Question

受this的启发我做了这个：

ISortable

public interface ISortable<T>
{
    IPageable<T> OrderBy<U>(Expression<Func<T, U>> orderBy);
    IPageable<T> OrderByDescending<U>(Expression<Func<T, U>> orderBy);
}

IPageable

public interface IPageable<T> : ISortable<T>, IEnumerable<T>
{
    IPageable<T> Page(int pageNumber, int pageSize);
    List<T> ToList();
    int TotalPages { get; }
    int TotalItemCount { get; }
    int PageNumber { get; }
    int? PageSize { get; }
}

可分页

public class Pageable<T> : IPageable<T>
{
    private readonly IQueryable<T> _countQuery;
    private IQueryable<T> _sourceQuery;

    /// <summary>
    /// A pageable result
    /// </summary>
    /// <param name="sourceQuery">Query which holdes all relevant items.</param>
    public Pageable(IQueryable<T> sourceQuery)
    {
        _sourceQuery = sourceQuery;
        _countQuery = sourceQuery;
        PageNumber = 1;
    }

    /// <summary>
    /// A pageable result
    /// </summary>
    /// <param name="sourceQuery">Query which holdes all relevant items.</param>
    /// <param name="countQuery">
    /// Alternative query optimized for counting.
    /// <see cref="countQuery"/> is required to give the same count as <see cref="sourceQuery"/> else paging will break. 
    /// <remarks>No checks if <see cref="sourceQuery"/> and <see cref="countQuery"/> return the same count are appiled.</remarks>
    /// </param>
    public Pageable(IQueryable<T> sourceQuery, IQueryable<T> countQuery)
        : this (sourceQuery)
    {
        _countQuery = countQuery;
    }

    #region Implementation of IEnumerable

    /// <summary>
    /// Returns an enumerator that iterates through the collection.
    /// </summary>
    /// <returns>
    /// A <see cref="T:System.Collections.Generic.IEnumerator`1"/> that can be used to iterate through the collection.
    /// </returns>
    /// <filterpriority>1</filterpriority>
    public IEnumerator<T> GetEnumerator()
    {
        return _sourceQuery.GetEnumerator();
    }

    /// <summary>
    /// Returns an enumerator that iterates through a collection.
    /// </summary>
    /// <returns>
    /// An <see cref="T:System.Collections.IEnumerator"/> object that can be used to iterate through the collection.
    /// </returns>
    /// <filterpriority>2</filterpriority>
    IEnumerator IEnumerable.GetEnumerator()
    {
        return GetEnumerator();
    }

    #endregion

    #region Implementation of ISortable

    public IPageable<T> OrderBy<U>(Expression<Func<T, U>> orderBy)
    {
        _sourceQuery = _sourceQuery.OrderBy(orderBy);
        return this;
    }

    public IPageable<T> OrderByDescending<U>(Expression<Func<T, U>> orderBy)
    {
        _sourceQuery = _sourceQuery.OrderByDescending(orderBy);
        return this;
    }

    #endregion

    #region Implementation of IPageable

    public int PageNumber { get; private set; }
    public int? PageSize { get; private set; }
    public int TotalItemCount
    {
        get { return _countQuery.Count(); }
    }
    public int TotalPages
    {
        get { return (int) (Math.Ceiling((double) TotalItemCount/PageSize ?? 1)); }
    }

    /// <summary>
    /// Chop a query result into pages.
    /// </summary>
    /// <param name="pageNumber">Page number to fetch. Starting from 1.</param>
    /// <param name="pageSize">Items per page.</param>
    /// <returns></returns>
    public IPageable<T> Page(int pageNumber, int pageSize)
    {
        PageNumber = pageNumber;
        PageSize = pageSize;

        _sourceQuery = _sourceQuery
            .Skip((pageNumber - 1) * pageSize)
            .Take(pageSize);

        return this;
    }

    public List<T> ToList()
    {
        return _sourceQuery.ToList();
    }

    #endregion
}

以上作品。巨大的成功！：）

但是我遇到了实现方法.ThenBy()的问题。问题是只有在.ThenBy()被调用时才能访问.OrderBy()。

我注意到IQueryable.OrderBy返回了一个IOrderedQueryable，而.ThenBy()的访问权来自于{{1}}。但是为了使我当前的解决方案工作，我需要制作一个IOrderedPageable和一个新的OrderedPagable。 OrderedPagable几乎就是Pageable的副本，这真的是非常糟糕的设计。

我非常怀疑它是如何在LINQ中完成的。所以我的问题是，他们是怎么做到的？我很好奇：）

我注意到的一件事是，几乎所有的LINQ方法都是扩展方法，是“技巧”的一部分：）？

Answer 1

听起来您的班级OrderedPageable可能是Pageable的子类，另外还会实现IOrderedPageable界面。

继承方法似乎有意义，因为处理Pageable的任何内容都应该能够以相同的方式处理OrderedPageable。

通过稍微更改接口并使用类似于上面的接口继承方法，您可以使用不可变的可查询类和扩展方法来实现您正在寻找的功能。

在我看来，用法更清晰，更符合LINQ。

示例：

query.AsPageable(100).Page(1);
query.AsPageable(100).OrderBy(x => x.Name).ThenBy(x => x.Age).Page(1).ToList();

我没有测试过这个，但这个概念应该有效。请注意：

总项目计数是从原始查询（不是任何已排序的查询）计算的
总项目数是懒惰的，只计算一次
根据您的查询提供程序，您可能需要公开SourceQuery IPageableQuery属性才能在PageableExtensions中使用，因为您的查询提供程序可能无法成功翻译针对此新PageableQuery的查询1}}类型。

接口：

public interface IPageableQuery<T> : IQueryable<T>
{
    int TotalPages { get; }
    int TotalItemCount { get; }

    int PageSize { get; }
}

public interface IOrderedPageableQuery<T> : IPageableQuery<T>, IOrderedQueryable<T>
{
}

实现：

public class PageableQuery<T> : IPageableQuery<T>
{
    readonly IQueryable<T> _sourceQuery;
    readonly Lazy<int> _totalItemCount; 

    public int TotalPages { get { return (int)Math.Ceiling((double)TotalItemCount / PageSize); } }
    public int TotalItemCount { get { return _totalItemCount.Value; } }
    public int PageSize { get; private set; }

    public PageableQuery(IQueryable<T> sourceQuery, int pageSize)
    {
        _sourceQuery = sourceQuery;
        _totalItemCount = new Lazy<int>(() => _sourceQuery.Count());

        PageSize = pageSize;
    }

    public IEnumerator<T> GetEnumerator() { return _sourceQuery.GetEnumerator();}
    IEnumerator IEnumerable.GetEnumerator() { return GetEnumerator(); }

    public Expression Expression { get { return _sourceQuery.Expression; } }
    public Type ElementType { get { return _sourceQuery.ElementType; } }
    public IQueryProvider Provider { get { return _sourceQuery.Provider; } }
}

public class OrderedPageableQuery<T> : IOrderedPageableQuery<T>
{
    readonly IPageableQuery<T> _sourcePageableQuery;
    readonly IOrderedQueryable<T> _sourceQuery;

    public int TotalPages { get { return (int)Math.Ceiling((double)TotalItemCount / PageSize); } }
    public int TotalItemCount { get { return _sourcePageableQuery.TotalItemCount; } }
    public int PageSize { get { return _sourcePageableQuery.PageSize; } }

    public OrderedPageableQuery(IPageableQuery<T> sourcePageableQuery, IOrderedQueryable<T> newSourceQuery)
    {
        _sourcePageableQuery = sourcePageableQuery;
        _sourceQuery = newSourceQuery;
    }

    public IEnumerator<T> GetEnumerator() { return _sourceQuery.GetEnumerator();}
    IEnumerator IEnumerable.GetEnumerator() { return GetEnumerator(); }

    public Expression Expression { get { return _sourceQuery.Expression; } }
    public Type ElementType { get { return _sourceQuery.ElementType; } }
    public IQueryProvider Provider { get { return _sourceQuery.Provider; } }
}

扩展方法：

public static class PageableExtension
{
    public static IPageableQuery<T> AsPageable<T>(this IQueryable<T> sourceQuery, int pageSize)
    {
        return new PageableQuery<T>(sourceQuery, pageSize);
    }

    public static IOrderedPageableQuery<T> OrderBy<T, U>(this IPageableQuery<T> sourcePageableQuery, Expression<Func<T, U>> orderBy)
    {
        return new OrderedPageableQuery<T>(sourcePageableQuery, Queryable.OrderBy(sourcePageableQuery, orderBy));
    }

    public static IOrderedPageableQuery<T> OrderByDescending<T, U>(this IPageableQuery<T> sourcePageableQuery, Expression<Func<T, U>> orderBy)
    {
        return new OrderedPageableQuery<T>(sourcePageableQuery, Queryable.OrderByDescending(sourcePageableQuery, orderBy));
    }

    public static IOrderedPageableQuery<T> ThenBy<T, U>(this IOrderedPageableQuery<T> sourcePageableQuery, Expression<Func<T, U>> orderBy)
    {
        return new OrderedPageableQuery<T>(sourcePageableQuery, Queryable.ThenBy(sourcePageableQuery, orderBy));
    }

    public static IOrderedPageableQuery<T> ThenByDescending<T, U>(this IOrderedPageableQuery<T> sourcePageableQuery, Expression<Func<T, U>> orderBy)
    {
        return new OrderedPageableQuery<T>(sourcePageableQuery, Queryable.ThenByDescending(sourcePageableQuery, orderBy));
    }

    public static IQueryable<T> Page<T>(this IPageableQuery<T> sourceQuery, int pageNumber)
    {
        return sourceQuery.Skip((pageNumber - 1) * sourceQuery.PageSize)
                          .Take(sourceQuery.PageSize);
    }
}

Answer 2

我建议的设计方法是明确表示你的方法实际上是在改变源对象，故意不反映LINQ方法名称以避免混淆。为了清楚起见，我省略了IPageable接口和一堆其他东西，因为代码已经有点冗长了：

public interface ISortable<T>
{
    Pageable<T> ResetOrder();
    Pageable<T> AddOrder(Expression<Func<T, object>> orderBy);
    Pageable<T> AddOrderDescending(Expression<Func<T, object>> orderBy);
}

public class Pageable<T> : ISortable<T>, IEnumerable<T> {
    class SortKey {
        public Expression<Func<T, object>> Expression { get; set; }
        public bool Descending { get; set; }
    }

    List<SortKey> _sortKeys = new List<SortKey>();

    System.Linq.IQueryable<T> _sourceQuery;

    int _pageNumber;
    int _pageSize;

    public Pageable<T> SetPage(int pageNumber, int pageSize) {
        _pageNumber = pageNumber;
        _pageSize = pageSize;
        return this;
    }

    public Pageable<T> ResetOrder()
    {
        _sortKeys.Clear();
        return this;
    }

    public Pageable<T> AddOrder(Expression<Func<T, object>> orderBy)
    {
        _sortKeys.Add(new SortKey {
            Expression = orderBy, 
            Descending = false
        });
        return this;
    }

    public Pageable<T> AddOrderDescending(Expression<Func<T, object>> orderBy)
    {
        _sortKeys.Add(new SortKey {
            Expression = orderBy, 
            Descending = true
        });
        return this;
    }

    IEnumerable<T> SortAndPage()
    {
        if (_sortKeys.Count == 0) 
        {
            return Page(_sourceQuery);
        }

        var firstKey = _sortKeys[0];
        var orderedQuery = firstKey.Descending 
            ? _sourceQuery.OrderByDescending(firstKey.Expression) 
            : _sourceQuery.OrderBy(firstKey.Expression);

        foreach (var key in _sortKeys.Skip(1)) 
        {
            orderedQuery = key.Descending ? orderedQuery.ThenByDescending(key.Expression) : orderedQuery.ThenBy(key.Expression);
        }
        return Page(orderedQuery);
    }

    IEnumerable<T> Page(IQueryable<T> query) 
    {
        return query.Skip((_pageNumber - 1) * _pageSize)
                    .Take (_pageSize);
    }

    public IEnumerator<T> GetEnumerator()
    {
        return SortAndPage().GetEnumerator();
    }

    System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
    {
        return this.GetEnumerator();
    }
}

请注意，源查询保持不变，因此您可以在枚举元素之前更改参数。如果您希望在执行此操作的方法中基于当前的Pageable而不是this返回新的SortAndPage()，也可以实现链接行为，但这会使代码更加笨拙，因为您' d创建一个复制构造函数并添加代码来创建那些派生对象。

另外我相信{{1}}中烦人的冗余代码可以使用一些FP方法进行重构（或者至少是高尔夫球化）但是阅读它的方式可能更直接。

怎么样.ThenBy实施了吗？

2 个答案: