c# - RowSelected - NullReferenceException

Question

我正在尝试这样做，当我在 UITableView 上按下一行时，它会将一些数据发送到另一个 UIViewController 并生成一个序列。

但是在测试时，它给了我:

System.NullReferenceException has been thrown Object reference not set to an instance of an object

错误似乎显示在这里:

viewController.NavigationController.PushViewController (detail, true);

这是我的“RowSelected”

public override void RowSelected (UITableView tableView, NSIndexPath indexPath)
{
    Console.WriteLine ("Row: " + tableItems [indexPath.Row].Heading);

    //new UIAlertView("Row Selected", tableItems[indexPath.Row].Heading, null, "OK", null).Show();
    tableView.DeselectRow (indexPath, true);

    // Specially for Storyboard !!
    var detail = viewController.Storyboard.InstantiateViewController("detail") as HolsteinItemViewController;
    detail.Title = tableItems[indexPath.Row].Heading;
    detail.LoadUrl (tableItems[indexPath.Row].SubHeading);
    viewController.NavigationController.PushViewController (detail, true);
}

异常捕获:

System.NullReferenceException: Object reference not set to an instance of an object
  at Holstein.TableSource.RowSelected (MonoTouch.UIKit.UITableView tableView, MonoTouch.Foundation.NSIndexPath indexPath) [0x0008e] in /Users/Emil/Projects/Holstein/Holstein/Code/TableSource.cs:41
  at at (wrapper managed-to-native) MonoTouch.UIKit.UIApplication:UIApplicationMain (int,string[],intptr,intptr)
  at MonoTouch.UIKit.UIApplication.Main (System.String[] args, System.String principalClassName, System.String delegateClassName) [0x0004c] in /Developer/MonoTouch/Source/monotouch/src/UIKit/UIApplication.cs:38
  at Holstein.Application.Main (System.String[] args) [0x00008] in /Users/Emil/Projects/Holstein/Holstein/Main.cs:16

TableSource.cs 第 46 行

viewController.NavigationController.PushViewController (detail, true);

Main.cs 第 16 行

UIApplication.Main (args, null, "AppDelegate");

Answer 1

您的 viewController.NavigationController 属性为空。根据 iOS UIViewController 引用 here

If the receiver or one of its ancestors is a child of a navigation controller, this property contains the owning navigation controller. This property is nil if the view controller is not embedded inside a navigation controller.

显然，该 View Controller 不是 UINavigationController 堆栈的一部分。