[C++] Exception thrown: read access violation. **this** was nullptr.

Question

Hello,

I have a settings page with a "back" button. When It is clicked, it calls the "CollapseFrame" method in MainPage.xaml. That method collapses the frame containing the Settings Page. However, when debugging, I received this error: "Exception thrown: read access violation. this was nullptr":

void MainPage::CollapseSettings()
{
    SettingsHolder->Visibility = ::Visibility::Collapsed; // This line is where I encountered the error.
}

Answer 1

Hello,

Welcome to Microsoft Q&A.

The instance of SettinsPage created in MainPage::MainPage() is not the instance of SettingsPage contained by frame SettingsHolder. Therefore, the statement s->ParentMainPage = this; does not work for the instance of SettingsPage contained by frame SettingsHolder and the ParentMainPage is null.

You could initiate the value of ParentMainPage in OnNavigatedTo() method with the parameter e which contains the parameter passed from Frame.Navigate(Type,Object) method.

For example:

MainPage.xaml.cpp

//Show the SettingsPage page in frame SettingsHolder by using Navigate method  
SettingsHolder->Navigate(TypeName(SettingsPage::typeid),this);  

SettingsPage.xaml.h

   protected:  
          virtual void OnNavigatedTo(Windows::UI::Xaml::Navigation::NavigationEventArgs^ e) override;  

SettingsPage.xaml.cpp

void AccessViolation::SettingsPage::OnNavigatedTo(Windows::UI::Xaml::Navigation::NavigationEventArgs^ e)  
{  
       MainPage^ mainPage = (MainPage^)e->Parameter;  
       if (mainPage != nullptr)  
       {  
              ParentMainPage = mainPage;  
       }  
}  

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