我的MiscUtil库包含一个ProjectionComparer，可从一个投影委托中构建一个IComparer
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 pre for the given value by projecting it and /// then asking the equality comparer to find the hash pre of /// the resulting key. /// </summary> public int GetHashCode(TSource obj) { if (obj == null) { throw new ArgumentNullException("obj"); } return comparer.GetHashCode(projection(obj)); }}这是一个示例用法：
var f3 = f1.Except(f2, ProjectionEqualityComparer<Foo>.Create(a => a.key));
