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我实现了提供给该方法的动态排序逻辑（请参见此处） 。Expression<Func<T, IComperable>>OrderBy

现在我遇到了问题，EF 无法将IComperablein转换Func<T, IComperable>为其实际类型：

无法将类型“System.Int32”转换为类型“System.IComparable”。LINQ to Entities 仅支持转换 EDM 基元或枚举类型。

在

System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.ValidateAndAdjustCastTypes（TypeUsage toType，TypeUsage fromType，Type toClrType，Type fromClrType）

有没有办法解决这个问题？

我目前发现的唯一方法是将Func<>'s 作为它们的真实类型“实现”，将此类型保存在旁边Func<>并调用OrderByvia 反射：

public static IQueryable<T> OrderBy<T>(this IQueryable<T> source, Type orderType, object expression)
{
    return typeof(Queryable).GetMethods().First(m => m.Name == "OrderBy")
       .MakeGenericMethod(typeof(T), orderType)
       .Invoke(null, new object[] { source, expression }) as IQueryable<T>
}

但这对我来说似乎很丑陋（而且很慢？），并且不像当前的（遗憾的是仅适用于 LINQ to objects 工作）解决方案那样好用......

更新：
这个问题似乎只在返回int或bool在时发生Func<T, IComperable>，因为它适用于string...

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1 回答 1

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我现在通过以下方式解决了它：

var orderDelegates = new Dictionary<string, LambdaExpression>();

Expression<Func<Image, int>> id = i => i.Id;
orderDelegates.Add(ContentItem.ORDER_BY_ID, id);
Expression<Func<Image, IComperable>> title = i => i.Title;
orderDelegates.Add(ContentItem.ORDER_BY_Title, title);
//more items...

（我希望这有点短 - 见这里）

在我自己的OrderBy：

var first = orderDelegates[orderKey ?? defaultKey];
Type firstType = first.GetType().GetGenericArguments()[0].GetGenericArguments()[1];

IOrderedQueryable<T> firstOrder;
if (firstType == typeof(int))
    firstOrder = items.OrderBy<T, int>(first, direction);
else if (firstType == typeof(bool))
    firstOrder = items.OrderBy<T, bool>(first, direction);
else
    firstOrder = items.OrderBy<T, IComparable>(first, direction);

var second = orderDelegates[defaultKey];
Type secondType = second.GetType().GetGenericArguments()[0].GetGenericArguments()[1];
if (secondType == typeof(int))
    return firstOrder.ThenBy<T, int>(second, direction);
else if (secondType == typeof(bool))
    return firstOrder.ThenBy<T, bool>(second, direction);
else
    return firstOrder.ThenBy<T, IComparable>(second, direction);

和

public static IOrderedQueryable<T> OrderBy<T, K>(this IQueryable<T> items, LambdaExpression expression, OrderDirection? direction)
{
    if (direction == OrderDirection.Ascending || !direction.HasValue)
        return items.OrderBy(expression as Expression<Func<T, K>>);
    else
        return items.OrderByDescending(expression as Expression<Func<T, K>>);
}

public static IQueryable<T> ThenBy<T, K>(this IOrderedQueryable<T> items, LambdaExpression expression, OrderDirection? direction)
{
    if (direction == OrderDirection.Ascending || !direction.HasValue)
        return items.ThenBy(expression as Expression<Func<T, K>>);
    else
        return items.ThenByDescending(expression as Expression<Func<T, K>>);
}
于 2013-08-17T09:13:44.267 回答