Question

我正在寻找一种方法来存储用于订购元素的Expression<Func<T, TProperty>>集合，然后针对IQueryable<T>对象执行存储列表（底层提供者是实体框架）。

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);
    }
}

order by子句（要订购的属性）：

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; }
    }
}

使用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（要对其进行排序的属性名称）。所以基本上，不是存储类型列表（由于TProperty不是常量而不可能），我存储了一个字符串列表，其中包含要排序的属性。

但这不起作用因为我无法重建Expression（我需要它，因为IQueryable.OrderBy以Expression<Func<T, TKey>>作为参数）。

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

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

Answer 1

这是少数dynamic /反射解决方案适合的情况之一。

我想你想要这样的东西？（我已经阅读了这些内容，并对我认为必要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();
    }
}

关键的诀窍是让C＃dynamic为我们做出可怕的重载决策和类型推断。更重要的是，我相信上述内容，尽管使用dynamic，实际上是类型安全的！

Answer 2

执行此操作的一种方法是将所有sort子句“存储”在Func<IQueryable<T>, IOrderedQueryable<T>>之类的内容中（即调用排序方法的函数）：

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，您不必处理反射或dynamic，所有这些代码都是类型安全且相对容易理解的。

Answer 3

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

甚至更好，存储排序定义，除了表达式之外，每个排序定义都会存储排序方向。

假设您有以下扩展方法

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));

以下是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

CHECK

，然后将其用作..下例与名为Player的类以及以下成员一起使用。

NOT VALID

您可以随意组合顺序，按性别排序，游戏体验递减（注意元组的 true 值）和名字。

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);
    }
}

只需执行以下命令

 public class Player
 {
    ...

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

表达式列表<func <t，tproperty =“”>＆gt; </func <t，>

4 个答案: