实体框架：LINQ to Entities仅支持转换实体数据模型基元类型

Question

我编写了一个方法来允许为orderby子句传递Expression，但我遇到了这个问题。

  

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

基本上表达式是：

Expression<Func<K, IComparable>> orderBy

使用方式如下：

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

我的想法是，我可以使用字典来保存字符串匹配，例如：

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

然后我有一个方法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它可以被命令的方式）现在这适用于String属性，但到目前为止还没有为datetime或整数，因为我得到上面的错误消息。

到目前为止，我（松散地）理解问题是实体框架需要它是主/ EDM类型，因为它必须将C＃DateTime转换为数据库可以处理的内容。

有没有办法将日期时间转换为基本类型，以便这仍然有效？

解决方案

通过方法获取订单的方法:(接受查询并以“有序的形式”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, ...)

使用传入的方法：

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施OrderBy和OrderByDescending委托。

我最终为IQueryable创建了一个名为ObjectSort的扩展方法，它只是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，以便LINQ to Entities不会吓坏了。

我不确定这是否是一个好的解决方案，但下面的示例适用于DateTime和int所以希望如果您想要完成某些事情，它可以给您一些想法类似！

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

实体框架使这很困难，我不确定是否有办法用单一的返回值类型（IComparable，object等）做你想做的事情。您可以考虑将设计重新编写为名称到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

遇到与原始海报类似的问题，其中“Order By”表达式写为Expression＆lt; Func＆lt; T，object＆gt;＆gt;类型的lambda。这些由NHibernate linq提供程序正确解释，但迁移到EF 5导致“无法将类型'System.DateTime'转换为'System.IComparable'类型.LINQ to Entities仅支持转换实体数据模型基元类型。”< / p>

以下方法提供对表达式＆lt; Func＆lt; T，TKey＆gt;＆gt;的转换。当调用各种“OrderBy”方法时（使用反射 - 道歉......）注意它们最初封装在一个通用类OrderBy＆lt; T＆gt;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

我找到了解决问题的非常简单的解决方案（我的也是）。在创建搜索表达式时，应该传递属性类型（然后你知道），但将表达式存储在动态变量中：

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

现在你可以将dynExpr存储在一个表或任何你想要的地方，以及int表达式，字符串表达式......当时间到了，你可以在OrderBy方法中使用它。但不是标准方式（扩展方法）：

query=query.OrderBy(dynExpr);

，只有这样：

query=Queryable.OrderBy(query, dynExpr);

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

Answer 5

在OldNic的启发下，我创建了几种扩展方法，供成员排序。这对我很有效。我也使用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);
    }