LINQ 和异常已被调用的目标抛出答案

【问题标题】：LINQ and Exception has been thrown by the target of an invocationLINQ 和异常已被调用的目标抛出
【发布时间】：2012-08-04 14:54:09
【问题描述】：

我有带有 linq 和 SQL CE 4.0 的 WPF .NET FW4 应用程序。我在客户的笔记本上随机收到此错误：

调用的目标已抛出异常。

我已经知道，重要的是内部异常（带有堆栈跟踪）：

对象引用未设置为对象的实例。在 lambda_method(闭包)

代码块：

private void LoadCustomersDetail(CustomersView cw)
    {
        Persons customer = db.Persons.Where(c => c.Person_id == cw.Customer_id).FirstOrDefault();
        reg = new RegistrationWindow(db, customer, false);
        reg.ShowDialog();
        if (reg.DialogResult.Value)
        {
            if (!onlyDebtors)
                LoadCustomers();
            else
                LoadDebtors();
        }
    }

原始异常的堆栈跟踪：

at System.RuntimeMethodHandle._InvokeMethodFast(IRuntimeMethodInfo method, Object target, Object[] arguments, SignatureStruct& sig, MethodAttributes methodAttributes, RuntimeType typeOwner)
   at System.RuntimeMethodHandle.InvokeMethodFast(IRuntimeMethodInfo method, Object target, Object[] arguments, Signature sig, MethodAttributes methodAttributes, RuntimeType typeOwner)
   at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture, Boolean skipVisibilityChecks)
   at System.Delegate.DynamicInvokeImpl(Object[] args)
   at System.Data.Linq.CommonDataServices.GetKeyFromPredicate(MetaType type, Dictionary`2 keys, Expression mex, Expression vex)
   at System.Data.Linq.CommonDataServices.GetKeysFromPredicate(MetaType type, Dictionary`2 keys, Expression expr)
   at System.Data.Linq.CommonDataServices.GetKeyValues(MetaType type, LambdaExpression predicate)
   at System.Data.Linq.CommonDataServices.GetCachedObject(Expression query)
   at System.Data.Linq.CommonDataServices.GetCachedObject(Expression query)
   at System.Data.Linq.SqlClient.SqlProvider.GetCachedResult(Expression query)
   at System.Data.Linq.SqlClient.SqlProvider.System.Data.Linq.Provider.IProvider.Execute(Expression query)
   at System.Data.Linq.DataQuery`1.System.Linq.IQueryProvider.Execute[S](Expression expression)
   at System.Linq.Queryable.FirstOrDefault[TSource](IQueryable`1 source)
   at ProjectName.Core.ViewWindow.LoadCustomersDetail(CustomersView cw)
   at ProjectName.Core.ViewWindow.dataGridCustomersList_MouseDoubleClick(Object sender, MouseButtonEventArgs e)
   at System.Windows.Input.MouseButtonEventArgs.InvokeEventHandler(Delegate genericHandler, Object genericTarget)
   at System.Windows.RoutedEventArgs.InvokeHandler(Delegate handler, Object target)
   at System.Windows.RoutedEventHandlerInfo.InvokeHandler(Object target, RoutedEventArgs routedEventArgs)
   at System.Windows.EventRoute.InvokeHandlersImpl(Object source, RoutedEventArgs args, Boolean reRaised)
   at System.Windows.UIElement.RaiseEventImpl(DependencyObject sender, RoutedEventArgs args)
   at System.Windows.UIElement.RaiseEvent(RoutedEventArgs e)
   at System.Windows.Controls.Control.OnMouseDoubleClick(MouseButtonEventArgs e)
   at System.Windows.Controls.Control.HandleDoubleClick(Object sender, MouseButtonEventArgs e)
   at System.Windows.Input.MouseButtonEventArgs.InvokeEventHandler(Delegate genericHandler, Object genericTarget)
   at System.Windows.RoutedEventArgs.InvokeHandler(Delegate handler, Object target)
   at System.Windows.RoutedEventHandlerInfo.InvokeHandler(Object target, RoutedEventArgs routedEventArgs)
   at System.Windows.EventRoute.InvokeHandlersImpl(Object source, RoutedEventArgs args, Boolean reRaised)
   at System.Windows.UIElement.ReRaiseEventAs(DependencyObject sender, RoutedEventArgs args, RoutedEvent newEvent)
   at System.Windows.UIElement.OnMouseDownThunk(Object sender, MouseButtonEventArgs e)
   at System.Windows.Input.MouseButtonEventArgs.InvokeEventHandler(Delegate genericHandler, Object genericTarget)
   at System.Windows.RoutedEventArgs.InvokeHandler(Delegate handler, Object target)
   at System.Windows.RoutedEventHandlerInfo.InvokeHandler(Object target, RoutedEventArgs routedEventArgs)
   at System.Windows.EventRoute.InvokeHandlersImpl(Object source, RoutedEventArgs args, Boolean reRaised)
   at System.Windows.UIElement.RaiseEventImpl(DependencyObject sender, RoutedEventArgs args)
   at System.Windows.UIElement.RaiseTrustedEvent(RoutedEventArgs args)
   at System.Windows.UIElement.RaiseEvent(RoutedEventArgs args, Boolean trusted)
   at System.Windows.Input.InputManager.ProcessStagingArea()
   at System.Windows.Input.InputManager.ProcessInput(InputEventArgs input)
   at System.Windows.Input.InputProviderSite.ReportInput(InputReport inputReport)
   at System.Windows.Interop.HwndMouseInputProvider.ReportInput(IntPtr hwnd, InputMode mode, Int32 timestamp, RawMouseActions actions, Int32 x, Int32 y, Int32 wheel)
   at System.Windows.Interop.HwndMouseInputProvider.FilterMessage(IntPtr hwnd, WindowMessage msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
   at System.Windows.Interop.HwndSource.InputFilterMessage(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
   at MS.Win32.HwndWrapper.WndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
   at MS.Win32.HwndSubclass.DispatcherCallbackOperation(Object o)
   at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
   at MS.Internal.Threading.ExceptionFilterHelper.TryCatchWhen(Object source, Delegate method, Object args, Int32 numArgs, Delegate catchHandler)

【问题讨论】：

一个空客户将是我要测试的第一件事 - 您的 RegistrationWindow 构造函数是否处理这种情况？

标签： c# wpf linq

【解决方案1】：

异常似乎与Where-表达式有关，因此c 或cw 为空。我建议在方法的开头检查参数（如果cw == null，则抛出ArgumentNullException）。还要确保Persons 集合不包含空值（如果可能的话）。

【讨论】：

我尝试将 cw 设置为 null，现在原始异常是未设置对象引用...，但具有不同的堆栈跟踪，并且在 RegistrationWindow 构造函数的后续步骤中发生。如果您检查堆栈跟踪，您将看到它在 FirstOrDefault() 方法调用时失败。
对不起，我试过这个：Persons customer = db.Persons.Where(c => c.Person_id == NULL).FirstOrDefault();而且它不一样： cw = null;个人客户 = db.Persons.Where(c => c.Person_id == cw.Customer_id).FirstOrDefault();

【解决方案2】：

您的错误在这里cw.Customer_id。你的cw 是null。

【讨论】：