表达式列表<Func<T, TProperty>>

Question

我正在寻找一种方法来存储用于对元素排序的Expression<Func<T, TProperty>> 集合，然后针对IQueryable<T> 对象（底层提供程序是实体框架）执行存储的列表。

例如，我想做这样的事情（这是伪代码）：

public class Program
{
    public static void Main(string[] args)
    {
        OrderClause<User> orderBys = new OrderClause<User>();
        orderBys.AddOrderBy(u => u.Firstname);
        orderBys.AddOrderBy(u => u.Lastname);
        orderBys.AddOrderBy(u => u.Age);

        Repository<User> userRepository = new Repository<User>();
        IEnumerable<User> result = userRepository.Query(orderBys.OrderByClauses);
    }
}

一个 order by 子句（订购的属性）：

public class OrderClause<T>
{
    public void AddOrderBy<TProperty>(Expression<Func<T, TProperty>> orderBySelector)
    {
        _list.Add(orderBySelector);
    }

    public IEnumerable<Expression<Func<T, ???>>> OrderByClauses
    {
        get { return _list; }
    }
}

带有我的查询方法的存储库：

public class Repository<T>
{
    public IEnumerable<T> Query(IEnumerable<OrderClause<T>> clauses)
    {
        foreach (OrderClause<T, ???> clause in clauses)
        {
            _query = _query.OrderBy(clause);
        }

        return _query.ToList();
    }
}

我的第一个想法是将Expression<Func<T, TProperty>> 转换为字符串（要排序的属性名称）。所以基本上，我不是存储一个类型化的列表（这是不可能的，因为 TProperty 不是常量），而是存储一个字符串列表以及要排序的属性。

但这不起作用，因为那时我无法重建 Expression 回来（我需要它，因为 IQueryable.OrderBy 将 Expression<Func<T, TKey>> 作为参数）。

我还尝试动态创建表达式（在 Expression.Convert 的帮助下），以获得 Expression<Func<T, object>> 但后来我从实体框架中得到一个异常，它说它无法处理 Expression.Convert 语句.

如果可能，我不想使用像 Dynamic Linq Library 这样的外部库。

Answer 1

这是dynamic/反射解决方案可能适用的少数情况之一。

我想你想要这样的东西？ （我已经阅读了字里行间，并在我认为必要的地方对您的结构进行了一些更改）。

public class OrderClauseList<T>
{
    private readonly List<LambdaExpression> _list = new List<LambdaExpression>();

    public void AddOrderBy<TProperty>(Expression<Func<T, TProperty>> orderBySelector)
    {
        _list.Add(orderBySelector);
    }

    public IEnumerable<LambdaExpression> OrderByClauses
    {
        get { return _list; }
    }
}

public class Repository<T>
{
    private IQueryable<T> _source = ... // Don't know how this works

    public IEnumerable<T> Query(OrderClause<T> clauseList)
    {
        // Needs validation, e.g. null-reference or empty clause-list. 

        var clauses = clauseList.OrderByClauses;

        IOrderedQueryable<T> result = Queryable.OrderBy(_source, 
                                                        (dynamic)clauses.First());

        foreach (var clause in clauses.Skip(1))
        {
            result = Queryable.ThenBy(result, (dynamic)clause);
        }

        return result.ToList();
    }
}

关键技巧是让 C# dynamic 为我们执行可怕的重载解析和类型推断。更重要的是，我相信上面的内容，尽管使用了dynamic，实际上是类型安全的！

Answer 2

这样做的一种方法是将所有排序子句“存储”在类似Func<IQueryable<T>, IOrderedQueryable<T>>（即调用排序方法的函数）中：

public class OrderClause<T>
{
    private Func<IQueryable<T>, IOrderedQueryable<T>> m_orderingFunction;

    public void AddOrderBy<TProperty>(Expression<Func<T, TProperty>> orderBySelector)
    {
        if (m_orderingFunction == null)
        {
            m_orderingFunction = q => q.OrderBy(orderBySelector);
        }
        else
        {
            // required so that m_orderingFunction doesn't reference itself
            var orderingFunction = m_orderingFunction;
            m_orderingFunction = q => orderingFunction(q).ThenBy(orderBySelector);
        }
    }

    public IQueryable<T> Order(IQueryable<T> source)
    {
        if (m_orderingFunction == null)
            return source;

        return m_orderingFunction(source);
    }
}

这样，你不必处理反射或dynamic，所有这些代码都是类型安全的并且相对容易理解。

Answer 3

您可以将 lambda 表达式作为 LambdaExpression 类型的实例存储在集合中。

或者更好的是，存储排序定义，每个定义，除了一个表达式，还存储一个排序方向。

假设你有以下扩展方法

public static IQueryable<T> OrderBy<T>(
    this IQueryable<T> source,
    SortDefinition sortDefinition) where T : class
{
    MethodInfo method;
    Type sortKeyType = sortDefinition.Expression.ReturnType;
    if (sortDefinition.Direction == SortDirection.Ascending)
    {
        method = MethodHelper.OrderBy.MakeGenericMethod(
            typeof(T),
            sortKeyType);
    }
    else
    {
        method = MethodHelper.OrderByDescending.MakeGenericMethod(
            typeof(T),
            sortKeyType);
    }

    var result = (IQueryable<T>)method.Invoke(
        null,
        new object[] { source, sortDefinition.Expression });
    return result;
}

和ThenBy 的类似方法。然后你可以做类似的事情

myQueryable = myQueryable.OrderBy(sortDefinitions.First());

myQueryable = sortDefinitions.Skip(1).Aggregate(
   myQueryable,
   (current, sortDefinition) => current.ThenBy(sortDefinition));

这里是SortDefinition和MethodHelper的定义

public class SortDefinition
{
    public SortDirection Direction
    {
        get;
        set;
    }

    public LambdaExpression Expression
    {
        get;
        set;
    }
}

internal static class MethodHelper
{
    static MethodHelper()
    {
        OrderBy = GetOrderByMethod();
        ThenBy = GetThenByMethod();
        OrderByDescending = GetOrderByDescendingMethod();
        ThenByDescending = GetThenByDescendingMethod();
    }

    public static MethodInfo OrderBy
    {
        get;
        private set;
    }

    public static MethodInfo ThenBy
    {
        get;
        private set;
    }

    public static MethodInfo OrderByDescending
    {
        get;
        private set;
    }

    public static MethodInfo ThenByDescending
    {
        get;
        private set;
    }

    private static MethodInfo GetOrderByMethod()
    {
        Expression<Func<IQueryable<object>, IOrderedQueryable<object>>> expr =
            q => q.OrderBy((Expression<Func<object, object>>)null);

        return ((MethodCallExpression)expr.Body).Method.GetGenericMethodDefinition();
    }

    private static MethodInfo GetThenByMethod()
    {
        Expression<Func<IOrderedQueryable<object>, IOrderedQueryable<object>>> expr =
            q => q.ThenBy((Expression<Func<object, object>>)null);

        return ((MethodCallExpression)expr.Body).Method.GetGenericMethodDefinition();
    }

    private static MethodInfo GetOrderByDescendingMethod()
    {
        Expression<Func<IQueryable<object>, IOrderedQueryable<object>>> expr =
            q => q.OrderByDescending((Expression<Func<object, object>>)null);

        return ((MethodCallExpression)expr.Body).Method.GetGenericMethodDefinition();
    }

    private static MethodInfo GetThenByDescendingMethod()
    {
        Expression<Func<IOrderedQueryable<object>, IOrderedQueryable<object>>> expr =
            q => q.ThenByDescending((Expression<Func<object, object>>)null);

        return ((MethodCallExpression)expr.Body).Method.GetGenericMethodDefinition();
    }
}

Answer 4

在 E.F. Core 中，您可以使用以下帮助程序类

public static class OrderBuilder
{
    public static IQueryable<TSource> OrderBy<TSource>(this IQueryable<TSource> queryable, params Tuple<Expression<Func<TSource, object>>, bool>[] keySelectors)
    {
        if (keySelectors == null || keySelectors.Length == 0) return queryable;

        return keySelectors.Aggregate(queryable, (current, keySelector) => keySelector.Item2 ? current.OrderDescending(keySelector.Item1) : current.Order(keySelector.Item1));
    }

    private static bool IsOrdered<TSource>(this IQueryable<TSource> queryable)
    {
        if (queryable == null) throw new ArgumentNullException(nameof(queryable));

        return queryable.Expression.Type == typeof(IOrderedQueryable<TSource>);
    }

    private static IQueryable<TSource> Order<TSource, TKey>(this IQueryable<TSource> queryable, Expression<Func<TSource, TKey>> keySelector)
    {
        if (!queryable.IsOrdered()) return queryable.OrderBy(keySelector);

        var orderedQuery = queryable as IOrderedQueryable<TSource>;

        return (orderedQuery ?? throw new InvalidOperationException()).ThenBy(keySelector);
    }

    private static IQueryable<TSource> OrderDescending<TSource, TKey>(this IQueryable<TSource> queryable, Expression<Func<TSource, TKey>> keySelector)
    {
        if (!queryable.IsOrdered()) return queryable.OrderByDescending(keySelector);

        var orderedQuery = queryable as IOrderedQueryable<TSource>;
        return (orderedQuery ?? throw new InvalidOperationException()).ThenByDescending(keySelector);
    }
}

然后将其用作.. 下面的示例带有一个名为 Player 的类，该类具有以下成员..（为简洁起见，代码已简化）

 public class Player
 {
    ...

    public string FirstName { get; set; 
    public int GenderTypeId { get; set; } 
    public int PlayingExperience { get; set; }

您可以随意组合排序、按性别排序、演奏经验降序（注意元组的 true 值）和名字..

    var combinedOrder = new[]
    {
        new Tuple<Expression<Func<Player, object>>, bool>(p => p.GenderTypeId, false),
        new Tuple<Expression<Func<Player, object>>, bool>(p => p.PlayingExperience, true),
        new Tuple<Expression<Func<Player, object>>, bool>(p => p.FirstName, false),
    };

只需按以下顺序进行操作

        var data = context.Set<Player>()
            .OrderBy(combinedOrder)
            .ToArray();