c# – 已经有一个必须先关闭的开放DataReader

我尝试使用以下代码示例中的查询来检索品牌名称列表以填充SelectListItem,但触发了以下异常：

There is already an open DataReader associated with this Command which
must be closed first.

制图

public class MedicalProductMapper
{
    private MvcMedicalStoreDb _db; // DataContext class

    public MedicalProductMapper(MvcMedicalStoreDb db)
    {
        _db = db;
    }    
    public MedicalProductViewModel GetMedicalProductViewModel(MedicalProduct source)
    {
        MedicalProductViewModel viewModel = new MedicalProductViewModel();

        viewModel.ID = source.ID; 
        viewModel.Name = source.Name;
        viewModel.Price = source.Price;
        viewModel.BrandID = source.BrandID;

        // This following line produces the exception
        viewModel.BrandName = _db.Brands.Single(b => b.ID == source.BrandID).Name;

        var queryBrands = from b in _db.Brands
                          select b;

        viewModel.BrandSelectListItem = queryBrands as IEnumerable<SelectListItem>;

        return viewModel;
    }
}

我知道通过在连接字符串中启用多个活动结果集(MARS)可以轻松解决问题,但我想知道是否有办法在不修改连接字符串的情况下执行我想要的操作.

这里有一些更多的类,以防它们有助于解决这个问题：

编辑视图

@model MvcMedicalStore.Models.MedicalProductViewModel

@{
    ViewBag.Title = "Edit";
}

<h2>Edit</h2>

@using (Html.BeginForm()) {
    @Html.AntiForgeryToken()
    @Html.ValidationSummary(true)

    <fieldset>
        <legend>MedicalProduct</legend>

        @Html.HiddenFor(model => model.ID)

        <div class="editor-label">
            @Html.LabelFor(model => model.Name)
        </div>
        <div class="editor-field">
            @Html.EditorFor(model => model.Name)
            @Html.ValidationMessageFor(model => model.Name)
        </div>

        <div class="editor-label">
            @Html.LabelFor(model => model.Price)
        </div>
        <div class="editor-field">
            @Html.EditorFor(model => model.Price)
            @Html.ValidationMessageFor(model => model.Price)
        </div>

        // BRAND NAME
        <div class="editor-label">
            @Html.LabelFor(model => model.BrandName)
        </div>
        <div class="editor-field">
            @Html.DropDownListFor(model => model.BrandName,Model.BrandSelectListItem)
            @Html.ValidationMessageFor(model => model.BrandName)
        </div>

        <p>
            <input type="submit" value="Save" />
        </p>
    </fieldset>
}

<div>
    @Html.ActionLink("Back to List","Index")
</div>

@section Scripts {
    @Scripts.Render("~/bundles/jqueryval")
}

控制器：

public class MedicalProductController : Controller
{
    private MvcMedicalStoreDb _db = new MvcMedicalStoreDb();

    //
    // GET: /MedicalSupply/

    public ActionResult Index()
    {
        var viewModel = _db.Products.AsEnumerable()
            .Select(product => GetMedicalProductViewModel(product));
        return View(viewModel);
    }

    public MedicalProductViewModel GetMedicalProductViewModel(MedicalProduct product)
    {
        var mapper = new MedicalProductMapper(_db);

        return mapper.GetMedicalProductViewModel(product);            
    }
    public MedicalProduct GetMedicalProduct(MedicalProductViewModel viewModel)
    {
        var mapper = new MedicalProductMapper(_db);

        return mapper.GetMedicalProduct(viewModel);
    }

    //
    // GET: /MedicalSupply/Edit/5

    public ActionResult Edit(int id = 0)
    {
        MedicalProduct medicalProduct = _db.Products.Find(id);
        if (medicalProduct == null)
        {
            return HttpNotFound();
        }

        var viewModel = GetMedicalProductViewModel(medicalProduct);
        return View(viewModel);
    }

    //
    // POST: /MedicalSupply/Edit/5

    [HttpPost]
    [ValidateAntiForgeryToken]
    public ActionResult Edit(MedicalProduct medicalProduct)
    {
        if (ModelState.IsValid)
        {
            _db.Entry(medicalProduct).State = EntityState.Modified;
            _db.SaveChanges();
            return RedirectToAction("Index");
        }

        var viewModel = GetMedicalProductViewModel(medicalProduct);
        return View(viewModel);
    }
}

堆栈跟踪

[InvalidOperationException: There is already an open DataReader
associated with this Command which must be closed first.]
System.Data.SqlClient.SqlInternalConnectionTds.ValidateConnectionForExecute(SqlCommand
command) +5287423
System.Data.SqlClient.SqlConnection.ValidateConnectionForExecute(String
method,SqlCommand command) +20
System.Data.SqlClient.SqlCommand.ValidateCommand(String method,
Boolean async) +155
System.Data.SqlClient.SqlCommand.RunExecuteReader(CommandBehavior
cmdBehavior,RunBehavior runBehavior,Boolean returnStream,String
method,TaskCompletionSource`1 completion,Int32 timeout,Task& task,
Boolean asyncWrite) +82
System.Data.SqlClient.SqlCommand.RunExecuteReader(CommandBehavior
cmdBehavior,String
method) +53
System.Data.SqlClient.SqlCommand.ExecuteReader(CommandBehavior
behavior,String method) +134
System.Data.SqlClient.SqlCommand.ExecuteDbDataReader(CommandBehavior
behavior) +41
System.Data.Common.DbCommand.ExecuteReader(CommandBehavior behavior)
+10 System.Data.EntityClient.EntityCommandDefinition.ExecuteStoreCommands(EntityCommand
entityCommand,CommandBehavior behavior) +437

[EntityCommandExecutionException: An error occurred while executing
the command definition. See the inner exception for details.]
System.Data.EntityClient.EntityCommandDefinition.ExecuteStoreCommands(EntityCommand
entityCommand,CommandBehavior behavior) +507
System.Data.Objects.Internal.ObjectQueryExecutionPlan.Execute(ObjectContext
context,ObjectParameterCollection parameterValues) +730
System.Data.Objects.ObjectQuery1.GetResults(Nullable1
forMergeOption) +131
System.Data.Objects.ObjectQuery1.System.Collections.Generic.IEnumerable<T>.GetEnumerator()
+36 System.Linq.Enumerable.Single(IEnumerable1 source) +179 System.Data.Objects.ELinq.ObjectQueryProvider.b_3(IEnumerable1
sequence) +41
System.Data.Objects.ELinq.ObjectQueryProvider.ExecuteSingle(IEnumerable1
query,Expression queryRoot) +59
System.Data.Objects.ELinq.ObjectQueryProvider.System.Linq.IQueryProvider.Execute(Expression
expression) +133
System.Data.Entity.Internal.Linq.DbQueryProvider.Execute(Expression
expression) +123 System.Linq.Queryable.Single(IQueryable1 source,
Expression1 predicate) +287
MvcMedicalStore.Mappers.MedicalProductMapper.GetMedicalProductViewModel(MedicalProduct
source) in c:UsersMattDocumentsVisual Studio
2012ProjectsMvcMedicalStoreMvcMedicalStoreMappersMedicalProductMapper.cs:28
MvcMedicalStore.Controllers.<>c_DisplayClass1.b_0(MedicalProduct
product) in c:UsersMattDocumentsVisual Studio
2012ProjectsMvcMedicalStoreMvcMedicalStoreControllersHomeController.cs:28
System.Linq.WhereSelectEnumerableIterator2.MoveNext() +145
ASP._Page_Views_Home_Index_cshtml.Execute() in
c:UsersMattDocumentsVisual Studio
2012ProjectsMvcMedicalStoreMvcMedicalStoreViewsHomeIndex.cshtml:25
System.Web.WebPages.WebPageBase.ExecutePageHierarchy() +197
System.Web.Mvc.WebViewPage.ExecutePageHierarchy() +119
System.Web.WebPages.StartPage.RunPage() +17
System.Web.WebPages.StartPage.ExecutePageHierarchy() +62
System.Web.WebPages.WebPageBase.ExecutePageHierarchy(WebPageContext
pageContext,TextWriter writer,WebPageRenderingBase startPage) +76
System.Web.Mvc.RazorView.RenderView(ViewContext viewContext,
TextWriter writer,Object instance) +743
System.Web.Mvc.BuildManagerCompiledView.Render(ViewContext
viewContext,TextWriter writer) +382
System.Web.Mvc.ViewResultBase.ExecuteResult(ControllerContext context)
+431 System.Web.Mvc.ControllerActionInvoker.InvokeActionResult(ControllerContext
controllerContext,ActionResult actionResult) +39
System.Web.Mvc.<>c__DisplayClass1a.<InvokeActionResultWithFilters>b__17()
+74 System.Web.Mvc.ControllerActionInvoker.InvokeActionResultFilter(IResultFilter
filter,ResultExecutingContext preContext,Func1 continuation) +388
System.Web.Mvc.<>c_DisplayClass1c.b_19()
+72 System.Web.Mvc.ControllerActionInvoker.InvokeActionResultWithFilters(ControllerContext
controllerContext,IList1 filters,ActionResult actionResult) +303
System.Web.Mvc.Async.<>c__DisplayClass2a.<BeginInvokeAction>b__20()
+155 System.Web.Mvc.Async.<>c__DisplayClass25.<BeginInvokeAction>b__22(IAsyncResult
asyncResult) +184 System.Web.Mvc.Async.WrappedAsyncResult1.End()
+136 System.Web.Mvc.Async.AsyncResultWrapper.End(IAsyncResult asyncResult,Object tag) +56
System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeAction(IAsyncResult
asyncResult) +40
System.Web.Mvc.<>c_DisplayClass1d.b_18(IAsyncResult
asyncResult) +40
System.Web.Mvc.Async.<>c_DisplayClass4.b_3(IAsyncResult
ar) +47 System.Web.Mvc.Async.WrappedAsyncResult1.End() +151
System.Web.Mvc.Async.AsyncResultWrapper.End(IAsyncResult asyncResult,
Object tag) +59
System.Web.Mvc.Async.AsyncResultWrapper.End(IAsyncResult asyncResult,
Object tag) +40
System.Web.Mvc.Controller.EndExecuteCore(IAsyncResult asyncResult) +44
System.Web.Mvc.Async.<>c__DisplayClass4.<MakeVoidDelegate>b__3(IAsyncResult
ar) +47 System.Web.Mvc.Async.WrappedAsyncResult1.End() +151
System.Web.Mvc.Async.AsyncResultWrapper.End(IAsyncResult asyncResult,
Object tag) +40 System.Web.Mvc.Controller.EndExecute(IAsyncResult
asyncResult) +39
System.Web.Mvc.Controller.System.Web.Mvc.Async.IAsyncController.EndExecute(IAsyncResult
asyncResult) +39
System.Web.Mvc.<>c_DisplayClass8.b_3(IAsyncResult
asyncResult) +45
System.Web.Mvc.Async.<>c_DisplayClass4.b__3(IAsyncResult
ar) +47 System.Web.Mvc.Async.WrappedAsyncResult`1.End() +151
System.Web.Mvc.Async.AsyncResultWrapper.End(IAsyncResult asyncResult,
Object tag) +40
System.Web.Mvc.MvcHandler.EndProcessRequest(IAsyncResult asyncResult)
+40 System.Web.Mvc.MvcHandler.System.Web.IHttpAsyncHandler.EndProcessRequest(IAsyncResult
result) +38
System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
+9628700 System.Web.HttpApplication.ExecuteStep(IExecutionStep step,Boolean& completedSynchronously) +155