c#编译的Lambda表达式访问一个对象的属性，该属性是另一个对象的属性，抛出异常

Question

按照我之前的问题：

Lambda 表达式访问一个对象的属性，该对象是 c# 中另一个对象的属性

我现在遇到另一个问题：

var param = Expression.Parameter(typeof(GAcordos.Models.Contratos), "x");
                    var body = Expression.Equal(Expression.PropertyOrField(Expression.PropertyOrField(param, propName[0]), columnName.ToString()), fixedItem, false, Type.GetType("GAcordos.Helpers.Comparators").GetMethod(oper, new Type[] { propType, propType }));
                    var lambda = Expression.Lambda<Func<GAcordos.Models.Contratos, bool>>(body, param);

                    contratosList = contratosList.Where(lambda).AsQueryable();

将 lambda 传递给 Expression.Where 方法时，它不会执行我给出的替换方法到等式表达式，并执行标准 Equal 比较。

如果我编译 lambda：

contratosList = contratosList.Where(lambda.Compile()).AsQueryable();

它会抛出异常 未将对象引用设置为对象的实例。 稍后在视图中，并显示以下堆栈跟踪：

 at lambda_method(Closure , Contratos )
 at System.Linq.Enumerable.WhereEnumerableIterator`1.MoveNext()
 at System.Linq.Buffer`1..ctor(IEnumerable`1 source)
 at System.Linq.OrderedEnumerable`1.<GetEnumerator>d__0.MoveNext()
 at System.Linq.Enumerable.Count[TSource](IEnumerable`1 source)
 at lambda_method(Closure )
 at System.Linq.EnumerableExecutor`1.Execute()
 at System.Linq.EnumerableQuery`1.System.Linq.IQueryProvider.Execute[S](Expression expression)
 at System.Linq.Queryable.Count[TSource](IQueryable`1 source)
 at MvcContrib.Pagination.LazyPagination`1.TryExecuteQuery() in C:\Users\daniel.almeida\Downloads\MVCContrib.source\src\MVCContrib\Pagination\LazyPagination.cs:line 62
 at MvcContrib.Pagination.LazyPagination`1.get_TotalItems() in C:\Users\daniel.almeida\Downloads\MVCContrib.source\src\MVCContrib\Pagination\LazyPagination.cs:line 85
 at MvcContrib.UI.Pager.Pager.ToHtmlString() in C:\Users\daniel.almeida\Downloads\MVCContrib.source\src\MVCContrib\UI\Pager\Pager.cs:line 130
 at MvcContrib.UI.Pager.Pager.ToString() in C:\Users\daniel.almeida\Downloads\MVCContrib.source\src\MVCContrib\UI\Pager\Pager.cs:line 125
 at ASP._Page_Views_Shared_Pager_cshtml.Execute() in c:\inetpub\wwwroot\Empresas\Proactivos\GAcordos\GAcordos\Views\Shared\Pager.cshtml:line 4
 at System.Web.WebPages.WebPageBase.ExecutePageHierarchy()
 at System.Web.Mvc.WebViewPage.ExecutePageHierarchy()
 at System.Web.WebPages.WebPageBase.ExecutePageHierarchy(WebPageContext pageContext, TextWriter writer, WebPageRenderingBase startPage)
 at System.Web.Mvc.RazorView.RenderView(ViewContext viewContext, TextWriter writer, Object instance)
 at System.Web.Mvc.BuildManagerCompiledView.Render(ViewContext viewContext, TextWriter writer)
 at System.Web.Mvc.HtmlHelper.RenderPartialInternal(String partialViewName, ViewDataDictionary viewData, Object model, TextWriter writer, ViewEngineCollection viewEngineCollection)
 at System.Web.Mvc.Html.RenderPartialExtensions.RenderPartial(HtmlHelper htmlHelper, String partialViewName, Object model)
 at ASP._Page_Views_Contratos_Index_cshtml.Execute() in c:\inetpub\wwwroot\Empresas\Proactivos\GAcordos\GAcordos\Views\Contratos\Index.cshtml:line 29
 at System.Web.WebPages.WebPageBase.ExecutePageHierarchy()
 at System.Web.Mvc.WebViewPage.ExecutePageHierarchy()
 at System.Web.WebPages.StartPage.RunPage()
 at System.Web.WebPages.StartPage.ExecutePageHierarchy()
 at System.Web.WebPages.WebPageBase.ExecutePageHierarchy(WebPageContext pageContext, TextWriter writer, WebPageRenderingBase startPage)
 at System.Web.Mvc.RazorView.RenderView(ViewContext viewContext, TextWriter writer, Object instance)
 at System.Web.Mvc.BuildManagerCompiledView.Render(ViewContext viewContext, TextWriter writer)
 at System.Web.Mvc.ViewResultBase.ExecuteResult(ControllerContext context)
 at System.Web.Mvc.ControllerActionInvoker.InvokeActionResult(ControllerContext controllerContext, ActionResult actionResult)
 at System.Web.Mvc.ControllerActionInvoker.<>c__DisplayClass1c.<InvokeActionResultWithFilters>b__19()
 at System.Web.Mvc.ControllerActionInvoker.InvokeActionResultFilter(IResultFilter filter, ResultExecutingContext preContext, Func`1 continuation)

可能出了什么问题？

Answer 1

很难说，但如果我不得不猜测，我会说你的 GetMethod 调用返回 null。

您能否将这一行分解为临时变量并检查其值？

Type.GetType("GAcordos.Helpers.Comparators").GetMethod(oper, new Type[] { propType, propType })

或者更好的是，制作一些独立的重现，供这里的人们运行。

Answer 2

问题不在于 lambda 表达式。这是从数据库加载信息时引起的，在迭代主表查询结果时正在查询数据库中的相关表。

将 MultipleActiveResultSets=true 添加到连接字符串的提供程序部分即可解决此问题。
请参阅解决方案：

已经有一个打开的 DataReader 与此命令关联，必须先将其关闭。