Xamarin navigate to page throws "Object reference not set to an instance of an object."

tim 120 Reputation points
2024-04-10T23:23:39.3666667+00:00

I'm trying to navigate and pass data to an edit page when I select a row.

I select a row in the checklistpage and what to pass that rows data to the treeinfopage. (has two points of entry 1 from menu and 1 from ChecklistPage)

the page structures are as follows:

treeInfoPage.xaml

treeInfoPage.xaml.cs

treeInfoPageModel.cs

checklistpageModel.cs

    private INavigationService _navigationService;

// not sure if this should be ChecklistpageModel or Checklistpage or Treeinfo...

    public void TreeInfoPageModel(INavigationService navigationService)

    {

        _navigationService = navigationService;           

    }

    public async void OnEditCommandAction(object SelectedObj)

    {          

        var treeInfoPage = new TreeInfoPage();

        treeInfoPage.BindingContext = SelectedObj;

     // everything work till the here , then the Object reference not set is thrown.

        await _navigationService.NavigateToAsync<TreeInfoPageModel>();

//

       /*tried the following, which in debug mode, will execute the TreeInfoPageModel code and Sector get set with show that the data is being passed but If I select the TreeInfoPage from the menu, the fields are empty, I assume that from the menu the data is cleared and the navigation is what is needed */

       // var pd2pm = new TreeInfoPageModel((CheckListItem)SelectedObj);

        return;

    }

TreeInfoPageModel.cs

public TreeInfoPageModel(CheckListItem data)

    {

        StatusMsg = "Ready";

        WorkTreeLocationItems = new ObservableCollection<WorkTreeLocationItem>();

        SaveLocationDataModel = new ButtonModel("Save Data", OnSaveLocationDataAction);

        Close_App_Clicked = new ButtonModel("Close App", CloseAppAction);

// prevents field from filling with 0 when entry from menu.

        if (data.TreeNumber > 0)

        {

            StatusMsg = data.Sector.ToString();

            Sector = data.Sector.ToString();

Thanks

Tim

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Accepted answer
  1. Leon Lu (Shanghai Wicresoft Co,.Ltd.) 68,656 Reputation points Microsoft Vendor
    2024-04-11T06:03:48.1366667+00:00

    Hello,

    Xamarin support will end on May 1, 2024, it's recommended that you migrate your Xamarin app to .NET MAUI. Please see Xamarin official support policy .

    =============Update==================

    You used Enterprise App Navigation, please refer to the following steps to make the navigation and fix errors.

    Step 1: please open MenuPageModel.cs , remove other content, just keep one constructor like following code.

    public class MenuPageModel : PageModelBase
    {
         public MenuPageModel() {
    
         
    
         }  
    }
    

    Step 2: Open MenuPage.xaml remove all of BindingContext= yourModel like following code.

    <?xml version="1.0" encoding="utf-8" ?>
    <TabbedPage xmlns="http://xamarin.com/schemas/2014/forms"
                xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
                xmlns:d="http://xamarin.com/schemas/2014/forms/design"
                xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
                xmlns:android="clr-namespace:Xamarin.Forms.PlatformConfiguration.AndroidSpecific;assembly=Xamarin.Forms.Core"
                android:TabbedPage.ToolbarPlacement="Bottom"
                mc:Ignorable="d"
                xmlns:pages="clr-namespace:MapleSugar.Pages"
                x:Class="MapleSugar.Pages.MenuPage">
        <TabbedPage.Children>
            <NavigationPage Title="Collection">
                <x:Arguments>
                    <pages:CollectionPage />
                </x:Arguments>
            </NavigationPage>
            <NavigationPage Title="Tree Location">
                <x:Arguments>
                    <pages:TreeInfoPage />
                </x:Arguments>
            </NavigationPage>
           <NavigationPage Title="Check List">
                <x:Arguments>
                    <pages:CheckListPage />
                </x:Arguments>
            </NavigationPage>
            <NavigationPage Title="Map Page">
                <x:Arguments>
                    <pages:MapPage />
                </x:Arguments>
            </NavigationPage>       
          <!--  <NavigationPage Title="File Picker">
                <x:Arguments>
                    <pages:FilePickerPage BindingContext="{Binding FilePickerPage}"/>
                </x:Arguments>
            </NavigationPage>
            -->
            <NavigationPage Title="File IO">
                <x:Arguments>
                    <pages:FileSystemPage />
                </x:Arguments>
            </NavigationPage>
    
    
       </TabbedPage.Children>
    </TabbedPage>
    

    Step 3: Open CheckListPage.xaml, add CheckListPageModel in the ContentPage's xml. As Note, please add pageModel for other pages like FileSystemPage , CollectionPage , TreeInfoPage CheckListPage .

    <ContentPage 
    ...
                 add this content
                 xmlns:local="clr-namespace:MapleSugar.PageModels"
    
    ...
                 x:Class="MapleSugar.Pages.CheckListPage">
    
    add page model here
    
        <ContentPage.BindingContext>
            <local:CheckListPageModel />
        </ContentPage.BindingContext>
    

    Step 4: open CheckListPageModel.cs, remove navigationService in constructor like following code.

    public CheckListPageModel()
      {
    
                  
    
          WorkTreeLocationItems = new ObservableCollection<WorkTreeLocationItem>();
    
          Load_Button_Clicked = new ButtonModel("Load Data", LoadCheckListAction);
    
           OnEditCommand = new Command(OnEditCommandAction);
    
          Close_App_Clicked = new ButtonModel("Close App", CloseAppAction);           
    
      }
    

    And use following code to make a Navigation and send data to TreeInfoPageModel

     public async void OnEditCommandAction(object SelectedObj)
      {
          CheckListItem checkListItem = SelectedObj as CheckListItem;
          // navigationService.PushAsync(new TreeInfoPage(checkListItem));      
    
    
         var navService = PageModelLocator.Resolve<INavigationService>();
    
    
    
         await navService.NavigateToAsync<TreeInfoPageModel>(checkListItem, true);
          return;
      }
    

    Step 5: In this TreeInfoPageModel.cs, please chanage the TreeInfoPageModel's constructor like following code and override InitializeAsync, you can get the navigated data by object navigationData

    /*  main code */
    public TreeInfoPageModel( )
    {
         StatusMsg = "Ready";
         WorkTreeLocationItems = new ObservableCollection<WorkTreeLocationItem>();
         SaveLocationDataModel = new ButtonModel("Save Data", OnSaveLocationDataAction);
         Close_App_Clicked = new ButtonModel("Close App", CloseAppAction);
    
    
    }
    
    
    public override Task InitializeAsync(object navigationData)
    {
         if (navigationData is WorkCollectedItem)
         {
             WorkCollectedItem workCollectedItem = navigationData as WorkCollectedItem;
             //  for testing.
    
    
            TreeNumber = workCollectedItem.TreeNumber.ToString();
         }
    
    
        if (navigationData is CheckListItem)
         {
             CheckListItem workCollectedItem = navigationData as CheckListItem;
    
             //  for testing.
    
    
            TreeNumber = workCollectedItem.TreeNumber.ToString();
         }
    
    
         return base.InitializeAsync(navigationData);
    }
    

    Best Regards,

    Leon Lu


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  2. Muhammad Nomann 0 Reputation points
    2024-04-17T08:02:13.5166667+00:00

    I was having the same issue but you described it pretty well. Thanks alot!