Question

因为LINQ提供了所有简洁的功能，.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。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？类似的东西：

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

我很确定以上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库包含用于构建IComparer＆lt; T＆gt;的ProjectionComparer。来自投影代表。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，可以做你想做的事情

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上提供额外的助手是一种更简洁的方法。

请参阅：this question

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 - lambda compare selector for Linq，它也有LINQ标准查询运算符的扩展。

Answer 5

为什么不能这样：

    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();
        }
    }

然后你可以做类似的事情（例如Intersect中的IEnumerable<T>）：

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

Comparer类可以放在实用程序项目中，并在需要的地方使用。

我现在才看到Sam Saffron的答案（与此非常相似）。

Answer 6

所以我知道这是解决您问题的方法，但是当我发现遇到这种情况时（合并列表并过滤重复项），Distinct需要一个我没有的IEquityComparer，我通常会选择Concat->组->选择。

原始

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

新

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，您可以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来自哪个列表，您可以说：

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

希望有帮助！

Answer 7

对于小型套装，你可以这样做：

f3 = f1.Where(x1 => f2.All(x2 => x2.key != x1.key));

对于大型集合，您将需要在搜索中更有效的内容，如：

var tmp = new HashSet<string>(f2.Select(f => f.key));
f3 = f1.Where(f => tmp.Add(f.key));

但是，在这里，键的Type必须实现IEqualityComparer（上面我假设它是string）。所以，在这种情况下，这并没有真正回答你关于使用lambda的问题，但它确实使用的代码少了一些答案。

您可能依赖优化程序并将第二个解决方案缩短为：

f3 = f1.Where(x1 => (new HashSet<string>(f2.Select(x2 => x2.key))).Add(x1.key));

但是，我还没有通过测试来了解它是否以相同的速度运行。并且那个班轮可能太聪明了。

Answer 8

基于其他答案，我最喜欢创建一个通用比较器。但是我遇到了Linq Enumerable.Union（msdn .Net reference）的问题，那就是它直接使用GetHashCode而不考虑Equals覆盖。

这使我将比较器实现为：

public class Comparer<T> : IEqualityComparer<T>
{
    private readonly Func<T, int> _hashFunction;

    public Comparer(Func<T, int> hashFunction)
    {
        _hashFunction = hashFunction;
    }

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

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

像这样使用它：

list.Union(otherList, new Comparer<T>( x => x.StringValue.GetHashCode()));

请注意，由于所比较的信息已映射到int值，因此比较可能会产生误报。

Answer 9

与其他答案类似，但更简洁的C＃7：

public class LambdaComparer<T> : IEqualityComparer<T> {
  private readonly Func<T, T, bool> lambdaComparer;
  private readonly Func<T, int> lambdaHash;
  public LambdaComparer(Func<T, T, bool> lambdaComparer) : this(lambdaComparer, o => o.GetHashCode()) {}
  public LambdaComparer(Func<T, T, bool> lambdaComparer, Func<T, int> lambdaHash) { this.lambdaComparer = lambdaComparer; this.lambdaHash = lambdaHash; }
  public bool Equals(T x, T y) => lambdaComparer is null ? false : lambdaComparer(x, y);
  public int GetHashCode(T obj) => lambdaHash is null ? 0 : lambdaHash(obj);
}

然后：

var a=List<string> { "a", "b" };
var b=List<string> { "a", "*" };
return a.SequenceEquals(b, new LambdaComparer<string>((s1, s2) => s1 is null ? s2 is null : s1 == s2 || s2 == "*");

我可以指定内联显式类型比较器吗？

9 个答案: