Question

当我向查询添加.Skip（）和.Take（）linq方法时，我一直在努力理解为什么会出现DBContext错误。我有一个运行良好的版本，没有上面提到的方法，只是无法掌握有什么区别。

这是两种方法，它们将数据返回到Model Binded GridView的select方法：

// This is the method that causes the DBContext disposed exception

            public IQueryable<ProductCategory> GetCategoriesByPage(int pageNumber, int pageSize)
        {
            using (var dataModel = new MagicDayEntities())
            {
                var pagedCategories = (from categories in dataModel.ProductCategories
                                       orderby categories.CategoryName ascending
                                       select categories)
                        .Skip((pageSize - 1) * pageNumber)
                        .Take(pageSize);
                pagedCategories.ToList().AsQueryable();
                return pagedCategories;
            }
        }

//This method returns the data properly without error

        public IQueryable<ProductCategory> GetAllCategories()
        {
            using (var dataModel = new MagicDayEntities())
            {
                var allCategories = (from categories in dataModel.ProductCategories
                                     orderby categories.CategoryName ascending
                                     select categories)
                                     .ToList()
                                     .AsQueryable();
                return allCategories;
            }
        }

据我所知，ToList（）应该强制查询运行，并克服延迟加载行为并使DbContext处于＆＃34;使用＆＃34;声明，这是一项要求。

以下是例外情况：

The operation cannot be completed because the DbContext has been disposed. 
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 

Exception Details: System.InvalidOperationException: The operation cannot be completed because the DbContext has been disposed.

Source Error: 

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below. 

Stack Trace: 


[InvalidOperationException: The operation cannot be completed because the DbContext has been disposed.]
   System.Data.Entity.Internal.LazyInternalContext.InitializeContext() +4249201
   System.Data.Entity.Internal.Linq.DbQueryProvider.Execute(Expression expression) +40
   System.Linq.Queryable.Count(IQueryable`1 source) +196
   System.Web.UI.WebControls.QueryableHelpers.CountHelper(IQueryable`1 queryable) +48

[TargetInvocationException: Exception has been thrown by the target of an invocation.]
   System.RuntimeMethodHandle.InvokeMethod(Object target, Object[] arguments, Signature sig, Boolean constructor) +0
   System.Reflection.RuntimeMethodInfo.UnsafeInvokeInternal(Object obj, Object[] parameters, Object[] arguments) +160
   System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture) +101
   System.Web.UI.WebControls.ModelDataSourceView.ProcessSelectMethodResult(DataSourceSelectArguments arguments, DataSourceSelectResultProcessingOptions selectResultProcessingOptions, ModelDataMethodResult result) +249
   System.Web.UI.WebControls.ModelDataSourceView.GetSelectMethodResult(DataSourceSelectArguments arguments) +92
   System.Web.UI.WebControls.ModelDataSourceView.ExecuteSelect(DataSourceSelectArguments arguments) +15
   System.Web.UI.DataSourceView.Select(DataSourceSelectArguments arguments, DataSourceViewSelectCallback callback) +22
   System.Web.UI.WebControls.ModelDataSourceView.Select(DataSourceSelectArguments arguments, DataSourceViewSelectCallback callback) +80
   System.Web.UI.WebControls.DataBoundControl.PerformSelect() +143
   System.Web.UI.WebControls.BaseDataBoundControl.DataBind() +74
   System.Web.UI.WebControls.GridView.DataBind() +9
   System.Web.UI.WebControls.BaseDataBoundControl.EnsureDataBound() +114
   System.Web.UI.WebControls.CompositeDataBoundControl.CreateChildControls() +75
   System.Web.UI.Control.EnsureChildControls() +92
   System.Web.UI.Control.PreRenderRecursiveInternal() +42
   System.Web.UI.Control.PreRenderRecursiveInternal() +160
   System.Web.UI.Control.PreRenderRecursiveInternal() +160
   System.Web.UI.Control.PreRenderRecursiveInternal() +160
   System.Web.UI.Control.PreRenderRecursiveInternal() +160
   System.Web.UI.Control.PreRenderRecursiveInternal() +160
   System.Web.UI.Control.PreRenderRecursiveInternal() +160
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +883


Version Information: Microsoft .NET Framework Version:4.0.30319; ASP.NET Version:4.6.1586.0

有人可以帮助我，并解释这个例外的原因吗？提前致谢。彼得

Answer 1

您可以致电ToList：

来执行查询

pagedCategories.ToList().AsQueryable();

然后忽略结果并返回原始查询，该查询将在您的视图绑定到它时执行：

return pagedCategories;

更改以返回列表：

return pagedCategories.ToList().AsQueryable();

我还建议将返回类型更改为IEnumerable<ProductCategory>并删除对AsQueryable的调用。这样做没有任何意义。

模型绑定DBContext在页面加载时处理

1 个答案: