c# - EntitySet.Where(myPredicate) 抛出 NotSupportedException

Question

编辑:让我们再试一次。这次我使用了 AdventureWorks 示例数据库，因此您可以一起玩。这将排除我在自己的数据库中所做的任何疯狂的事情。这是一个新示例，演示什么有效以及我期望什么有效(但无效)。任何人都可以解释为什么它不起作用或建议实现我的目标的不同方法(重构通用表达式以便它可以在其他地方重用)？

using (AdventureWorksDataContext db = new AdventureWorksDataContext())
{
    // For simplicity's sake we'll just grab the first result.
    // The result should have the name of the SubCategory and an array of Products with ListPrice greater than zero.
    var result = db.ProductSubcategories.Select(subCategory => new
    {
        Name = subCategory.Name,
        ProductArray = subCategory.Products.Where(product => product.ListPrice > 0).ToArray()
    }).First();
    Console.WriteLine("There are {0} products in SubCategory {1} with ListPrice > 0.", result.ProductArray.Length, result.Name);
    // Output should say: There are 3 products in SubCategory Bib-Shorts with ListPrice > 0.

    // This won't work.  I want to pull the expression out so that I can reuse it in several other places.
    Expression<Func<Product, bool>> expression = product => product.ListPrice > 0;
    result = db.ProductSubcategories.Select(subCategory => new
    {
        Name = subCategory.Name,
        ProductArray = subCategory.Products.Where(expression).ToArray() // This won't compile because Products is an EntitySet<Product> and that doesn't have an overload of Where that accepts an Expression.
    }).First();
    Console.WriteLine("There are {0} products in SubCategory {1} with ListPrice > 0.", result.ProductArray.Length, result.Name);
}

</Edit>

以下 LINQ to SQL 运行良好:

var result = from subAccount in db.SubAccounts
             select new ServiceTicket
             {
                 MaintenancePlans = subAccount.Maintenances.Where(plan => plan.CancelDate == null && plan.UpgradeDate == null).Select(plan => plan.ToString()).ToArray()
                 // Set other properties...
             };

但是，我想分解传递给 Where 的谓词因为它在整个代码中使用。但是如果我尝试将定义的谓词传递给 Where它失败了，例如:

Func<DatabaseAccess.Maintenance, bool> activePlanPredicate = plan => plan.CancelDate == null && plan.UpgradeDate == null;
var result = from subAccount in db.SubAccounts
             select new ServiceTicket
             {
                 MaintenancePlans = subAccount.Maintenances.Where(activePlanPredicate).Select(plan => plan.ToString()).ToArray()
                 // Set other properties...
             };

这对我来说毫无意义。任何人都可以解释发生了什么事吗？ Maintenances类型为 EntitySet<DatabaseAccess.Maintenance> .我得到的错误是:

System.NotSupportedException: Unsupported overload used for query operator 'Where'..

编辑:对于那些感兴趣的人，这里是 Reflector 为优化设置为 .NET 2.0 的第一个(工作)示例提供的内容:

using (BugsDatabaseDataContext db = new BugsDatabaseDataContext())
{
    ParameterExpression CS$0$0001;
    ParameterExpression CS$0$0006;
    ParameterExpression CS$0$0010;
    return db.SubAccounts.Select<SubAccount, ServiceTicket>(Expression.Lambda<Func<SubAccount, ServiceTicket>>(
        Expression.MemberInit(
            Expression.New(
                (ConstructorInfo) methodof(ServiceTicket..ctor), 
                new Expression[0]), 
                new MemberBinding[] 
                { 
                    Expression.Bind(
                        (MethodInfo) methodof(ServiceTicket.set_MaintenancePlans), 
                        Expression.Call(
                            null, 
                            (MethodInfo) methodof(Enumerable.ToArray), 
                            new Expression[] 
                            { 
                                Expression.Call(
                                    null, 
                                    (MethodInfo) methodof(Enumerable.Select), 
                                    new Expression[] 
                                    { 
                                        Expression.Call(
                                            null, 
                                            (MethodInfo) methodof(Enumerable.Where), 
                                            new Expression[] 
                                            { 
                                                Expression.Property(CS$0$0001 = Expression.Parameter(typeof(SubAccount), "subAccount"), (MethodInfo) methodof(SubAccount.get_Maintenances)), 
                                                Expression.Lambda<Func<Maintenance, bool>>(
                                                    Expression.AndAlso(
                                                        Expression.Equal(
                                                            Expression.Property(CS$0$0006 = Expression.Parameter(typeof(Maintenance), "plan"), (MethodInfo) methodof(Maintenance.get_CancelDate)), 
                                                            Expression.Convert(Expression.Constant(null, typeof(DateTime?)), typeof(DateTime?)), false, (MethodInfo) methodof(DateTime.op_Equality)
                                                        ), 
                                                        Expression.Equal(
                                                            Expression.Property(CS$0$0006, (MethodInfo) methodof(Maintenance.get_UpgradeDate)), 
                                                            Expression.Convert(Expression.Constant(null, typeof(DateTime?)), typeof(DateTime?)), false, (MethodInfo) methodof(DateTime.op_Equality)
                                                        )
                                                    ), 
                                                    new ParameterExpression[] { CS$0$0006 }
                                                ) 
                                            }
                                        ), 
                                        Expression.Lambda<Func<Maintenance, string>>(
                                            Expression.Call(
                                                CS$0$0010 = Expression.Parameter(typeof(Maintenance), "plan"), 
                                                (MethodInfo) methodof(object.ToString), 
                                                new Expression[0]
                                            ), 
                                            new ParameterExpression[] { CS$0$0010 }
                                        ) 
                                    }
                                ) 
                            }
                        )
                    )
                }
            ), 
            new ParameterExpression[] { CS$0$0001 }
        )
    ).ToList<ServiceTicket>();
}

编辑:第二个示例(使用谓词)的 Reflector 输出大部分相似。最大的区别在于，在调用 Enumerable.Where 时，而不是传递 Expression.Lambda它通过 Expression.Constant(activePlanPredicate) .

Answer 1

我不完全理解 Linq to Entities 的核心内容，但是有一个开源(可在专有软件中使用)工具包专门用于帮助解决这个问题，称为 LinqKit，链接来自这篇与 O'Reilly 相关的文章:

http://www.albahari.com/nutshell/predicatebuilder.aspx

因为我不是很懂，所以我就直接引用一下吧

Entity Framework's query processing pipeline cannot handle invocation expressions, which is why you need to call AsExpandable on the first object in the query. By calling AsExpandable, you activate LINQKit's expression visitor class which substitutes invocation expressions with simpler constructs that Entity Framework can understand.

这里是 a direct link to LinqKit .

这是该项目支持的代码类型:

using LinqKit;

// ...

Expression<Func<Product, bool>> expression = product => product.ListPrice > 0;

var result = db.ProductSubcategories
    .AsExpandable() // This is the magic that makes it all work
    .Select(
        subCategory => new
        {
            Name = subCategory.Name,
            ProductArray = subCategory.Products
                // Products isn't IQueryable, so we must call expression.Compile
                .Where(expression.Compile())
        })
    .First();

Console.WriteLine("There are {0} products in SubCategory {1} with ListPrice > 0."
    , result.ProductArray.Count()
    , result.Name
    );

结果是:

There are 3 products in SubCategory Bib-Shorts with ListPrice > 0.

是的，没有异常(exception)，我们可以提取谓词!