我可以指定我的显式类型比较内联吗？
因此.NET 3.0 / 3.5为我们提供了许多新的查询，排序和操作数据的方法，这要归功于LINQ提供的所有整齐的函数。 有时，我需要比较没有内置比较运算符的用户定义类型。 在很多情况下，比较非常简单 - 例如foo1.key？= foo2.key。 我可以使用匿名代理/ lambda函数简单地指定内联比较，而不是为类型创建新的IEqualityComparer吗？ 就像是：
var f1 = ...,
f2 = ...;
var f3 = f1.Except(
f2, new IEqualityComparer(
(Foo a, Foo b) => a.key.CompareTo(b.key)
) );
我非常肯定上述不实际工作。 我只是不想让整个班级变得“沉重”，只是为了告诉程序如何比较苹果和苹果。
我的MiscUtil库包含一个ProjectionComparer，用于从投影委托构建一个IComparer <T>。 这将是10分钟的工作，使一个ProjectionEqualityComparer做同样的事情。
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));
}
}
这里有一个示例使用：
var f3 = f1.Except(f2, ProjectionEqualityComparer<Foo>.Create(a => a.key));
这里是一个简单的助手类，应该做你想做的事
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));
我发现在IEnumerable上提供额外的帮助是一种更简单的方法。
见：这个问题
所以你可以有：
var f3 = f1.Except(
f2,
(a, b) => a.key.CompareTo(b.key)
);
如果您正确定义扩展方法
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