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11/18/2011

Lookup<TKey, TElement> Class

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Represents a collection of keys each mapped to one or more values.

Inheritance Hierarchy

System.Object
System.Linq.Lookup<TKey, TElement>

Namespace: System.Linq
Assembly: System.Core (in System.Core.dll)

Syntax

'Declaration
<DefaultMemberAttribute("Item")> _
Public Class Lookup(Of TKey, TElement) _
    Implements ILookup(Of TKey, TElement), IEnumerable(Of IGrouping(Of TKey, TElement)),  _
    IEnumerable

[DefaultMemberAttribute("Item")]
public class Lookup<TKey, TElement> : ILookup<TKey, TElement>, 
    IEnumerable<IGrouping<TKey, TElement>>, IEnumerable

Type Parameters

TKey
The type of the keys in the Lookup<TKey, TElement>.

TElement
The type of the elements of each IEnumerable<T> value in the Lookup<TKey, TElement>.

The Lookup<TKey, TElement> type exposes the following members.

Properties

	Name	Description
	Count	Gets the number of key/value collection pairs in the Lookup<TKey, TElement>.
	Item	Gets the collection of values indexed by the specified key.

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Methods

	Name	Description
	ApplyResultSelector<TResult>	Applies a transform function to each key and its associated values and returns the results.
	Contains	Determines whether a specified key is in the Lookup<TKey, TElement>.
	Equals(Object)	Determines whether the specified Object is equal to the current Object. (Inherited from Object.)
	Finalize	Allows an object to try to free resources and perform other cleanup operations before the Object is reclaimed by garbage collection. (Inherited from Object.)
	GetEnumerator	Returns a generic enumerator that iterates through the Lookup<TKey, TElement>.
	GetHashCode	Serves as a hash function for a particular type. (Inherited from Object.)
	GetType	Gets the Type of the current instance. (Inherited from Object.)
	MemberwiseClone	Creates a shallow copy of the current Object. (Inherited from Object.)
	ToString	Returns a string that represents the current object. (Inherited from Object.)

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Extension Methods

	Name	Description
	Aggregate<IGrouping<TKey, TElement>>(Func<IGrouping<TKey, TElement>, IGrouping<TKey, TElement>, IGrouping<TKey, TElement>>)	Overloaded. Applies an accumulator function over a sequence. (Defined by Enumerable.)
	Aggregate<IGrouping<TKey, TElement>, TAccumulate>(TAccumulate, Func<TAccumulate, IGrouping<TKey, TElement>, TAccumulate>)	Overloaded. Applies an accumulator function over a sequence. The specified seed value is used as the initial accumulator value. (Defined by Enumerable.)
	Aggregate<IGrouping<TKey, TElement>, TAccumulate, TResult>(TAccumulate, Func<TAccumulate, IGrouping<TKey, TElement>, TAccumulate>, Func<TAccumulate, TResult>)	Overloaded. 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. (Defined by Enumerable.)
	All<IGrouping<TKey, TElement>>	Determines whether all elements of a sequence satisfy a condition. (Defined by Enumerable.)
	Any<IGrouping<TKey, TElement>>()	Overloaded. Determines whether a sequence contains any elements. (Defined by Enumerable.)
	Any<IGrouping<TKey, TElement>>(Func<IGrouping<TKey, TElement>, Boolean>)	Overloaded. Determines whether any element of a sequence satisfies a condition. (Defined by Enumerable.)
	AsEnumerable<IGrouping<TKey, TElement>>	Returns the input typed as IEnumerable<T>. (Defined by Enumerable.)
	AsQueryable()	Overloaded. Converts an IEnumerable to an IQueryable. (Defined by Queryable.)
	AsQueryable<IGrouping<TKey, TElement>>()	Overloaded. Converts a generic IEnumerable<T> to a generic IQueryable<T>. (Defined by Queryable.)
	Average<IGrouping<TKey, TElement>>(Func<IGrouping<TKey, TElement>, Nullable<Decimal>>)	Overloaded. Computes the average of a sequence of nullable Decimal values that are obtained by invoking a transform function on each element of the input sequence. (Defined by Enumerable.)
	Average<IGrouping<TKey, TElement>>(Func<IGrouping<TKey, TElement>, Nullable<Double>>)	Overloaded. Computes the average of a sequence of nullable Double values that are obtained by invoking a transform function on each element of the input sequence. (Defined by Enumerable.)
	Average<IGrouping<TKey, TElement>>(Func<IGrouping<TKey, TElement>, Int32>)	Overloaded. Computes the average of a sequence of Int32 values that are obtained by invoking a transform function on each element of the input sequence. (Defined by Enumerable.)
	Average<IGrouping<TKey, TElement>>(Func<IGrouping<TKey, TElement>, Nullable<Int32>>)	Overloaded. Computes the average of a sequence of nullable Int32 values that are obtained by invoking a transform function on each element of the input sequence. (Defined by Enumerable.)
	Average<IGrouping<TKey, TElement>>(Func<IGrouping<TKey, TElement>, Int64>)	Overloaded. Computes the average of a sequence of Int64 values that are obtained by invoking a transform function on each element of the input sequence. (Defined by Enumerable.)
	Average<IGrouping<TKey, TElement>>(Func<IGrouping<TKey, TElement>, Nullable<Int64>>)	Overloaded. Computes the average of a sequence of nullable Int64 values that are obtained by invoking a transform function on each element of the input sequence. (Defined by Enumerable.)
	Average<IGrouping<TKey, TElement>>(Func<IGrouping<TKey, TElement>, Single>)	Overloaded. Computes the average of a sequence of Single values that are obtained by invoking a transform function on each element of the input sequence. (Defined by Enumerable.)
	Average<IGrouping<TKey, TElement>>(Func<IGrouping<TKey, TElement>, Nullable<Single>>)	Overloaded. Computes the average of a sequence of nullable Single values that are obtained by invoking a transform function on each element of the input sequence. (Defined by Enumerable.)
	Average<IGrouping<TKey, TElement>>(Func<IGrouping<TKey, TElement>, Double>)	Overloaded. Computes the average of a sequence of Double values that are obtained by invoking a transform function on each element of the input sequence. (Defined by Enumerable.)
	Average<IGrouping<TKey, TElement>>(Func<IGrouping<TKey, TElement>, Decimal>)	Overloaded. Computes the average of a sequence of Decimal values that are obtained by invoking a transform function on each element of the input sequence. (Defined by Enumerable.)
	Cast<TResult>	Converts the elements of an IEnumerable to the specified type. (Defined by Enumerable.)
	Concat<IGrouping<TKey, TElement>>	Concatenates two sequences. (Defined by Enumerable.)
	Contains<IGrouping<TKey, TElement>>(IGrouping<TKey, TElement>)	Overloaded. Determines whether a sequence contains a specified element by using the default equality comparer. (Defined by Enumerable.)
	Contains<IGrouping<TKey, TElement>>(IGrouping<TKey, TElement>, IEqualityComparer<IGrouping<TKey, TElement>>)	Overloaded. Determines whether a sequence contains a specified element by using a specified IEqualityComparer<T>. (Defined by Enumerable.)
	Count<IGrouping<TKey, TElement>>()	Overloaded. Returns the number of elements in a sequence. (Defined by Enumerable.)
	Count<IGrouping<TKey, TElement>>(Func<IGrouping<TKey, TElement>, Boolean>)	Overloaded. Returns a number that represents how many elements in the specified sequence satisfy a condition. (Defined by Enumerable.)
	DefaultIfEmpty<IGrouping<TKey, TElement>>()	Overloaded. Returns the elements of the specified sequence or the type parameter's default value in a singleton collection if the sequence is empty. (Defined by Enumerable.)
	DefaultIfEmpty<IGrouping<TKey, TElement>>(IGrouping<TKey, TElement>)	Overloaded. Returns the elements of the specified sequence or the specified value in a singleton collection if the sequence is empty. (Defined by Enumerable.)
	Distinct<IGrouping<TKey, TElement>>()	Overloaded. Returns distinct elements from a sequence by using the default equality comparer to compare values. (Defined by Enumerable.)
	Distinct<IGrouping<TKey, TElement>>(IEqualityComparer<IGrouping<TKey, TElement>>)	Overloaded. Returns distinct elements from a sequence by using a specified IEqualityComparer<T> to compare values. (Defined by Enumerable.)
	ElementAt<IGrouping<TKey, TElement>>	Returns the element at a specified index in a sequence. (Defined by Enumerable.)
	ElementAtOrDefault<IGrouping<TKey, TElement>>	Returns the element at a specified index in a sequence or a default value if the index is out of range. (Defined by Enumerable.)
	Except<IGrouping<TKey, TElement>>(IEnumerable<IGrouping<TKey, TElement>>)	Overloaded. Produces the set difference of two sequences by using the default equality comparer to compare values. (Defined by Enumerable.)
	Except<IGrouping<TKey, TElement>>(IEnumerable<IGrouping<TKey, TElement>>, IEqualityComparer<IGrouping<TKey, TElement>>)	Overloaded. Produces the set difference of two sequences by using the specified IEqualityComparer<T> to compare values. (Defined by Enumerable.)
	First<IGrouping<TKey, TElement>>()	Overloaded. Returns the first element of a sequence. (Defined by Enumerable.)
	First<IGrouping<TKey, TElement>>(Func<IGrouping<TKey, TElement>, Boolean>)	Overloaded. Returns the first element in a sequence that satisfies a specified condition. (Defined by Enumerable.)
	FirstOrDefault<IGrouping<TKey, TElement>>()	Overloaded. Returns the first element of a sequence, or a default value if the sequence contains no elements. (Defined by Enumerable.)
	FirstOrDefault<IGrouping<TKey, TElement>>(Func<IGrouping<TKey, TElement>, Boolean>)	Overloaded. Returns the first element of the sequence that satisfies a condition or a default value if no such element is found. (Defined by Enumerable.)
	GroupBy<IGrouping<TKey, TElement>, TKey>(Func<IGrouping<TKey, TElement>, TKey>)	Overloaded. Groups the elements of a sequence according to a specified key selector function. (Defined by Enumerable.)
	GroupBy<IGrouping<TKey, TElement>, TKey>(Func<IGrouping<TKey, TElement>, TKey>, IEqualityComparer<TKey>)	Overloaded. Groups the elements of a sequence according to a specified key selector function and compares the keys by using a specified comparer. (Defined by Enumerable.)
	GroupBy<IGrouping<TKey, TElement>, TKey, TElement>(Func<IGrouping<TKey, TElement>, TKey>, Func<IGrouping<TKey, TElement>, TElement>)	Overloaded. 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. (Defined by Enumerable.)
	GroupBy<IGrouping<TKey, TElement>, TKey, TResult>(Func<IGrouping<TKey, TElement>, TKey>, Func<TKey, IEnumerable<IGrouping<TKey, TElement>>, TResult>)	Overloaded. Groups the elements of a sequence according to a specified key selector function and creates a result value from each group and its key. (Defined by Enumerable.)
	GroupBy<IGrouping<TKey, TElement>, TKey, TElement>(Func<IGrouping<TKey, TElement>, TKey>, Func<IGrouping<TKey, TElement>, TElement>, IEqualityComparer<TKey>)	Overloaded. Groups the elements of a sequence according to a key selector function. The keys are compared by using a comparer and each group's elements are projected by using a specified function. (Defined by Enumerable.)
	GroupBy<IGrouping<TKey, TElement>, TKey, TResult>(Func<IGrouping<TKey, TElement>, TKey>, Func<TKey, IEnumerable<IGrouping<TKey, TElement>>, TResult>, IEqualityComparer<TKey>)	Overloaded. 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 keys are compared by using a specified comparer. (Defined by Enumerable.)
	GroupBy<IGrouping<TKey, TElement>, TKey, TElement, TResult>(Func<IGrouping<TKey, TElement>, TKey>, Func<IGrouping<TKey, TElement>, TElement>, Func<TKey, IEnumerable<TElement>, TResult>)	Overloaded. 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. (Defined by Enumerable.)
	GroupBy<IGrouping<TKey, TElement>, TKey, TElement, TResult>(Func<IGrouping<TKey, TElement>, TKey>, Func<IGrouping<TKey, TElement>, TElement>, Func<TKey, IEnumerable<TElement>, TResult>, IEqualityComparer<TKey>)	Overloaded. Groups the elements of a sequence according to a specified key selector function and creates a result value from each group and its key. Key values are compared by using a specified comparer, and the elements of each group are projected by using a specified function. (Defined by Enumerable.)
	GroupJoin<IGrouping<TKey, TElement>, TInner, TKey, TResult>(IEnumerable<TInner>, Func<IGrouping<TKey, TElement>, TKey>, Func<TInner, TKey>, Func<IGrouping<TKey, TElement>, IEnumerable<TInner>, TResult>)	Overloaded. Correlates the elements of two sequences based on equality of keys and groups the results. The default equality comparer is used to compare keys. (Defined by Enumerable.)
	GroupJoin<IGrouping<TKey, TElement>, TInner, TKey, TResult>(IEnumerable<TInner>, Func<IGrouping<TKey, TElement>, TKey>, Func<TInner, TKey>, Func<IGrouping<TKey, TElement>, IEnumerable<TInner>, TResult>, IEqualityComparer<TKey>)	Overloaded. Correlates the elements of two sequences based on key equality and groups the results. A specified IEqualityComparer<T> is used to compare keys. (Defined by Enumerable.)
	Intersect<IGrouping<TKey, TElement>>(IEnumerable<IGrouping<TKey, TElement>>)	Overloaded. Produces the set intersection of two sequences by using the default equality comparer to compare values. (Defined by Enumerable.)
	Intersect<IGrouping<TKey, TElement>>(IEnumerable<IGrouping<TKey, TElement>>, IEqualityComparer<IGrouping<TKey, TElement>>)	Overloaded. Produces the set intersection of two sequences by using the specified IEqualityComparer<T> to compare values. (Defined by Enumerable.)
	Join<IGrouping<TKey, TElement>, TInner, TKey, TResult>(IEnumerable<TInner>, Func<IGrouping<TKey, TElement>, TKey>, Func<TInner, TKey>, Func<IGrouping<TKey, TElement>, TInner, TResult>)	Overloaded. Correlates the elements of two sequences based on matching keys. The default equality comparer is used to compare keys. (Defined by Enumerable.)
	Join<IGrouping<TKey, TElement>, TInner, TKey, TResult>(IEnumerable<TInner>, Func<IGrouping<TKey, TElement>, TKey>, Func<TInner, TKey>, Func<IGrouping<TKey, TElement>, TInner, TResult>, IEqualityComparer<TKey>)	Overloaded. Correlates the elements of two sequences based on matching keys. A specified IEqualityComparer<T> is used to compare keys. (Defined by Enumerable.)
	Last<IGrouping<TKey, TElement>>()	Overloaded. Returns the last element of a sequence. (Defined by Enumerable.)
	Last<IGrouping<TKey, TElement>>(Func<IGrouping<TKey, TElement>, Boolean>)	Overloaded. Returns the last element of a sequence that satisfies a specified condition. (Defined by Enumerable.)
	LastOrDefault<IGrouping<TKey, TElement>>()	Overloaded. Returns the last element of a sequence, or a default value if the sequence contains no elements. (Defined by Enumerable.)
	LastOrDefault<IGrouping<TKey, TElement>>(Func<IGrouping<TKey, TElement>, Boolean>)	Overloaded. Returns the last element of a sequence that satisfies a condition or a default value if no such element is found. (Defined by Enumerable.)
	LongCount<IGrouping<TKey, TElement>>()	Overloaded. Returns an Int64 that represents the total number of elements in a sequence. (Defined by Enumerable.)
	LongCount<IGrouping<TKey, TElement>>(Func<IGrouping<TKey, TElement>, Boolean>)	Overloaded. Returns an Int64 that represents how many elements in a sequence satisfy a condition. (Defined by Enumerable.)
	Max<IGrouping<TKey, TElement>>()	Overloaded. Returns the maximum value in a generic sequence. (Defined by Enumerable.)
	Max<IGrouping<TKey, TElement>>(Func<IGrouping<TKey, TElement>, Nullable<Decimal>>)	Overloaded. Invokes a transform function on each element of a sequence and returns the maximum nullable Decimal value. (Defined by Enumerable.)
	Max<IGrouping<TKey, TElement>>(Func<IGrouping<TKey, TElement>, Nullable<Double>>)	Overloaded. Invokes a transform function on each element of a sequence and returns the maximum nullable Double value. (Defined by Enumerable.)
	Max<IGrouping<TKey, TElement>>(Func<IGrouping<TKey, TElement>, Int32>)	Overloaded. Invokes a transform function on each element of a sequence and returns the maximum Int32 value. (Defined by Enumerable.)
	Max<IGrouping<TKey, TElement>>(Func<IGrouping<TKey, TElement>, Nullable<Int32>>)	Overloaded. Invokes a transform function on each element of a sequence and returns the maximum nullable Int32 value. (Defined by Enumerable.)
	Max<IGrouping<TKey, TElement>>(Func<IGrouping<TKey, TElement>, Int64>)	Overloaded. Invokes a transform function on each element of a sequence and returns the maximum Int64 value. (Defined by Enumerable.)
	Max<IGrouping<TKey, TElement>>(Func<IGrouping<TKey, TElement>, Nullable<Int64>>)	Overloaded. Invokes a transform function on each element of a sequence and returns the maximum nullable Int64 value. (Defined by Enumerable.)
	Max<IGrouping<TKey, TElement>>(Func<IGrouping<TKey, TElement>, Single>)	Overloaded. Invokes a transform function on each element of a sequence and returns the maximum Single value. (Defined by Enumerable.)
	Max<IGrouping<TKey, TElement>>(Func<IGrouping<TKey, TElement>, Nullable<Single>>)	Overloaded. Invokes a transform function on each element of a sequence and returns the maximum nullable Single value. (Defined by Enumerable.)
	Max<IGrouping<TKey, TElement>>(Func<IGrouping<TKey, TElement>, Double>)	Overloaded. Invokes a transform function on each element of a sequence and returns the maximum Double value. (Defined by Enumerable.)
	Max<IGrouping<TKey, TElement>>(Func<IGrouping<TKey, TElement>, Decimal>)	Overloaded. Invokes a transform function on each element of a sequence and returns the maximum Decimal value. (Defined by Enumerable.)
	Max<IGrouping<TKey, TElement>, TResult>(Func<IGrouping<TKey, TElement>, TResult>)	Overloaded. Invokes a transform function on each element of a generic sequence and returns the maximum resulting value. (Defined by Enumerable.)
	Min<IGrouping<TKey, TElement>>()	Overloaded. Returns the minimum value in a generic sequence. (Defined by Enumerable.)
	Min<IGrouping<TKey, TElement>>(Func<IGrouping<TKey, TElement>, Nullable<Decimal>>)	Overloaded. Invokes a transform function on each element of a sequence and returns the minimum nullable Decimal value. (Defined by Enumerable.)
	Min<IGrouping<TKey, TElement>>(Func<IGrouping<TKey, TElement>, Nullable<Double>>)	Overloaded. Invokes a transform function on each element of a sequence and returns the minimum nullable Double value. (Defined by Enumerable.)
	Min<IGrouping<TKey, TElement>>(Func<IGrouping<TKey, TElement>, Int32>)	Overloaded. Invokes a transform function on each element of a sequence and returns the minimum Int32 value. (Defined by Enumerable.)
	Min<IGrouping<TKey, TElement>>(Func<IGrouping<TKey, TElement>, Nullable<Int32>>)	Overloaded. Invokes a transform function on each element of a sequence and returns the minimum nullable Int32 value. (Defined by Enumerable.)
	Min<IGrouping<TKey, TElement>>(Func<IGrouping<TKey, TElement>, Int64>)	Overloaded. Invokes a transform function on each element of a sequence and returns the minimum Int64 value. (Defined by Enumerable.)
	Min<IGrouping<TKey, TElement>>(Func<IGrouping<TKey, TElement>, Nullable<Int64>>)	Overloaded. Invokes a transform function on each element of a sequence and returns the minimum nullable Int64 value. (Defined by Enumerable.)
	Min<IGrouping<TKey, TElement>>(Func<IGrouping<TKey, TElement>, Single>)	Overloaded. Invokes a transform function on each element of a sequence and returns the minimum Single value. (Defined by Enumerable.)
	Min<IGrouping<TKey, TElement>>(Func<IGrouping<TKey, TElement>, Nullable<Single>>)	Overloaded. Invokes a transform function on each element of a sequence and returns the minimum nullable Single value. (Defined by Enumerable.)
	Min<IGrouping<TKey, TElement>>(Func<IGrouping<TKey, TElement>, Double>)	Overloaded. Invokes a transform function on each element of a sequence and returns the minimum Double value. (Defined by Enumerable.)
	Min<IGrouping<TKey, TElement>>(Func<IGrouping<TKey, TElement>, Decimal>)	Overloaded. Invokes a transform function on each element of a sequence and returns the minimum Decimal value. (Defined by Enumerable.)
	Min<IGrouping<TKey, TElement>, TResult>(Func<IGrouping<TKey, TElement>, TResult>)	Overloaded. Invokes a transform function on each element of a generic sequence and returns the minimum resulting value. (Defined by Enumerable.)
	OfType<TResult>	Filters the elements of an IEnumerable based on a specified type. (Defined by Enumerable.)
	OrderBy<IGrouping<TKey, TElement>, TKey>(Func<IGrouping<TKey, TElement>, TKey>)	Overloaded. Sorts the elements of a sequence in ascending order according to a key. (Defined by Enumerable.)
	OrderBy<IGrouping<TKey, TElement>, TKey>(Func<IGrouping<TKey, TElement>, TKey>, IComparer<TKey>)	Overloaded. Sorts the elements of a sequence in ascending order by using a specified comparer. (Defined by Enumerable.)
	OrderByDescending<IGrouping<TKey, TElement>, TKey>(Func<IGrouping<TKey, TElement>, TKey>)	Overloaded. Sorts the elements of a sequence in descending order according to a key. (Defined by Enumerable.)
	OrderByDescending<IGrouping<TKey, TElement>, TKey>(Func<IGrouping<TKey, TElement>, TKey>, IComparer<TKey>)	Overloaded. Sorts the elements of a sequence in descending order by using a specified comparer. (Defined by Enumerable.)
	Reverse<IGrouping<TKey, TElement>>	Inverts the order of the elements in a sequence. (Defined by Enumerable.)
	Select<IGrouping<TKey, TElement>, TResult>(Func<IGrouping<TKey, TElement>, TResult>)	Overloaded. Projects each element of a sequence into a new form. (Defined by Enumerable.)
	Select<IGrouping<TKey, TElement>, TResult>(Func<IGrouping<TKey, TElement>, Int32, TResult>)	Overloaded. Projects each element of a sequence into a new form by incorporating the element's index. (Defined by Enumerable.)
	SelectMany<IGrouping<TKey, TElement>, TResult>(Func<IGrouping<TKey, TElement>, IEnumerable<TResult>>)	Overloaded. Projects each element of a sequence to an IEnumerable<T> and flattens the resulting sequences into one sequence. (Defined by Enumerable.)
	SelectMany<IGrouping<TKey, TElement>, TResult>(Func<IGrouping<TKey, TElement>, Int32, IEnumerable<TResult>>)	Overloaded. Projects each element of a sequence to an IEnumerable<T>, and flattens the resulting sequences into one sequence. The index of each source element is used in the projected form of that element. (Defined by Enumerable.)
	SelectMany<IGrouping<TKey, TElement>, TCollection, TResult>(Func<IGrouping<TKey, TElement>, Int32, IEnumerable<TCollection>>, Func<IGrouping<TKey, TElement>, TCollection, TResult>)	Overloaded. Projects each element of a sequence to an IEnumerable<T>, flattens the resulting sequences into one sequence, and invokes a result selector function on each element therein. The index of each source element is used in the intermediate projected form of that element. (Defined by Enumerable.)
	SelectMany<IGrouping<TKey, TElement>, TCollection, TResult>(Func<IGrouping<TKey, TElement>, IEnumerable<TCollection>>, Func<IGrouping<TKey, TElement>, TCollection, TResult>)	Overloaded. Projects each element of a sequence to an IEnumerable<T>, flattens the resulting sequences into one sequence, and invokes a result selector function on each element therein. (Defined by Enumerable.)
	SequenceEqual<IGrouping<TKey, TElement>>(IEnumerable<IGrouping<TKey, TElement>>)	Overloaded. Determines whether two sequences are equal by comparing the elements by using the default equality comparer for their type. (Defined by Enumerable.)
	SequenceEqual<IGrouping<TKey, TElement>>(IEnumerable<IGrouping<TKey, TElement>>, IEqualityComparer<IGrouping<TKey, TElement>>)	Overloaded. Determines whether two sequences are equal by comparing their elements by using a specified IEqualityComparer<T>. (Defined by Enumerable.)
	Single<IGrouping<TKey, TElement>>()	Overloaded. Returns the only element of a sequence, and throws an exception if there is not exactly one element in the sequence. (Defined by Enumerable.)
	Single<IGrouping<TKey, TElement>>(Func<IGrouping<TKey, TElement>, Boolean>)	Overloaded. Returns the only element of a sequence that satisfies a specified condition, and throws an exception if more than one such element exists. (Defined by Enumerable.)
	SingleOrDefault<IGrouping<TKey, TElement>>()	Overloaded. 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. (Defined by Enumerable.)
	SingleOrDefault<IGrouping<TKey, TElement>>(Func<IGrouping<TKey, TElement>, Boolean>)	Overloaded. 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. (Defined by Enumerable.)
	Skip<IGrouping<TKey, TElement>>	Bypasses a specified number of elements in a sequence and then returns the remaining elements. (Defined by Enumerable.)
	SkipWhile<IGrouping<TKey, TElement>>(Func<IGrouping<TKey, TElement>, Boolean>)	Overloaded. Bypasses elements in a sequence as long as a specified condition is true and then returns the remaining elements. (Defined by Enumerable.)
	SkipWhile<IGrouping<TKey, TElement>>(Func<IGrouping<TKey, TElement>, Int32, Boolean>)	Overloaded. 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. (Defined by Enumerable.)
	Sum<IGrouping<TKey, TElement>>(Func<IGrouping<TKey, TElement>, Nullable<Decimal>>)	Overloaded. Computes the sum of the sequence of nullable Decimal values that are obtained by invoking a transform function on each element of the input sequence. (Defined by Enumerable.)
	Sum<IGrouping<TKey, TElement>>(Func<IGrouping<TKey, TElement>, Nullable<Double>>)	Overloaded. Computes the sum of the sequence of nullable Double values that are obtained by invoking a transform function on each element of the input sequence. (Defined by Enumerable.)
	Sum<IGrouping<TKey, TElement>>(Func<IGrouping<TKey, TElement>, Int32>)	Overloaded. Computes the sum of the sequence of Int32 values that are obtained by invoking a transform function on each element of the input sequence. (Defined by Enumerable.)
	Sum<IGrouping<TKey, TElement>>(Func<IGrouping<TKey, TElement>, Nullable<Int32>>)	Overloaded. Computes the sum of the sequence of nullable Int32 values that are obtained by invoking a transform function on each element of the input sequence. (Defined by Enumerable.)
	Sum<IGrouping<TKey, TElement>>(Func<IGrouping<TKey, TElement>, Int64>)	Overloaded. Computes the sum of the sequence of Int64 values that are obtained by invoking a transform function on each element of the input sequence. (Defined by Enumerable.)
	Sum<IGrouping<TKey, TElement>>(Func<IGrouping<TKey, TElement>, Nullable<Int64>>)	Overloaded. Computes the sum of the sequence of nullable Int64 values that are obtained by invoking a transform function on each element of the input sequence. (Defined by Enumerable.)
	Sum<IGrouping<TKey, TElement>>(Func<IGrouping<TKey, TElement>, Single>)	Overloaded. Computes the sum of the sequence of Single values that are obtained by invoking a transform function on each element of the input sequence. (Defined by Enumerable.)
	Sum<IGrouping<TKey, TElement>>(Func<IGrouping<TKey, TElement>, Nullable<Single>>)	Overloaded. Computes the sum of the sequence of nullable Single values that are obtained by invoking a transform function on each element of the input sequence. (Defined by Enumerable.)
	Sum<IGrouping<TKey, TElement>>(Func<IGrouping<TKey, TElement>, Double>)	Overloaded. Computes the sum of the sequence of Double values that are obtained by invoking a transform function on each element of the input sequence. (Defined by Enumerable.)
	Sum<IGrouping<TKey, TElement>>(Func<IGrouping<TKey, TElement>, Decimal>)	Overloaded. Computes the sum of the sequence of Decimal values that are obtained by invoking a transform function on each element of the input sequence. (Defined by Enumerable.)
	Take<IGrouping<TKey, TElement>>	Returns a specified number of contiguous elements from the start of a sequence. (Defined by Enumerable.)
	TakeWhile<IGrouping<TKey, TElement>>(Func<IGrouping<TKey, TElement>, Boolean>)	Overloaded. Returns elements from a sequence as long as a specified condition is true. (Defined by Enumerable.)
	TakeWhile<IGrouping<TKey, TElement>>(Func<IGrouping<TKey, TElement>, Int32, Boolean>)	Overloaded. 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. (Defined by Enumerable.)
	ToArray<IGrouping<TKey, TElement>>	Creates an array from a IEnumerable<T>. (Defined by Enumerable.)
	ToDictionary<IGrouping<TKey, TElement>, TKey>(Func<IGrouping<TKey, TElement>, TKey>)	Overloaded. Creates a Dictionary<TKey, TValue> from an IEnumerable<T> according to a specified key selector function. (Defined by Enumerable.)
	ToDictionary<IGrouping<TKey, TElement>, TKey>(Func<IGrouping<TKey, TElement>, TKey>, IEqualityComparer<TKey>)	Overloaded. Creates a Dictionary<TKey, TValue> from an IEnumerable<T> according to a specified key selector function and key comparer. (Defined by Enumerable.)
	ToDictionary<IGrouping<TKey, TElement>, TKey, TElement>(Func<IGrouping<TKey, TElement>, TKey>, Func<IGrouping<TKey, TElement>, TElement>)	Overloaded. Creates a Dictionary<TKey, TValue> from an IEnumerable<T> according to specified key selector and element selector functions. (Defined by Enumerable.)
	ToDictionary<IGrouping<TKey, TElement>, TKey, TElement>(Func<IGrouping<TKey, TElement>, TKey>, Func<IGrouping<TKey, TElement>, TElement>, IEqualityComparer<TKey>)	Overloaded. Creates a Dictionary<TKey, TValue> from an IEnumerable<T> according to a specified key selector function, a comparer, and an element selector function. (Defined by Enumerable.)
	ToList<IGrouping<TKey, TElement>>	Creates a List<T> from an IEnumerable<T>. (Defined by Enumerable.)
	ToLookup<IGrouping<TKey, TElement>, TKey>(Func<IGrouping<TKey, TElement>, TKey>)	Overloaded. Creates a Lookup<TKey, TElement> from an IEnumerable<T> according to a specified key selector function. (Defined by Enumerable.)
	ToLookup<IGrouping<TKey, TElement>, TKey>(Func<IGrouping<TKey, TElement>, TKey>, IEqualityComparer<TKey>)	Overloaded. Creates a Lookup<TKey, TElement> from an IEnumerable<T> according to a specified key selector function and key comparer. (Defined by Enumerable.)
	ToLookup<IGrouping<TKey, TElement>, TKey, TElement>(Func<IGrouping<TKey, TElement>, TKey>, Func<IGrouping<TKey, TElement>, TElement>)	Overloaded. Creates a Lookup<TKey, TElement> from an IEnumerable<T> according to specified key selector and element selector functions. (Defined by Enumerable.)
	ToLookup<IGrouping<TKey, TElement>, TKey, TElement>(Func<IGrouping<TKey, TElement>, TKey>, Func<IGrouping<TKey, TElement>, TElement>, IEqualityComparer<TKey>)	Overloaded. Creates a Lookup<TKey, TElement> from an IEnumerable<T> according to a specified key selector function, a comparer and an element selector function. (Defined by Enumerable.)
	Union<IGrouping<TKey, TElement>>(IEnumerable<IGrouping<TKey, TElement>>)	Overloaded. Produces the set union of two sequences by using the default equality comparer. (Defined by Enumerable.)
	Union<IGrouping<TKey, TElement>>(IEnumerable<IGrouping<TKey, TElement>>, IEqualityComparer<IGrouping<TKey, TElement>>)	Overloaded. Produces the set union of two sequences by using a specified IEqualityComparer<T>. (Defined by Enumerable.)
	Where<IGrouping<TKey, TElement>>(Func<IGrouping<TKey, TElement>, Boolean>)	Overloaded. Filters a sequence of values based on a predicate. (Defined by Enumerable.)
	Where<IGrouping<TKey, TElement>>(Func<IGrouping<TKey, TElement>, Int32, Boolean>)	Overloaded. Filters a sequence of values based on a predicate. Each element's index is used in the logic of the predicate function. (Defined by Enumerable.)
	Zip<IGrouping<TKey, TElement>, TSecond, TResult>	Merges two sequences by using the specified predicate function. (Defined by Enumerable.)

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Explicit Interface Implementations

	Name	Description
	IEnumerable.GetEnumerator	Returns an enumerator that iterates through the Lookup<TKey, TElement>. This class cannot be inherited.

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Remarks

A Lookup<TKey, TElement> resembles a Dictionary<TKey, TValue>. The difference is that a Dictionary<TKey, TValue> maps keys to single values, whereas a Lookup<TKey, TElement> maps keys to collections of values.

You can create an instance of a Lookup<TKey, TElement> by calling ToLookup on an object that implements IEnumerable<T>.

Note:
There is no public constructor to create a new instance of a Lookup<TKey, TElement>. Additionally, Lookup<TKey, TElement> objects are immutable, that is, you cannot add or remove elements or keys from a Lookup<TKey, TElement> object after it has been created.

Examples

The following example creates a Lookup<TKey, TElement> from a collection of objects. It then enumerates the Lookup<TKey, TElement> and outputs each key and each value in the key's associated collection of values. It also demonstrates how to use the properties Count and Item and the methods Contains and GetEnumerator.

Structure Package
   Public Company As String
   Public Weight As Double
   Public TrackingNumber As Long
End Structure

Sub LookupExample()
   ' Create a list of Packages to put into a Lookup data structure.
   Dim packages As New System.Collections.Generic.List(Of Package)(New Package() _
       {New Package With {.Company = "Coho Vineyard", .Weight = 25.2, .TrackingNumber = 89453312L}, _
         New Package With {.Company = "Lucerne Publishing", .Weight = 18.7, .TrackingNumber = 89112755L}, _
         New Package With {.Company = "Wingtip Toys", .Weight = 6.0, .TrackingNumber = 299456122L}, _
         New Package With {.Company = "Contoso Pharmaceuticals", .Weight = 9.3, .TrackingNumber = 670053128L}, _
         New Package With {.Company = "Wide World Importers", .Weight = 33.8, .TrackingNumber = 4665518773L}})

   ' Create a Lookup to organize the packages. Use the first character of Company as the key value.
   ' Select Company appended to TrackingNumber for each element value in the Lookup.
   Dim lookup As ILookup(Of Char, String) = _
       packages.ToLookup(Function(p) Convert.ToChar(p.Company.Substring(0, 1)), _
                         Function(p) p.Company & " " & p.TrackingNumber)

   Dim output As New System.Text.StringBuilder
   ' Iterate through each Example in the Lookup and output the contents.
   For Each packageGroup As IGrouping(Of Char, String) In lookup
      ' Print the key value of the Example.
      output.AppendLine(packageGroup.Key)
      ' Iterate through each value in the Example and print its value.
      For Each str As String In packageGroup
         output.AppendLine(String.Format("    {0}", str))
      Next
   Next

   ' Display the output.
   outputBlock.Text &= output.ToString() & vbCrLf

   ' This code produces the following output:
   '
   ' C
   '     Coho Vineyard 89453312
   '     Contoso Pharmaceuticals 670053128
   ' L
   '     Lucerne Publishing 89112755
   ' W
   '     Wingtip Toys 299456122
   '     Wide World Importers 4665518773

   ' Get the number of key-collection pairs in the Lookup.
   Dim count As Integer = lookup.Count

   ' Select a collection of Packages by indexing directly into the Lookup.
   Dim cgroup As System.Collections.Generic.IEnumerable(Of String) = lookup("C"c)

   output = New System.Text.StringBuilder
   ' Output the results.
   output.AppendLine(vbCrLf & "Packages that have a key of 'C':")
   For Each str As String In cgroup
      output.AppendLine(str)
   Next

   ' Display the output.
   outputBlock.Text &= output.ToString() & vbCrLf

   ' This code produces the following output:
   '
   ' Packages that have a key of 'C'
   ' Coho Vineyard 89453312
   ' Contoso Pharmaceuticals 670053128

   ' Determine if there is a key with the value 'G' in the Lookup.
   Dim hasG As Boolean = lookup.Contains("G"c)
End Sub

class Package
{
   public string Company;
   public double Weight;
   public long TrackingNumber;
}

public static void LookupExample(System.Windows.Controls.TextBlock outputBlock)
{
   // Create a list of Packages to put into a Lookup data structure.
   List<Package> packages = new List<Package> { new Package { Company = "Coho Vineyard", Weight = 25.2, TrackingNumber = 89453312L },
                                                new Package { Company = "Lucerne Publishing", Weight = 18.7, TrackingNumber = 89112755L },
                                                new Package { Company = "Wingtip Toys", Weight = 6.0, TrackingNumber = 299456122L },
                                                new Package { Company = "Contoso Pharmaceuticals", Weight = 9.3, TrackingNumber = 670053128L },
                                                new Package { Company = "Wide World Importers", Weight = 33.8, TrackingNumber = 4665518773L } };

   // Create a Lookup to organize the packages. Use the first character of Company as the key value.
   // Select Company appended to TrackingNumber for each element value in the Lookup.
   Lookup<char, string> lookup = (Lookup<char, string>)packages.ToLookup(p => Convert.ToChar(p.Company.Substring(0, 1)),
                                                   p => p.Company + " " + p.TrackingNumber);

   // Iterate through each Example in the Lookup and output the contents.
   foreach (IGrouping<char, string> packageGroup in lookup)
   {
      // Print the key value of the Example.
      outputBlock.Text += packageGroup.Key + "\n";
      // Iterate through each value in the Example and print its value.
      foreach (string str in packageGroup)
         outputBlock.Text += String.Format("    {0}", str) + "\n";
   }

   // This code produces the following output:
   //
   // C
   //     Coho Vineyard 89453312
   //     Contoso Pharmaceuticals 670053128
   // L
   //     Lucerne Publishing 89112755
   // W
   //     Wingtip Toys 299456122
   //     Wide World Importers 4665518773

   // Get the number of key-collection pairs in the Lookup.
   int count = lookup.Count;

   // Select a collection of Packages by indexing directly into the Lookup.
   IEnumerable<string> cgroup = lookup['C'];

   // Output the results.
   outputBlock.Text += "\nPackages that have a key of 'C':" + "\n";
   foreach (string str in cgroup)
      outputBlock.Text += str + "\n";

   // This code produces the following output:
   //
   // Packages that have a key of 'C'
   // Coho Vineyard 89453312
   // Contoso Pharmaceuticals 670053128

   // Determine if there is a key with the value 'G' in the Lookup.
   bool hasG = lookup.Contains('G');
}

Version Information

Silverlight

Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1, Windows Phone OS 7.0

XNA Framework

Supported in: Xbox 360, Windows Phone OS 7.0

Platforms

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.

Thread Safety

Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.

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