为什么有OrderBy和OrderByDescending而不是OrderBy(SortOrder)？

Question

目前,如果我们将排序方向作为外部依赖关系,我们必须使用if来应用这个方向:

public static IEnumerable<FileInfo> getlist(string directory, string searchPattern, string order)
{
    var files = new DirectoryInfo(directory).EnumerateFiles(searchPattern);

    if (order == "A")
        return files.OrderBy(f => f.CreationTime);

    return files.OrderByDescending(f => f.CreationTime);
}

为什么OrderBy不会将订单方向作为参数？在Reflector我看到它或多或少在内部实现,但由于一些奇怪的原因没有暴露.

我更愿意写这样的东西:

public static IEnumerable<FileInfo> getlist(string directory, string searchPattern, string order)
{
    return new DirectoryInfo(directory)
        .EnumerateFiles(searchPattern)
        .OrderBy(f => f.CreationTime, order == "A" ? SortOrder.Ascending : SortOrder.Descending);
}

更新:

我可以自己写这个,只是希望它已经在框架中:

public static IOrderedEnumerable<TSource> OrderBy<TSource, TKey>(
    this IEnumerable<TSource> source,
    Func<TSource, TKey> keySelector,
    ListSortDirection order)
{
    switch (order)
    {
        case ListSortDirection.Ascending: return source.OrderBy(keySelector);
        case ListSortDirection.Descending: return source.OrderByDescending(keySelector);
    }

    throw new ArgumentOutOfRangeException("order");
}

Answer 1

由于SortOrder枚举在技术上可以承担超过2个值(想想(SortOrder) 35),因此它不会完全捕获二元性.有两种方法可以确保没有歧义或需要进行范围检查(从您的示例btw中遗漏).

也就是说,这是你想要的方法:

public static IOrderedEnumerable<TSource> OrderBy<TSource, TKey>(
    this IEnumerable<TSource> source,
    Func<TSource, TKey> keySelector,
    SortOrder order)
{
    if(order < SortOrder.Ascending || order > SortOrder.Descending)
    {
        throw new ArgumentOutOfRangeException("order");
    }

    return order == SortOrder.Ascending
        ? source.OrderBy(keySelector)
        : source.OrderByDescending(keySelector);
}

Answer 2

该OrderBy方法已具有您需要的灵活性,以及​​更多,因为它可以采用可选IComparer<T>参数:

return new DirectoryInfo(directory)
    .EnumerateFiles(searchPattern)
    .OrderBy(f => f.CreationTime, order == "A"
                                      ? Comparer<DateTime>.Default
                                      : new DescendingComparer<DateTime>);

// ...

public DescendingComparer<T> : Comparer<T>
{
    public override int Compare(T x, T y)
    {
        return Comparer<T>.Default.Compare(y, x);
    }
}