c# - .ThenBy 是如何实现的？

Question

灵感来自 this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，我在实现方法 .ThenBy() 时遇到了问题。问题是 .ThenBy() 应该只有在调用 .OrderBy() 时才可以访问。

我注意到 IQueryable.OrderBy返回一个 IOrderedQueryable，这就是访问 .ThenBy() 的来源。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 来配合它。 OrderedPagable 几乎是 Pageable 的副本，这是非常糟糕的设计。

我非常怀疑 LINQ 是如何做到的。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，这是“技巧”的一部分 :)？

Answer 1

听起来您的类 OrderedPageable 可能是 Pageable 的子类并且另外实现了 IOrderedPageable 接口(interface)。

继承方法似乎很有意义，因为任何处理 Pageable 的东西都应该能够以相同的方式处理 OrderedPageable。

---

通过稍微更改您的接口(interface)并使用与上述类似的接口(interface)继承方法，您可以使用不可变的可查询类和扩展方法实现您正在寻找的功能。

在我看来，用法更清晰，更符合LINQ。

例子:

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

我还没有对此进行测试，但这个概念应该可行。请注意:

• 项目总数是根据原始查询计算的(不是任何排序查询)
• 项目总数是惰性的，只计算一次
• 根据您的查询提供者，您可能必须公开 IPageableQuery 的 SourceQuery 属性以便在 PageableExtensions 中使用，因为您的查询提供者可能未能成功翻译针对这种新的 PageableQuery 类型的查询。

接口(interface):

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);
    }
}