Method LeftOuterHashJoin

LeftOuterHashJoin<TLeft, TRight, TKey>(IEnumerable<TLeft>, IEnumerable<TRight>, Func<TLeft, TKey>, Func<TRight, TKey>, IEqualityComparer<TKey>?)

Performs a left outer join on two heterogeneous sequences.

Declaration

public static IEnumerable<(TLeft Left, TRight? Right)> LeftOuterHashJoin<TLeft, TRight, TKey>(this IEnumerable<TLeft> left, IEnumerable<TRight> right, Func<TLeft, TKey> leftKeySelector, Func<TRight, TKey> rightKeySelector, IEqualityComparer<TKey>? comparer = null)

Parameters

Type	Name	Description
IEnumerable<TLeft>	left	The first sequence.
IEnumerable<TRight>	right	The second sequence.
Func<TLeft, TKey>	leftKeySelector	A function to extract the join key from each element of the first sequence.
Func<TRight, TKey>	rightKeySelector	A function to extract the join key from each element of the second sequence.
IEqualityComparer<TKey>	comparer	An IEqualityComparer<T> to hash and compare keys.

Returns

Type	Description
IEnumerable<(TLeft Left, TRight Right)>	A sequence containing values from a left outer join of the two input sequences.

Type Parameters

Name	Description
TLeft	The type of elements in the first sequence.
TRight	The type of elements in the second sequence.
TKey	The type of the key returned by the key selector functions.

Remarks

The result of this method is a `left outer`-join. All elements from left are returned, along with any matching elements from right, if any are present. If no values in right match, default is returned for the right element.

This method is implemented using a `hash`-join. The sequence right is cached as an ILookup<TKey, TElement>, and for-each element of left, the lookup is accessed to find any matching elements.

This method uses deferred execution and streams its results.

Examples

The following code example demonstrates how to execute an left outer hash join of two sequences using LeftOuterHashJoin.

var people = new Person[]
{
    new("John Doe", 1),
    new("Jane Doe", 6),
    new("Lucy Ricardo", 4),
    new("Ricky Ricardo", 2),
    new("Fred Mertz", 3),
    new("Ethel Mertz", 5),
};

var pets = new Pet[]
{
    new("Bear", 8),
    new("Polly", 2),
    new("Minnie", 2),
    new("Mittens", 1),
    new("Patches", 1),
    new("Paws", 1),
};

var results = people
    .LeftOuterHashJoin(
        pets,
        p => p.PersonId,
        p => p.PersonId);

foreach (var (person, pet) in results)
{
    Console.WriteLine($"({person.Name}, {pet?.Name ?? "No Pets"})");
}

// This code produces the following output:
// (John Doe, Mittens)
// (John Doe, Patches)
// (John Doe, Paws)
// (Jane Doe, No Pets)
// (Lucy Ricardo, No Pets)
// (Ricky Ricardo, Polly)
// (Ricky Ricardo, Minnie)
// (Fred Mertz, No Pets)
// (Ethel Mertz, No Pets)

record Person(string Name, int PersonId);
record Pet(string Name, int PersonId);

Exceptions

Type	Condition
ArgumentNullException	left, right, leftKeySelector, or rightKeySelector is null.

LeftOuterHashJoin<TLeft, TRight, TKey, TResult>(IEnumerable<TLeft>, IEnumerable<TRight>, Func<TLeft, TKey>, Func<TRight, TKey>, Func<TLeft, TResult>, Func<TLeft, TRight, TResult>, IEqualityComparer<TKey>?)

Performs a left outer join on two heterogeneous sequences.

Declaration

public static IEnumerable<TResult> LeftOuterHashJoin<TLeft, TRight, TKey, TResult>(this IEnumerable<TLeft> left, IEnumerable<TRight> right, Func<TLeft, TKey> leftKeySelector, Func<TRight, TKey> rightKeySelector, Func<TLeft, TResult> leftResultSelector, Func<TLeft, TRight, TResult> bothResultSelector, IEqualityComparer<TKey>? comparer = null)

Parameters

Type	Name	Description
IEnumerable<TLeft>	left	The first sequence.
IEnumerable<TRight>	right	The second sequence.
Func<TLeft, TKey>	leftKeySelector	A function to extract the join key from each element of the first sequence.
Func<TRight, TKey>	rightKeySelector	A function to extract the join key from each element of the second sequence.
Func<TLeft, TResult>	leftResultSelector	A function to create a result element from a left value that does not have a matching value in right.
Func<TLeft, TRight, TResult>	bothResultSelector	A function to create a result element from two matching elements.
IEqualityComparer<TKey>	comparer	An IEqualityComparer<T> to hash and compare keys.

Returns

Type	Description
IEnumerable<TResult>	A sequence containing values projected from a left outer join of the two input sequences.

Type Parameters

Name	Description
TLeft	The type of elements in the first sequence.
TRight	The type of elements in the second sequence.
TKey	The type of the key returned by the key selector functions.
TResult	The type of the result elements.

Remarks

The result of this method is a `left outer`-join. Values are projected using bothResultSelector when matching elements are found in both left and right sequences. Values are projected using leftKeySelector when elements in left do not have a matching element in right.

This method is implemented using a `hash`-join. The sequence right is cached as an ILookup<TKey, TElement>, and for-each element of left, the lookup is accessed to find any matching elements.

This method uses deferred execution and streams its results.

Examples

The following code example demonstrates how to execute an left outer hash join of two sequences using LeftOuterHashJoin.

var people = new Person[]
{
    new("John Doe", 1),
    new("Jane Doe", 6),
    new("Lucy Ricardo", 4),
    new("Ricky Ricardo", 2),
    new("Fred Mertz", 3),
    new("Ethel Mertz", 5),
};

var pets = new Pet[]
{
    new("Bear", 8),
    new("Polly", 2),
    new("Minnie", 2),
    new("Mittens", 1),
    new("Patches", 1),
    new("Paws", 1),
};

var results = people
    .LeftOuterHashJoin(
        pets,
        p => p.PersonId,
        p => p.PersonId,
        person => $"({person.Name}, No Pets)",
        (person, pet) => $"({person.Name}, {pet.Name})");

foreach (var str in results)
    Console.WriteLine(str);

// This code produces the following output:
// (John Doe, Mittens)
// (John Doe, Patches)
// (John Doe, Paws)
// (Jane Doe, No Pets)
// (Lucy Ricardo, No Pets)
// (Ricky Ricardo, Polly)
// (Ricky Ricardo, Minnie)
// (Fred Mertz, No Pets)
// (Ethel Mertz, No Pets)

record Person(string Name, int PersonId);
record Pet(string Name, int PersonId);

Exceptions

Type	Condition
ArgumentNullException	left, right, leftKeySelector, rightKeySelector, leftResultSelector, or bothResultSelector is null.