实体框架：LINQ to Entities 仅支持转换实体数据模型原始类型

Question

我写了一个方法来允许为 orderby 子句传递一个表达式，但是我遇到了这个问题。

无法转换类型 'System.DateTime' 键入 'System.IComparable'。 LINQ 到实体 仅支持转换实体数据 模型基元类型。

基本上表达式是这样的：

Expression<Func<K, IComparable>> orderBy

并且是这样使用的：

SomeEntities.SomeTable
.Where
(
   whereClause
)
.Select
(
   selectClause
)
.OrderBy(orderBy)

这个想法是为了让我可以使用字典来保存与表达式匹配的字符串，例如：

_possibleSortForForumItem.Add("CreateDate", item => item.CreateDate);

然后我有一个方法，它接受排序字符串并返回表达式，如果它匹配字典中的键，如果不返回一些默认值。 （这个想法是一种控制可以订购什么的方法）现在这适用于字符串属性，但到目前为止不适用于日期时间或整数，因为我收到上面的错误消息。

现在据我（大致）了解，问题在于实体框架需要它是 Primary/EDM 类型，因为它必须将 C# DateTime 转换为数据库可以处理的东西。

有没有办法将日期时间转换为原始类型，这样它仍然可以工作？

解决方案

按方法获取订单的方法：（取一个查询，以“有序形式”返回）

private static Func<IQueryable<ForumViewItem>, IOrderedQueryable<ForumViewItem>> GetMethodForSort(String sortBy)
{
  if (_methodForSort == null)
  {
    _methodForSort = new Dictionary<String, Func<IQueryable<ForumViewItem>, IOrderedQueryable<ForumViewItem>>>();
    _methodForSort.Add(SortForumViewItemCreatedOn, item => item.OrderBy(innerItem => innerItem.CreatedOn));
    ...
  }

  Func<IQueryable<ForumViewItem>, IOrderedQueryable<ForumViewItem>> orderMethod;

  if(String.IsNullOrEmpty(sortBy) || !_methodForSort.ContainsKey(sortBy))
  {
    orderMethod = _methodForSort["ForumName"];
  }
  else
  {
    orderMethod = _methodForSort[sortBy];
  }

  return orderMethod;
}

通用查询方法的方法签名：

IList<K> GetListForGrid<T, K>(this ObjectQuery<T> query, ... Func<IQueryable<K>, IOrderedQueryable<K>> orderBy, ...)

以及传入方法的使用：

initialQuery = query
  .Where
  (
    somethingEqualsSomething
  )
  .Select
  (
    selectClause
  );

var orderedQuery = orderBy(initialQuery);

returnValue = orderedQuery
  .Skip(numberToShow * realPage)
  .Take(numberToShow)
  .ToList();

Answer 1

我知道这是旧的，但我希望完成与 OP 完全相同的事情，并且不想在我的字典中使用 Func<IQueryable<T>, IOrderedQueryable<T>>。主要是因为我必须同时实现 OrderBy 和 OrderByDescending 委托。

我最终为 IQueryable 创建了一个名为 ObjectSort 的扩展方法，它将简单地检查表达式的返回类型应该是什么，然后使用该类型创建一个新的 lambda，这样 LINQ to Entities 就不会崩溃.

我不确定这是否是一个好的解决方案，但下面的示例确实适用于 DateTime 和 int，所以如果您想要完成类似的事情，希望它能给您一些想法！

public static IOrderedQueryable<T> ObjectSort<T>(this IQueryable<T> entities, Expression<Func<T, object>> expression, SortOrder order = SortOrder.Ascending)
{
    var unaryExpression = expression.Body as UnaryExpression;
    if (unaryExpression != null)
    {
        var propertyExpression = (MemberExpression)unaryExpression.Operand;
        var parameters = expression.Parameters;

        if (propertyExpression.Type == typeof(DateTime))
        {
            var newExpression = Expression.Lambda<Func<T, DateTime>>(propertyExpression, parameters);
            return order == SortOrder.Ascending ? entities.OrderBy(newExpression) : entities.OrderByDescending(newExpression);
        }

        if (propertyExpression.Type == typeof(int))
        {
            var newExpression = Expression.Lambda<Func<T, int>>(propertyExpression, parameters);
            return order == SortOrder.Ascending ? entities.OrderBy(newExpression) : entities.OrderByDescending(newExpression);
        }

        throw new NotSupportedException("Object type resolution not implemented for this type");
    }
    return entities.OrderBy(expression);
}

Answer 2

实体框架使这变得困难，我不确定是否有一种方法可以使用单个返回值类型（IComparable、对象等）来执行您想要执行的操作。您可能会考虑将您的设计重新设计成一个名称到Func<IQueryable<K>, IOrderedQueryable<K>> 值的字典：

_possibleSortForForumItem.Add("CreateDate", 
    query => query.OrderBy(item.CreateDate));

然后像这样应用它：

var orderedQuery = query.OrderBy(item => item.DefaultOrderColumn);

Func<IQueryable<K>, IOrderedQueryable<K>> assignOrderBy = null;

if (_possibleSortForForumItem.TryGetValue(orderColumnName, out assignOrderBy))
{
    orderedQuery = assignOrderBy(query);
}

Answer 3

遇到了与原始海报类似的问题，其中“Order By”表达式写为 Expression> 类型的 lambdas。 NHibernate linq 提供程序正确解释了这些内容，但迁移到 EF 5 导致“无法将类型 'System.DateTime' 转换为类型 'System.IComparable'。LINQ to Entities 仅支持转换实体数据模型原始类型。”

以下方法在调用各种“OrderBy”方法时提供了到 Expression> 的转换（使用反射 - 道歉...）请注意，它们最初封装在通用类 OrderBy 中。

    private static readonly Type QueryableType = typeof(Queryable);

    // HACK: Use reflection to call strongly-typed methods instead of object-based methods
    // This is to work around "Unable to cast the type 'System.DateTime' to type 'System.Object'. LINQ to Entities only supports casting Entity Data Model primitive types."
    private IOrderedQueryable<T> ApplyOrderByTo(
        IQueryable<T> query,
        Expression<Func<T, object>> keySelector,
        bool sortAscending,
        bool useReflection)
    {
        if (useReflection)
        {
            var body = keySelector.Body as UnaryExpression;
            var keyExpr = body.Operand as MemberExpression;

            return (IOrderedQueryable<T>)query.Provider.CreateQuery(
                Expression.Call(
                QueryableType,
                sortAscending ? "OrderBy" : "OrderByDescending",
                new Type[] { typeof(T), keyExpr.Type },
                query.Expression,
                Expression.Lambda(keyExpr, keySelector.Parameters)));
        }
        else
        {
            if (sortAscending)
                return query.OrderBy(keySelector);
            else
                return query.OrderByDescending(keySelector);
        }
    }

    // HACK: Use reflection to call strongly-typed methods instead of object-based methods
    // This is to work around "Unable to cast the type 'System.DateTime' to type 'System.Object'. LINQ to Entities only supports casting Entity Data Model primitive types."
    private IOrderedQueryable<T> ApplyOrderByTo(
        IOrderedQueryable<T> query,
        Expression<Func<T, object>> keySelector,
        bool sortAscending,
        bool useReflection)
    {
        if (useReflection)
        {
            var body = keySelector.Body as UnaryExpression;
            var keyExpr = body.Operand as MemberExpression;

            return (IOrderedQueryable<T>)query.Provider.CreateQuery(
                Expression.Call(
                QueryableType,
                sortAscending ? "ThenBy" : "ThenByDescending",
                new Type[] { typeof(T), keyExpr.Type },
                query.Expression,
                Expression.Lambda(keyExpr, keySelector.Parameters)));
        }
        else
        {
            if (sortAscending)
                return query.ThenBy(keySelector);
            else
                return query.ThenByDescending(keySelector);
        }
    }

Answer 4

我为您的问题找到了非常简单的解决方案（也是我的问题）。当你创建你的搜索表达式时，你应该传递属性的类型（那时你就知道了），但将表达式存储在动态变量中：

Expression<Func<TObject, DateTime>> Expr = obj=>obj.StartDate;
dynamic dynExpr=Expr;

现在您可以将 dynExpr 与 int 表达式、字符串表达式等一起存储在表中或任何您想要的地方，到时候您可以在 OrderBy 方法中使用它。但不是标准方式（扩展方式）：

query=query.OrderBy(dynExpr);

，只能这样：

query=Queryable.OrderBy(query, dynExpr);

通过这种方式，您可以在所有排序函数（OrderBy、OrderByDescending、ThenBy、ThenByDescending）中使用一个表达式。

Answer 5

在 OldNic 的启发下，我创建了几个用于按成员排序的扩展方法。这对我很有效。我也在使用 System.Data.SqlClient.SortOrder 枚举来定义排序顺序。

        /// <summary>
    ///     Supports sorting of a given member as an expression when type is not known. It solves problem with LINQ to Entities unable to
    ///     cast different types as 'System.DateTime', 'System.DateTime?' to type 'System.Object'.
    ///     LINQ to Entities only supports casting Entity Data Model primitive types.
    /// </summary>
    /// <typeparam name="T">entity type</typeparam>
    /// <param name="query">query to apply sorting on.</param>
    /// <param name="expression">the member expression to apply</param>
    /// <param name="sortOrder">the sort order to apply</param>
    /// <returns>Query with sorting applied as IOrderedQueryable of type T</returns>
    public static IOrderedQueryable<T> OrderByMember<T>(
        this IQueryable<T> query, 
        Expression<Func<T, object>> expression, 
        SortOrder sortOrder)
    {
        var body = expression.Body as UnaryExpression;

        if (body != null)
        {
            var memberExpression = body.Operand as MemberExpression;

            if (memberExpression != null)
            {
                return
                    (IOrderedQueryable<T>)
                    query.Provider.CreateQuery(
                        Expression.Call(
                            typeof(Queryable), 
                            sortOrder == SortOrder.Ascending ? "OrderBy" : "OrderByDescending",
                            new[] { typeof(T), memberExpression.Type }, 
                            query.Expression,
                            Expression.Lambda(memberExpression, expression.Parameters)));
            }
        }

        return sortOrder == SortOrder.Ascending ? query.OrderBy(expression) : query.OrderByDescending(expression);
    }

    /// <summary>
    ///     Supports sorting of a given member as an expression when type is not known. It solves problem with LINQ to Entities unable to
    ///     cast different types as 'System.DateTime', 'System.DateTime?' to type 'System.Object'.
    ///     LINQ to Entities only supports casting Entity Data Model primitive types.
    /// </summary>
    /// <typeparam name="T">entity type</typeparam>
    /// <param name="query">query to apply sorting on.</param>
    /// <param name="expression">the member expression to apply</param>
    /// <param name="sortOrder">the sort order to apply</param>
    /// <returns>Query with sorting applied as IOrderedQueryable of type T</returns>
    public static IOrderedQueryable<T> ThenByMember<T>(
        this IQueryable<T> query, 
        Expression<Func<T, object>> expression, 
        SortOrder sortOrder)
    {
        return ((IOrderedQueryable<T>)query).ThenByMember(expression, sortOrder);
    }

    /// <summary>
    ///     Supports sorting of a given member as an expression when type is not known. It solves problem with LINQ to Entities unable to
    ///     cast different types as 'System.DateTime', 'System.DateTime?' to type 'System.Object'.
    ///     LINQ to Entities only supports casting Entity Data Model primitive types.
    /// </summary>
    /// <typeparam name="T">entity type</typeparam>
    /// <param name="query">query to apply sorting on.</param>
    /// <param name="expression">the member expression to apply</param>
    /// <param name="sortOrder">the sort order to apply</param>
    /// <returns>Query with sorting applied as IOrderedQueryable of type T</returns>
    public static IOrderedQueryable<T> ThenByMember<T>(
        this IOrderedQueryable<T> query, 
        Expression<Func<T, object>> expression, 
        SortOrder sortOrder)
    {
        var body = expression.Body as UnaryExpression;

        if (body != null)
        {
            var memberExpression = body.Operand as MemberExpression;

            if (memberExpression != null)
            {
                return
                    (IOrderedQueryable<T>)
                    query.Provider.CreateQuery(
                        Expression.Call(
                            typeof(Queryable), 
                            sortOrder == SortOrder.Ascending ? "ThenBy" : "ThenByDescending",
                            new[] { typeof(T), memberExpression.Type }, 
                            query.Expression,
                            Expression.Lambda(memberExpression, expression.Parameters)));
            }
        }

        return sortOrder == SortOrder.Ascending ? query.ThenBy(expression) : query.ThenByDescending(expression);
    }