我可以指定内联显式类型比较器吗？

Question

因此,.NET 3.0/3.5为我们提供了许多查询,排序和操作数据的新方法,这要归功于LINQ提供的所有简洁功能.有时,我需要比较没有内置比较运算符的用户定义类型.在许多情况下,比较非常简单 - 比如foo1.key？= foo2.key.我可以使用匿名委托/ lambda函数简单地指定内联比较,而不是为该类型创建新的IEqualityComparer吗？就像是:

var f1 = ...,
    f2 = ...;
var f3 = f1.Except(
           f2, new IEqualityComparer(
             (Foo a, Foo b) => a.key.CompareTo(b.key)
           ) );

我很确定上面的内容实际上并不奏效.我只是不想让整个班级做一些"沉重"的事情只是为了告诉程序如何比较苹果和苹果.

Answer 1

我的MiscUtil库包含一个ProjectionComparer,用于从投影委托构建IComparer <T>.让ProjectionEqualityComparer做同样的事情将是10分钟的工作.

编辑:这是ProjectionEqualityComparer的代码:

using System;
using System.Collections.Generic;

/// <summary>
/// Non-generic class to produce instances of the generic class,
/// optionally using type inference.
/// </summary>
public static class ProjectionEqualityComparer
{
    /// <summary>
    /// Creates an instance of ProjectionEqualityComparer using the specified projection.
    /// </summary>
    /// <typeparam name="TSource">Type parameter for the elements to be compared</typeparam>
    /// <typeparam name="TKey">Type parameter for the keys to be compared,
    /// after being projected from the elements</typeparam>
    /// <param name="projection">Projection to use when determining the key of an element</param>
    /// <returns>A comparer which will compare elements by projecting 
    /// each element to its key, and comparing keys</returns>
    public static ProjectionEqualityComparer<TSource, TKey> Create<TSource, TKey>(Func<TSource, TKey> projection)
    {
        return new ProjectionEqualityComparer<TSource, TKey>(projection);
    }

    /// <summary>
    /// Creates an instance of ProjectionEqualityComparer using the specified projection.
    /// The ignored parameter is solely present to aid type inference.
    /// </summary>
    /// <typeparam name="TSource">Type parameter for the elements to be compared</typeparam>
    /// <typeparam name="TKey">Type parameter for the keys to be compared,
    /// after being projected from the elements</typeparam>
    /// <param name="ignored">Value is ignored - type may be used by type inference</param>
    /// <param name="projection">Projection to use when determining the key of an element</param>
    /// <returns>A comparer which will compare elements by projecting
    /// each element to its key, and comparing keys</returns>
    public static ProjectionEqualityComparer<TSource, TKey> Create<TSource, TKey>
        (TSource ignored,
         Func<TSource, TKey> projection)
    {
        return new ProjectionEqualityComparer<TSource, TKey>(projection);
    }

}

/// <summary>
/// Class generic in the source only to produce instances of the 
/// doubly generic class, optionally using type inference.
/// </summary>
public static class ProjectionEqualityComparer<TSource>
{
    /// <summary>
    /// Creates an instance of ProjectionEqualityComparer using the specified projection.
    /// </summary>
    /// <typeparam name="TKey">Type parameter for the keys to be compared,
    /// after being projected from the elements</typeparam>
    /// <param name="projection">Projection to use when determining the key of an element</param>
    /// <returns>A comparer which will compare elements by projecting each element to its key,
    /// and comparing keys</returns>        
    public static ProjectionEqualityComparer<TSource, TKey> Create<TKey>(Func<TSource, TKey> projection)
    {
        return new ProjectionEqualityComparer<TSource, TKey>(projection);
    }
}

/// <summary>
/// Comparer which projects each element of the comparison to a key, and then compares
/// those keys using the specified (or default) comparer for the key type.
/// </summary>
/// <typeparam name="TSource">Type of elements which this comparer 
/// will be asked to compare</typeparam>
/// <typeparam name="TKey">Type of the key projected
/// from the element</typeparam>
public class ProjectionEqualityComparer<TSource, TKey> : IEqualityComparer<TSource>
{
    readonly Func<TSource, TKey> projection;
    readonly IEqualityComparer<TKey> comparer;

    /// <summary>
    /// Creates a new instance using the specified projection, which must not be null.
    /// The default comparer for the projected type is used.
    /// </summary>
    /// <param name="projection">Projection to use during comparisons</param>
    public ProjectionEqualityComparer(Func<TSource, TKey> projection)
        : this(projection, null)
    {
    }

    /// <summary>
    /// Creates a new instance using the specified projection, which must not be null.
    /// </summary>
    /// <param name="projection">Projection to use during comparisons</param>
    /// <param name="comparer">The comparer to use on the keys. May be null, in
    /// which case the default comparer will be used.</param>
    public ProjectionEqualityComparer(Func<TSource, TKey> projection, IEqualityComparer<TKey> comparer)
    {
        if (projection == null)
        {
            throw new ArgumentNullException("projection");
        }
        this.comparer = comparer ?? EqualityComparer<TKey>.Default;
        this.projection = projection;
    }

    /// <summary>
    /// Compares the two specified values for equality by applying the projection
    /// to each value and then using the equality comparer on the resulting keys. Null
    /// references are never passed to the projection.
    /// </summary>
    public bool Equals(TSource x, TSource y)
    {
        if (x == null && y == null)
        {
            return true;
        }
        if (x == null || y == null)
        {
            return false;
        }
        return comparer.Equals(projection(x), projection(y));
    }

    /// <summary>
    /// Produces a hash code for the given value by projecting it and
    /// then asking the equality comparer to find the hash code of
    /// the resulting key.
    /// </summary>
    public int GetHashCode(TSource obj)
    {
        if (obj == null)
        {
            throw new ArgumentNullException("obj");
        }
        return comparer.GetHashCode(projection(obj));
    }
}

以下是一个示例用法:

var f3 = f1.Except(f2, ProjectionEqualityComparer<Foo>.Create(a => a.key));

Answer 2

这是一个简单的助手类,应该做你想要的

public class EqualityComparer<T> : IEqualityComparer<T>
{
    public EqualityComparer(Func<T, T, bool> cmp)
    {
        this.cmp = cmp;
    }
    public bool Equals(T x, T y)
    {
        return cmp(x, y);
    }

    public int GetHashCode(T obj)
    {
        return obj.GetHashCode();
    }

    public Func<T, T, bool> cmp { get; set; }
}

你可以像这样使用它:

processed.Union(suburbs, new EqualityComparer<Suburb>((s1, s2)
    => s1.SuburbId == s2.SuburbId));

Answer 3

我发现在IEnumerable上提供额外的帮助是一种更干净的方法.

见:这个问题

所以你可以:

var f3 = f1.Except(
           f2, 
             (a, b) => a.key.CompareTo(b.key)
            );

如果正确定义扩展方法

Answer 4

这个项目做了类似的事情:AnonymousComparer - Linq的lambda比较选择器,它也有LINQ标准查询运算符的扩展.

Answer 5

为什么不是这样的:

    public class Comparer<T> : IEqualityComparer<T>
    {
        private readonly Func<T, T, bool> _equalityComparer;

        public Comparer(Func<T, T, bool> equalityComparer)
        {
            _equalityComparer = equalityComparer;
        }

        public bool Equals(T first, T second)
        {
            return _equalityComparer(first, second);
        }

        public int GetHashCode(T value)
        {
            return value.GetHashCode();
        }
    }

然后你可以例如像(如在的情况下做Intersect的IEnumerable<T>):

list.Intersect(otherList, new Comparer<T>( (x, y) => x.Property == y.Property));

本Comparer类可以放在一个公共事业项目,被需要的地方使用.

我现在才看到Sam Saffron的答案(与此非常相似).

Answer 6

所以我知道这是您的问题的解决方法，但是当我发现我遇到了您在此处遇到的情况（组合列表并过滤重复项）并且 Distinct 需要一个我没有的 IEquityComparer 时，我通常会去使用 Concat -> 组 -> 选择。

原来的

var f1 = ...,
    f2 = ...;
var f3 = f1.Except(
           f2, new IEqualityComparer(
             (Foo a, Foo b) => a.key.CompareTo(b.key)
           ) );

新的

var f1 = ...,
    f2 = ...;
var distinctF = f1
    .Concat(f2)                       // Combine the lists
    .GroupBy(x => x.key)              // Group them up by our equity comparison key
    .Select(x => x.FirstOrDefault()); // Just grab one of them.

请注意，在 GroupBy() 中，您有机会添加逻辑来创建混合键，例如：

.GroupBy(f => new Uri(f.Url).PathAndQuery)  

以及在 Select() 中，如果您想指定结果项目来自哪个列表，您可以说：

.Select(x => x.FirstOrDefault(y => f1.Contains(y))

希望有帮助！