Method ExceptBy

ExceptBy<TSource, TKey>(IEnumerable<TSource>, IEnumerable<TSource>, Func<TSource, TKey>)

Returns the set of elements in the first sequence which aren't in the second sequence, according to a given key selector.

Declaration

public static IEnumerable<TSource> ExceptBy<TSource, TKey>(this IEnumerable<TSource> first, IEnumerable<TSource> second, Func<TSource, TKey> keySelector)

Parameters

Type	Name	Description
IEnumerable<TSource>	first	The sequence of potentially included elements.
IEnumerable<TSource>	second	The sequence of elements whose keys may prevent elements in first from being returned.
Func<TSource, TKey>	keySelector	The mapping from source element to key.

Returns

Type	Description
IEnumerable<TSource>	A sequence of elements from first whose key was not also a key for any element in second.

Type Parameters

Name	Description
TSource	The type of the elements in the input sequences.
TKey	The type of the key returned by keySelector.

Remarks

This is a set operation; if multiple elements in first have equal keys, only the first such element is returned. This operator uses deferred execution and streams the results, although a set of keys from second is immediately selected and retained.

Examples

The following code example demonstrates how to filter items from a sequence using a second sequence using ExceptBy.

var sequence = new[] { "aaa", "bb", "c", "dddd", };

// Determine if sequence ends with the ends sequence
var result = sequence.ExceptBy(new[] { "a", "b", }, s => s[0]);

Console.WriteLine(
    "[" +
    string.Join(", ", result) +
    "]");

// This code produces the following output:
// [c, dddd]

Exceptions

Type	Condition
ArgumentNullException	first, second, or keySelector is null.

ExceptBy<TSource, TKey>(IEnumerable<TSource>, IEnumerable<TSource>, Func<TSource, TKey>, IEqualityComparer<TKey>?)

Returns the set of elements in the first sequence which aren't in the second sequence, according to a given key selector.

Declaration

public static IEnumerable<TSource> ExceptBy<TSource, TKey>(this IEnumerable<TSource> first, IEnumerable<TSource> second, Func<TSource, TKey> keySelector, IEqualityComparer<TKey>? keyComparer)

Parameters

Type	Name	Description
IEnumerable<TSource>	first	The sequence of potentially included elements.
IEnumerable<TSource>	second	The sequence of elements whose keys may prevent elements in first from being returned.
Func<TSource, TKey>	keySelector	The mapping from source element to key.
IEqualityComparer<TKey>	keyComparer	The equality comparer to use to determine whether or not keys are equal. If null, the default equality comparer for TSource is used.

Returns

Type	Description
IEnumerable<TSource>	A sequence of elements from first whose key was not also a key for any element in second.

Type Parameters

Name	Description
TSource	The type of the elements in the input sequences.
TKey	The type of the key returned by keySelector.

Remarks

This is a set operation; if multiple elements in first have equal keys, only the first such element is returned. This operator uses deferred execution and streams the results, although a set of keys from second is immediately selected and retained.

Examples

The following code example demonstrates how to filter items from a sequence using a second sequence using ExceptBy.

var sequence = new[]
{
    (key: "aaa", value: 1),
    (key: "bb", value: 2),
    (key: "c", value: 3),
    (key: "dddd", value: 4),
};

// Determine if sequence ends with the ends sequence
var result = sequence
    .ExceptBy(
        new[] { (key: "A", value: 13), (key: "D", value: 14), },
        s => s.key[..1],
        StringComparer.OrdinalIgnoreCase);

Console.WriteLine(
    "[" +
    string.Join(", ", result) +
    "]");

// This code produces the following output:
// [(bb, 2), (c, 3)]

Exceptions

Type	Condition
ArgumentNullException	first, second, or keySelector, or keyComparer is null.