怎么样.然后通过实施

本文关键字：然后怎么样 | 更新日期: 2023-09-27 18:27:21

受此启发，我做了这个：

可存储

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

可设置IPage

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。大获成功！:)

但是我在实现该方法时遇到了问题 .ThenBy().问题是.ThenBy()应该只有在调用.OrderBy()时才能访问。

我注意到IQueryable.OrderBy返回了一个IOrderedQueryable，这就是访问.ThenBy()的来源。但是为了使我当前的解决方案正常工作，我需要创建一个IOrderedPageable和一个新的OrderedPagable来配合它。OrderedPagable将是Pageable的几乎完全相同的副本，这确实是非常糟糕的设计。

我非常怀疑这就是它在 LINQ 中完成的方式。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":)的一部分吗？

怎么样.然后通过实施

听起来你的类OrderedPageable可以是Pageable的子类，并另外实现IOrderedPageable接口。

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

通过稍微更改接口并使用类似于上述的接口继承方法，可以使用不可变的可查询类和扩展方法实现所需的功能。

在我看来，用法更清晰，与 LINQ 更一致。

例子：

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

我还没有测试过这个，但这个概念应该有效。请注意：

总项目计数是根据原始查询(而不是任何排序的查询(计算的
总项目计数是延迟的，只计算一次
根据您的查询提供程序，您可能必须公开 IPageableQuery 的 SourceQuery 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转换针对此新PageableQuery类型的查询。

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);
    }
}

我对此建议的设计是明确指出您的方法实际上是在改变源对象，并且故意不镜像 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。如果您愿意，这也可以让您通过基于当前Pageable返回新来实现链接行为，而不是在执行此操作的方法中this，但它会使代码更加笨拙，因为您必须创建一个复制构造函数并添加代码来创建这些派生对象。

此外，我相信可以使用一些 FP 方法重构(或至少降低(SortAndPage()中烦人的冗余代码，但阅读方式可能更直接。