ios - Xamarin iOS PushViewController 空引用异常

Question

我是 Xamarin iOS 开发的新手，遇到了一个我无法解决的问题。当下面这行代码运行时，它会抛出一个空引用异常，我不明白为什么。

this.NavigationController.PushViewController(location, true);

这是我的整个代码块。

partial void BtnLogin_TouchUpInside(UIButton sender)
    {
        if (txtPassword.Text != "" && txtUsername.Text != "")
        {
            //send UN & PD to webservice for validation.
            xxx.xxx.xxx.Services s = new xxx.xxx.xxx.Services();
            bool result = s.validateLogin(txtUsername.Text, txtPassword.Text);
            if (result)
            {
                try
                {
                    LocationViewController location = this.Storyboard.InstantiateViewController("LocationViewController") as LocationViewController;

                    if (location != null)
                        this.NavigationController.PushViewController(location, true);
                }
                catch (Exception ex)
                {
                    LoginFail("Error", ex.Message);
                }
            }
            else
            {
                LoginFail("Login", "Invalid username or password.");
            }
        }
        else
        {
            LoginFail("Login", "You must enter both your username and password.");
        }
    }

And my Storyboard

Answer 1

因为你登录的viewController不在NavigationController中，所以当你使用this.NavigationController时，会抛出空引用异常。

解决这个问题，这里有两种方法:

• 将登录ViewVontroller放到NavigationController中，使NavigationController(和登录页面的rootviewController)成为storyboard的初始Controller。

• 更改此代码 this.NavigationController.PushViewController(location, true);

  到 this.PresentViewController(location, true, null);