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using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
namespace DogmaMix.Core.Comparers
{
/// <summary>
/// Represents a generic equality comparison operation that compares keys extracted from source items.
/// </summary>
/// <typeparam name="TSource">The type of the source items whose keys are to be extracted.</typeparam>
/// <typeparam name="TKey">The type of the keys on which the comparison is internally performed.</typeparam>
/// <remarks>
/// <para>
/// Instances of this class may be created through the
/// <see cref="KeyEqualityComparer.Create{TSource, TKey}(Func{TSource, TKey}, IEqualityComparer{TKey})"/> factory method.
/// </para>
/// <para>
/// This class is defined separately from <see cref="KeyComparer{TSource, TKey}"/> for reasons
/// discussed under the remarks of the latter class.
/// </para>
/// </remarks>
public class KeyEqualityComparer<TSource, TKey> : EqualityComparerBase<TSource>
{
private readonly Func<TSource, TKey> _keySelector;
private readonly IEqualityComparer<TKey> _innerEqualityComparer;
/// <summary>
/// Initializes a new instance of the <see cref="KeyEqualityComparer{TSource,TKey}"/> class,
/// using the specified key extraction function and the specified or default
/// equality comparer for extracted keys.
/// </summary>
/// <param name="keySelector">A function to extract the key from a source item.</param>
/// <param name="innerEqualityComparer">
/// The equality comparison operation to apply to the extracted keys.
/// If the argument is omitted or specified as <see langword="null"/>,
/// the <see cref="EqualityComparer{T}.Default"/> comparer for type <typeparamref name="TKey"/> is used.
/// </param>
/// <exception cref="ArgumentNullException"><paramref name="keySelector"/> is <see langword="null"/>.</exception>
protected internal KeyEqualityComparer(
Func<TSource, TKey> keySelector,
IEqualityComparer<TKey> innerEqualityComparer = null)
{
ArgumentValidate.NotNull(keySelector, nameof(keySelector));
_keySelector = keySelector;
_innerEqualityComparer = innerEqualityComparer ?? EqualityComparer<TKey>.Default;
}
/// <summary>
/// Determines whether the keys extracted from the specified source items are equal.
/// </summary>
/// <param name="x">The first source item to compare.</param>
/// <param name="y">The second source item to compare.</param>
/// <returns><see langword="true"/> if the keys are equal; otherwise, <see langword="false"/>.</returns>
protected override bool EqualsNonNull(TSource x, TSource y)
{
TKey xKey = _keySelector(x);
TKey yKey = _keySelector(y);
return _innerEqualityComparer.Equals(xKey, yKey);
}
/// <summary>
/// Returns a hash code for the key extracted from the specified source item.
/// </summary>
/// <param name="item">The source item for which to get a hash code.</param>
/// <returns>A hash code for the extracted key.</returns>
protected override int GetHashCodeNonNull(TSource item)
{
TKey itemKey = _keySelector(item);
return _innerEqualityComparer.GetHashCode(itemKey);
}
}
}
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