动态创建LINQ到实体OrderBy表达式
我正在尝试动态添加orderby表达式。 但是当执行下面的查询时，我得到以下exception：
System.NotSupportedException：无法创建类型为“Closure type”的常量值。 在此上下文中仅支持基本类型（例如Int32，String和Guid’）。
奇怪的是，我只是查询那些原始类型。
string sortBy = HttpContext.Current.Request.QueryString["sidx"]; ParameterExpression prm = Expression.Parameter(typeof(buskerPosting), "posting"); Expression orderByProperty = Expression.Property(prm, sortBy); // get the paged records IQueryable query = (from posting in be.buskerPosting where posting.buskerAccount.cmsMember.nodeId == m.Id orderby orderByProperty //orderby posting.Created select new PostingListItemDto { Set = posting }).Skip((page - 1) * pageSize).Take(pageSize); 希望有人能对此有所了解！
由于它们的翻译方式，你基本上不能使用这样的查询表达式。 但是，您可以使用扩展方法显式执行此操作：
string sortBy = HttpContext.Current.Request.QueryString["sidx"]; ParameterExpression prm = Expression.Parameter(typeof(buskerPosting), "posting"); Expression orderByProperty = Expression.Property(prm, sortBy); // get the paged records IQueryable query = be.buskerPosting .Where(posting => posting.buskerAccount.cmsMember.nodeId == m.Id) .OrderBy(orderByExpression) .Select(posting => new PostingListItemDto { Set = posting }) .Skip ((page - 1) * pageSize) .Take (pageSize);
棘手的一点是获得正确的表达式树类型 – 这将进入编辑:)
编辑：由于各种原因，编辑将有所延迟。 基本上你可能需要使用reflection调用generics方法，因为Queryable.OrderBy需要一个通用的Expression ，虽然看起来你在编译时知道源类型，但你可能不知道密钥类型。 如果您确实知道它总是按（例如）int排序，您可以使用：
Expression orderByProperty = Expression.Property(prm, sortBy); var orderByExpression = Expression.Lambda> (orderByProperty, new[] { prm });
编辑：好的，看起来我毕竟有时间。 这是使用reflection调用OrderBy的简短示例：
using System; using System.Reflection; using System.Linq; using System.Linq.Expressions; public class Test { static void Main() { string[] names = { "Jon", "Holly", "Tom", "Robin", "Will" }; var query = names.AsQueryable(); query = CallOrderBy(query, "Length"); foreach (var name in query) { Console.WriteLine(name); } } private static readonly MethodInfo OrderByMethod = typeof(Queryable).GetMethods() .Where(method => method.Name == "OrderBy") .Where(method => method.GetParameters().Length == 2) .Single(); public static IQueryable CallOrderBy (IQueryable source, string propertyName) { ParameterExpression parameter = Expression.Parameter(typeof(TSource), "posting"); Expression orderByProperty = Expression.Property(parameter, propertyName); LambdaExpression lambda = Expression.Lambda(orderByProperty, new[] { parameter }); Console.WriteLine(lambda); MethodInfo genericMethod = OrderByMethod.MakeGenericMethod (new[] { typeof(TSource), orderByProperty.Type }); object ret = genericMethod.Invoke(null, new object[] {source, lambda}); return (IQueryable ) ret; } }
您可以轻松地将CallOrderBy重构为扩展方法（例如OrderByProperty ），如下所示：
public static class ReflectionQueryable { private static readonly MethodInfo OrderByMethod = typeof(Queryable).GetMethods() .Where(method => method.Name == "OrderBy") .Where(method => method.GetParameters().Length == 2) .Single(); public static IQueryable OrderByProperty (this IQueryable source, string propertyName) { ParameterExpression parameter = Expression.Parameter(typeof(TSource), "posting"); Expression orderByProperty = Expression.Property(parameter, propertyName); LambdaExpression lambda = Expression.Lambda(orderByProperty, new[] { parameter }); Console.WriteLine(lambda); MethodInfo genericMethod = OrderByMethod.MakeGenericMethod (new[] { typeof(TSource), orderByProperty.Type }); object ret = genericMethod.Invoke(null, new object[] {source, lambda}); return (IQueryable ) ret; } }
您的原始代码将变为：
string sortBy = HttpContext.Current.Request.QueryString["sidx"]; // get the paged records IQueryable query = be.buskerPosting .Where(posting => posting.buskerAccount.cmsMember.nodeId == m.Id) .OrderByProperty(sortBy) .Select(posting => new PostingListItemDto { Set = posting }) .Skip ((page - 1) * pageSize) .Take (pageSize);
（对于涉及水平滚动条的格式化道歉……如果有人关心，我会稍后重新格式化。如果你有足够的代表，你可以为我做这件事;）
我想用Jon的上述答案作为起点分享我的实现。 在这种情况下，不是通过来自表示层的字符串属性名称进行排序（因为此问题的标题不是特定于此），我正构建一个Entity Framework数据层并希望允许其使用者指定顺序作为lambda表达式的属性。 IE而不是传递"sidx" ，我希望能够使用p => p.sidx 。 我还希望能够通过属性传递无限数量的顺序，并能够指定升序或降序。
那么我的方法可以接受像Expression这样的lambda表达式。 这让我按照我想要的方式调用它，但问题是，除非第二个generics参数是强类型的，否则Entity Framework无法将表达式转换为SQL。 OrderBy扩展方法需要两个通用参数：T – 属性所属的类型，以及TKey – 属性返回的类型。 所以第一步是修改Jon的例子，将给定的Expression转换为Expression （一旦我们在查询的上下文中工作，我们就可以确定类型TKey ）：
internal static IQueryable OrderByDynamic (this IQueryable source, Expression> sortExp) { //We need to convert the key selector from Expression> to a strongly typed Expression> //in order for Entity Framework to be able to translate it to SQL MemberExpression orderByMemExp = ExpressionHelpers.RemoveConvert(sortExp.Body) as MemberExpression; ParameterExpression sourceParam = sortExp.Parameters[0]; LambdaExpression orderByLamda = Expression.Lambda(orderByMemExp, new[] { sourceParam }); MethodInfo orderByMethod = OrderByMethod.MakeGenericMethod(new[] { typeof(T), orderByMemExp.Type }); //Call OrderBy or OrderByDescending on the source IQueryable return (IQueryable )orderByMethod.Invoke(null, new object[] { source, orderByLamda }); }
正如我所提到的，我希望通过键选择器接受无限数量的顺序，并且还能够指定升序或降序方向，因此我为Expression创建了一个包装类，我将其命名为DynamicSortExpression：
public class DynamicSortExpression { /// /// Creates a new ascending DynamicSortExpression /// /// A MemberExpression identifying the property to sort on public DynamicSortExpression(Expression> keySelector) : this(keySelector, false) { } public DynamicSortExpression(Expression> keySelector, bool descending) { this.KeySelector = keySelector; this.Desc = descending; } /// /// Gets the expression that selects the property of T to sort on /// public Expression> KeySelector { get; } /// /// Gets sort expression is in ascending or descending order /// public bool Desc { get; } }
然后我更新了扩展方法以接受这种类型并为OrderBy创建了一个重载，它接收List并使用扩展方法将它们List添加到查询中。 这是最终结果：
public static class Extensions { private static readonly MethodInfo OrderByMethod = typeof(Queryable).GetMethods() .Where(method => method.Name == "OrderBy") .Where(method => method.GetParameters().Length == 2) .Single(); private static readonly MethodInfo OrderByDescMethod = typeof(Queryable).GetMethods() .Where(method => method.Name == "OrderByDescending") .Where(method => method.GetParameters().Length == 2) .Single(); private static readonly MethodInfo ThenByMethod = typeof(Queryable).GetMethods() .Where(method => method.Name == "ThenBy") .Where(method => method.GetParameters().Length == 2) .Single(); private static readonly MethodInfo ThenByDescMethod = typeof(Queryable).GetMethods() .Where(method => method.Name == "ThenByDescending") .Where(method => method.GetParameters().Length == 2) .Single(); internal static IQueryable OrderBy (this IQueryable sourceQuery, List> orderBy) { bool isFirst = true; foreach (var sortExpression in orderBy) { if (isFirst) { sourceQuery = sourceQuery.OrderByDynamic(sortExpression); isFirst = false; } else sourceQuery = sourceQuery.ThenByDynamic(sortExpression); } return sourceQuery; } internal static IQueryable OrderByDynamic (this IQueryable source, DynamicSortExpression sortExpression) { //We need to convert the key selector from Expression> to a strongly typed Expression> //in order for Entity Framework to be able to translate it to SQL MemberExpression orderByMemExp = ExpressionHelpers.RemoveConvert(sortExpression.KeySelector.Body) as MemberExpression; ParameterExpression sourceParam = sortExpression.KeySelector.Parameters[0]; LambdaExpression orderByLamda = Expression.Lambda(orderByMemExp, new[] { sourceParam }); MethodInfo orderByMethod = sortExpression.Desc ? OrderByDescMethod.MakeGenericMethod(new[] { typeof(T), orderByMemExp.Type }) : OrderByMethod.MakeGenericMethod(new[] { typeof(T), orderByMemExp.Type }); //Call OrderBy or OrderByDescending on the source IQueryable return (IQueryable )orderByMethod.Invoke(null, new object[] { source, orderByLamda }); } internal static IQueryable ThenByDynamic (this IQueryable source, DynamicSortExpression sortExpression) { //We need to convert the key selector from Expression> to a strongly typed Expression> //in order for Entity Framework to be able to translate it to SQL Expression orderByMemExp = ExpressionHelpers.RemoveConvert(sortExpression.KeySelector.Body) as MemberExpression; ParameterExpression sourceParam = sortExpression.KeySelector.Parameters[0]; LambdaExpression orderByLamda = Expression.Lambda(orderByMemExp, new[] { sourceParam }); MethodInfo orderByMethod = sortExpression.Desc ? ThenByDescMethod.MakeGenericMethod(new[] { typeof(T), orderByMemExp.Type }) : ThenByMethod.MakeGenericMethod(new[] { typeof(T), orderByMemExp.Type }); //Call OrderBy or OrderByDescending on the source IQueryable return (IQueryable )orderByMethod.Invoke(null, new object[] { source, orderByLamda }); } }
现在我的数据层可以有像List ，可以像
new MyClass().GetList(p => p.FirstName == "Billy", //where clause new DynamicSortExpression(p => p.FirstName), new DynamicSortExpression (p => p.LastName, true));
RemoveConvert方法是我从EntityFramework源代码中拉出来以递归方式从MemberExpression中删除转换调用的方法：
internal static Expression RemoveConvert(Expression expression) { System.Diagnostics.Debug.Assert(expression != null); while ((expression != null) && (expression.NodeType == ExpressionType.Convert || expression.NodeType == ExpressionType.ConvertChecked)) { expression = RemoveConvert(((UnaryExpression)expression).Operand); } return expression; }
我希望这是有帮助的！ 谢谢乔恩！