我有一个类(Patch),我想要排序,所以我实现了IComparer.

但是,它需要根据用户的需要进行排序,例如：
– key1,key2,key3
– key1,key3,key2

对于每个密钥比较,我写了一个IComparer类,但是,我想知道如何实现它的连接.即排序时我只能传递一个IComparer实例.

或者我应该为每种完整排序创建一个IComparer类,即IComparerKey1Key2Key3,IComparerKey1Key3Key2等？

解决方法:

您可以创建一个通用比较器,让代理人选择密钥：

class ByKeyComparer<T, TKey> : IComparer<T>
{
    private readonly Func<T, TKey> _keySelector;
    private readonly IComparer<TKey> _keyComparer;

    public ByKeyComparer(Func<T, TKey> keySelector, IComparer<TKey> keyComparer = null)
    {
        if (keySelector == null) throw new ArgumentNullException("keySelector");
        _keySelector = keySelector;
        _keyComparer = keyComparer ?? Comparer<TKey>.Default;
    }

    public int Compare(T x, T y)
    {
        return _keyComparer.Compare(_keySelector(x), _keySelector(y));
    }
}

使用帮助器类来利用类型推断(因此您不需要指定键的类型)：

static class ByKeyComparer<T>
{
    public static IComparer<T> Create<TKey>(Func<T, TKey> keySelector, IComparer<TKey> keyComparer = null)
    {
        return new ByKeyComparer<T, TKey>(keySelector, keyComparer);
    }
}

你可以像这样使用它：

var patchVersionComparer = ByKeyComparer<Patch>.Create(p => p.Version);
patches.Sort(patchVersionComparer);

如果需要组合多个比较键,可以创建一个使用其他比较器的比较器：

class CompositeComparer<T> : IComparer<T>
{
    private readonly IEnumerable<IComparer<T>> _comparers;

    public CompositeComparer(IEnumerable<IComparer<T>> comparers)
    {
        if (comparers == null) throw new ArgumentNullException("comparers");
        _comparers = comparers;
    }

    public CompositeComparer(params IComparer<T>[] comparers)
        : this((IEnumerable<IComparer<T>>)comparers)
    {
    }

    public int Compare(T x, T y)
    {
        foreach (var comparer in _comparers)
        {
            int result = comparer.Compare(x, y);
            if (result != 0)
                return result;
        }
        return 0;
    }
}

用法示例：

var comparer = new CompositeComparer<Patch>(
                       ByKeyComparer<Patch>.Create(p => p.Key1),
                       ByKeyComparer<Patch>.Create(p => p.Key2),
                       ByKeyComparer<Patch>.Create(p => p.Key3));
patches.Sort(comparer);

编辑：这是一个更流畅的API：

static class ByKeyComparer<T>
{
    public static IComparer<T> CompareBy<TKey>(Func<T, TKey> keySelector, IComparer<TKey> keyComparer = null)
    {
        return new ByKeyComparer<T, TKey>(keySelector, keyComparer);
    }
}

static class ComparerExtensions
{
    public static IComparer<T> ThenBy<T, TKey>(this IComparer<T> comparer, Func<T, TKey> keySelector, IComparer<TKey> keyComparer = null)
    {
        var newComparer = ByKeyComparer<T>.CompareBy(keySelector, keyComparer);

        var composite = comparer as CompositeComparer<T>;
        if (composite != null)
            return new CompositeComparer<T>(composite.Comparers.Concat(new[] { newComparer }));
        return new CompositeComparer<T>(comparer, newComparer);
    }
}

例：

var comparer = ByKeyComparer<Patch>.CompareBy(p => p.Key1)
                                   .ThenBy(p => p.Key2)
                                   .ThenBy(p => p.Key3);
patches.Sort(comparer);

(显然你可能想要添加* CompareBy和ThenBy方法的降序版本以允许按降序排序)