEqualityComparer<T> to compare values. | |

First<TSource>(IQueryable<TSource>) | Returns the first element of a sequence. | |

First<TSource>(IQueryable<TSource>, Expression<Func<TSource, Boolean>>) | Returns the first element of a sequence that satisfies a specified condition. | |

FirstOrDefault<TSource>(IQueryable<TSource>) | Returns the first element of a sequence, or a default value if the sequence contains no elements. | |

FirstOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource, Boolean>>) | Returns the first element of a sequence that satisfies a specified condition or a default value if no such element is found. | |

GroupBy<TSource, TKey>(IQueryable<TSource>, Expression<Func<TSource, TKey>>) | Groups the elements of a sequence according to a specified key selector function. | |

GroupBy<TSource, TKey>(IQueryable<TSource>, Expression<Func<TSource, TKey>>, IEqualityComparer<TKey>) | Groups the elements of a sequence according to a specified key selector function and compares the keys by using a specified comparer. | |

GroupBy<TSource, TKey, TElement>(IQueryable<TSource>, Expression<Func<TSource, TKey>>, Expression<Func<TSource, TElement>>) | Groups the elements of a sequence according to a specified key selector function and projects the elements for each group by using a specified function. | |

GroupBy<TSource, TKey, TResult>(IQueryable<TSource>, Expression<Func<TSource, TKey>>, Expression<Func<TKey, IEnumerable<TSource>, TResult>>) | Groups the elements of a sequence according to a specified key selector function and creates a result value from each group and its key. | |

GroupBy<TSource, TKey, TElement>(IQueryable<TSource>, Expression<Func<TSource, TKey>>, Expression<Func<TSource, TElement>>, IEqualityComparer<TKey>) | Groups the elements of a sequence and projects the elements for each group by using a specified function. Key values are compared by using a specified comparer. | |

GroupBy<TSource, TKey, TResult>(IQueryable<TSource>, Expression<Func<TSource, TKey>>, Expression<Func<TKey, IEnumerable<TSource>, TResult>>, IEqualityComparer<TKey>) | Groups the elements of a sequence according to a specified key selector function and creates a result value from each group and its key. Keys are compared by using a specified comparer. | |

GroupBy<TSource, TKey, TElement, TResult>(IQueryable<TSource>, Expression<Func<TSource, TKey>>, Expression<Func<TSource, TElement>>, Expression<Func<TKey, IEnumerable<TElement>, TResult>>) | Groups the elements of a sequence according to a specified key selector function and creates a result value from each group and its key. The elements of each group are projected by using a specified function. | |

GroupBy<TSource, TKey, TElement, TResult>(IQueryable<TSource>, Expression<Func<TSource, TKey>>, Expression<Func<TSource, TElement>>, Expression<Func<TKey, IEnumerable<TElement>, TResult>>, IEqualityComparer<TKey>) | Groups the elements of a sequence according to a specified key selector function and creates a result value from each group and its key. Keys are compared by using a specified comparer and the elements of each group are projected by using a specified function. | |

GroupJoin<TOuter, TInner, TKey, TResult>(IQueryable<TOuter>, IEnumerable<TInner>, Expression<Func<TOuter, TKey>>, Expression<Func<TInner, TKey>>, Expression<Func<TOuter, IEnumerable<TInner>, TResult>>) | Correlates the elements of two sequences based on key equality and groups the results. The default equality comparer is used to compare keys. | |

GroupJoin<TOuter, TInner, TKey, TResult>(IQueryable<TOuter>, IEnumerable<TInner>, Expression<Func<TOuter, TKey>>, Expression<Func<TInner, TKey>>, Expression<Func<TOuter, IEnumerable<TInner>, TResult>>, IEqualityComparer<TKey>) | Correlates the elements of two sequences based on key equality and groups the results. A specified IEqualityComparer<T> is used to compare keys. | |

Intersect<TSource>(IQueryable<TSource>, IEnumerable<TSource>) | Produces the set intersection of two sequences by using the default equality comparer to compare values. | |

Intersect<TSource>(IQueryable<TSource>, IEnumerable<TSource>, IEqualityComparer<TSource>) | Produces the set intersection of two sequences by using the specified IEqualityComparer<T> to compare values. | |

Join<TOuter, TInner, TKey, TResult>(IQueryable<TOuter>, IEnumerable<TInner>, Expression<Func<TOuter, TKey>>, Expression<Func<TInner, TKey>>, Expression<Func<TOuter, TInner, TResult>>) | Correlates the elements of two sequences based on matching keys. The default equality comparer is used to compare keys. | |

Join<TOuter, TInner, TKey, TResult>(IQueryable<TOuter>, IEnumerable<TInner>, Expression<Func<TOuter, TKey>>, Expression<Func<TInner, TKey>>, Expression<Func<TOuter, TInner, TResult>>, IEqualityComparer<TKey>) | Correlates the elements of two sequences based on matching keys. A specified IEqualityComparer<T> is used to compare keys. | |

Last<TSource>(IQueryable<TSource>) | Returns the last element in a sequence. | |

Last<TSource>(IQueryable<TSource>, Expression<Func<TSource, Boolean>>) | Returns the last element of a sequence that satisfies a specified condition. | |

LastOrDefault<TSource>(IQueryable<TSource>) | Returns the last element in a sequence, or a default value if the sequence contains no elements. | |

LastOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource, Boolean>>) | Returns the last element of a sequence that satisfies a condition or a default value if no such element is found. | |

LongCount<TSource>(IQueryable<TSource>) | Returns an Int64 that represents the total number of elements in a sequence. | |

LongCount<TSource>(IQueryable<TSource>, Expression<Func<TSource, Boolean>>) | Returns an Int64 that represents the number of elements in a sequence that satisfy a condition. | |

Max<TSource>(IQueryable<TSource>) | Returns the maximum value in a generic IQueryable<T>. | |

Max<TSource, TResult>(IQueryable<TSource>, Expression<Func<TSource, TResult>>) | Invokes a projection function on each element of a generic IQueryable<T> and returns the maximum resulting value. | |

Min<TSource>(IQueryable<TSource>) | Returns the minimum value of a generic IQueryable<T>. | |

Min<TSource, TResult>(IQueryable<TSource>, Expression<Func<TSource, TResult>>) | Invokes a projection function on each element of a generic IQueryable<T> and returns the minimum resulting value. | |

OfType<TResult> | Filters the elements of an IQueryable based on a specified type. | |

OrderBy<TSource, TKey>(IQueryable<TSource>, Expression<Func<TSource, TKey>>) | Sorts the elements of a sequence in ascending order according to a key. | |

OrderBy<TSource, TKey>(IQueryable<TSource>, Expression<Func<TSource, TKey>>, IComparer<TKey>) | Sorts the elements of a sequence in ascending order by using a specified comparer. | |

OrderByDescending<TSource, TKey>(IQueryable<TSource>, Expression<Func<TSource, TKey>>) | Sorts the elements of a sequence in descending order according to a key. | |

OrderByDescending<TSource, TKey>(IQueryable<TSource>, Expression<Func<TSource, TKey>>, IComparer<TKey>) | Sorts the elements of a sequence in descending order by using a specified comparer. | |

Reverse<TSource> | Inverts the order of the elements in a sequence. | |

Select<TSource, TResult>(IQueryable<TSource>, Expression<Func<TSource, TResult>>) | Projects each element of a sequence into a new form. | |

Select<TSource, TResult>(IQueryable<TSource>, Expression<Func<TSource, Int32, TResult>>) | Projects each element of a sequence into a new form by incorporating the element's index. | |

SelectMany<TSource, TResult>(IQueryable<TSource>, Expression<Func<TSource, IEnumerable<TResult>>>) | Projects each element of a sequence to an IEnumerable<T> and combines the resulting sequences into one sequence. | |

SelectMany<TSource, TResult>(IQueryable<TSource>, Expression<Func<TSource, Int32, IEnumerable<TResult>>>) | Projects each element of a sequence to an IEnumerable<T> and combines the resulting sequences into one sequence. The index of each source element is used in the projected form of that element. | |

SelectMany<TSource, TCollection, TResult>(IQueryable<TSource>, Expression<Func<TSource, IEnumerable<TCollection>>>, Expression<Func<TSource, TCollection, TResult>>) | Projects each element of a sequence to an IEnumerable<T> and invokes a result selector function on each element therein. The resulting values from each intermediate sequence are combined into a single, one-dimensional sequence and returned. | |

SelectMany<TSource, TCollection, TResult>(IQueryable<TSource>, Expression<Func<TSource, Int32, IEnumerable<TCollection>>>, Expression<Func<TSource, TCollection, TResult>>) | Projects each element of a sequence to an IEnumerable<T> that incorporates the index of the source element that produced it. A result selector function is invoked on each element of each intermediate sequence, and the resulting values are combined into a single, one-dimensional sequence and returned. | |

SequenceEqual<TSource>(IQueryable<TSource>, IEnumerable<TSource>) | Determines whether two sequences are equal by using the default equality comparer to compare elements. | |

SequenceEqual<TSource>(IQueryable<TSource>, IEnumerable<TSource>, IEqualityComparer<TSource>) | Determines whether two sequences are equal by using a specified IEqualityComparer<T> to compare elements. | |

Single<TSource>(IQueryable<TSource>) | Returns the only element of a sequence, and throws an exception if there is not exactly one element in the sequence. | |

Single<TSource>(IQueryable<TSource>, Expression<Func<TSource, Boolean>>) | Returns the only element of a sequence that satisfies a specified condition, and throws an exception if more than one such element exists. | |

SingleOrDefault<TSource>(IQueryable<TSource>) | Returns the only element of a sequence, or a default value if the sequence is empty; this method throws an exception if there is more than one element in the sequence. | |

SingleOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource, Boolean>>) | Returns the only element of a sequence that satisfies a specified condition or a default value if no such element exists; this method throws an exception if more than one element satisfies the condition. | |

Skip<TSource> | Bypasses a specified number of elements in a sequence and then returns the remaining elements. | |

SkipWhile<TSource>(IQueryable<TSource>, Expression<Func<TSource, Boolean>>) | Bypasses elements in a sequence as long as a specified condition is true and then returns the remaining elements. | |

SkipWhile<TSource>(IQueryable<TSource>, Expression<Func<TSource, Int32, Boolean>>) | Bypasses elements in a sequence as long as a specified condition is true and then returns the remaining elements. The element's index is used in the logic of the predicate function. | |

Sum(IQueryable<Decimal>) | Computes the sum of a sequence of Decimal values. | |

Sum(IQueryable<Nullable<Decimal>>) | Computes the sum of a sequence of nullable Decimal values. | |

Sum(IQueryable<Double>) | Computes the sum of a sequence of Double values. | |

Sum(IQueryable<Nullable<Double>>) | Computes the sum of a sequence of nullable Double values. | |

Sum(IQueryable<Int32>) | Computes the sum of a sequence of Int32 values. | |

Sum(IQueryable<Nullable<Int32>>) | Computes the sum of a sequence of nullable Int32 values. | |

Sum(IQueryable<Int64>) | Computes the sum of a sequence of Int64 values. | |

Sum(IQueryable<Nullable<Int64>>) | Computes the sum of a sequence of nullable Int64 values. | |

Sum(IQueryable<Nullable<Single>>) | Computes the sum of a sequence of nullable Single values. | |

Sum(IQueryable<Single>) | Computes the sum of a sequence of Single values. | |

Sum<TSource>(IQueryable<TSource>, Expression<Func<TSource, Decimal>>) | Computes the sum of the sequence of Decimal values that is obtained by invoking a projection function on each element of the input sequence. | |

Sum<TSource>(IQueryable<TSource>, Expression<Func<TSource, Double>>) | Computes the sum of the sequence of Double values that is obtained by invoking a projection function on each element of the input sequence. | |

Sum<TSource>(IQueryable<TSource>, Expression<Func<TSource, Int32>>) | Computes the sum of the sequence of Int32 values that is obtained by invoking a projection function on each element of the input sequence. | |

Sum<TSource>(IQueryable<TSource>, Expression<Func<TSource, Int64>>) | Computes the sum of the sequence of Int64 values that is obtained by invoking a projection function on each element of the input sequence. | |

Sum<TSource>(IQueryable<TSource>, Expression<Func<TSource, Nullable<Decimal>>>) | Computes the sum of the sequence of nullable Decimal values that is obtained by invoking a projection function on each element of the input sequence. | |

Sum<TSource>(IQueryable<TSource>, Expression<Func<TSource, Nullable<Double>>>) | Computes the sum of the sequence of nullable Double values that is obtained by invoking a projection function on each element of the input sequence. | |

Sum<TSource>(IQueryable<TSource>, Expression<Func<TSource, Nullable<Int32>>>) | Computes the sum of the sequence of nullable Int32 values that is obtained by invoking a projection function on each element of the input sequence. | |

Sum<TSource>(IQueryable<TSource>, Expression<Func<TSource, Nullable<Int64>>>) | Computes the sum of the sequence of nullable Int64 values that is obtained by invoking a projection function on each element of the input sequence. | |

Sum<TSource>(IQueryable<TSource>, Expression<Func<TSource, Nullable<Single>>>) | Computes the sum of the sequence of nullable Single values that is obtained by invoking a projection function on each element of the input sequence. | |

Sum<TSource>(IQueryable<TSource>, Expression<Func<TSource, Single>>) | Computes the sum of the sequence of Single values that is obtained by invoking a projection function on each element of the input sequence. | |

Take<TSource> | Returns a specified number of contiguous elements from the start of a sequence. | |

TakeWhile<TSource>(IQueryable<TSource>, Expression<Func<TSource, Boolean>>) | Returns elements from a sequence as long as a specified condition is true. | |

TakeWhile<TSource>(IQueryable<TSource>, Expression<Func<TSource, Int32, Boolean>>) | Returns elements from a sequence as long as a specified condition is true. The element's index is used in the logic of the predicate function. | |

ThenBy<TSource, TKey>(IOrderedQueryable<TSource>, Expression<Func<TSource, TKey>>) | Performs a subsequent ordering of the elements in a sequence in ascending order according to a key. | |

ThenBy<TSource, TKey>(IOrderedQueryable<TSource>, Expression<Func<TSource, TKey>>, IComparer<TKey>) | Performs a subsequent ordering of the elements in a sequence in ascending order by using a specified comparer. | |

ThenByDescending<TSource, TKey>(IOrderedQueryable<TSource>, Expression<Func<TSource, TKey>>) | Performs a subsequent ordering of the elements in a sequence in descending order, according to a key. | |

ThenByDescending<TSource, TKey>(IOrderedQueryable<TSource>, Expression<Func<TSource, TKey>>, IComparer<TKey>) | Performs a subsequent ordering of the elements in a sequence in descending order by using a specified comparer. | |

Union<TSource>(IQueryable<TSource>, IEnumerable<TSource>) | Produces the set union of two sequences by using the default equality comparer. | |

Union<TSource>(IQueryable<TSource>, IEnumerable<TSource>, IEqualityComparer<TSource>) | Produces the set union of two sequences by using a specified IEqualityComparer<T>. | |

Where<TSource>(IQueryable<TSource>, Expression<Func<TSource, Boolean>>) | Filters a sequence of values based on a predicate. | |

Where<TSource>(IQueryable<TSource>, Expression<Func<TSource, Int32, Boolean>>) | Filters a sequence of values based on a predicate. Each element's index is used in the logic of the predicate function. | |

Zip<TFirst, TSecond, TResult> | Merges two sequences by using the specified predicate function. |

The set of methods declared in the Queryable class provides an implementation of the standard query operators for querying data sources that implement IQueryable<T>. The standard query operators are general purpose methods that follow the LINQ pattern and enable you to express traversal, filter, and projection operations over data in any .NET-based programming language.

The majority of the methods in this class are defined as extension methods that extend the IQueryable<T> type. This means they can be called like an instance method on any object that implements IQueryable<T>. These methods that extend IQueryable<T> do not perform any querying directly. Instead, their functionality is to build an Expression object, which is an expression tree that represents the cumulative query. The methods then pass the new expression tree to either the Execute<TResult>(Expression) method or the CreateQuery<TElement>(Expression) method of the input IQueryable<T>. The method that is called depends on whether the Queryable method returns a singleton value, in which case Execute<TResult>(Expression) is called, or has enumerable results, in which case CreateQuery<TElement>(Expression) is called.

The actual query execution on the target data is performed by a class that implements IQueryable<T>. The expectation of any IQueryable<T> implementation is that the result of executing an expression tree that was constructed by a Queryable standard query operator method is equivalent to the result of calling the corresponding method in the Enumerable class, if the data source were an IEnumerable<T>.

In addition to the standard query operator methods that operate on IQueryable<T> objects, this class also contains a method, AsQueryable, which types IEnumerable objects as IQueryable objects.