How to solve the error "Object reference not set to an instance of an object."?

Question

I get the error "XDG0062 Object reference not set to an instance of an object. " but when I build, compile, and run, everything is checked normally. The code is posted here

Answer 1

Hi @Peter Fleischer (former MVP) , I fixed the bugs by adding a static class (ContactRef) to get the Contact property in all ViewModels(ContactListingViewModel and AddContactViewModel).
The code is below.

XAML

<Window x:Class="cManagement.MainWindow"  
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"  
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"  
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"  
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"  
        xmlns:local="clr-namespace:cManagement.ViewModels"   
        mc:Ignorable="d"  
        Title="cManagement"  
               Height="530" MinHeight="530" Width="725" MinWidth="725"   
               WindowStartupLocation="CenterScreen">  
     
    <Window.DataContext>  
        <local:ContactListingViewModel/>  
    </Window.DataContext>  
      
    <Grid Margin="10" >  
        <Grid.ColumnDefinitions>  
            <ColumnDefinition Width="*"/>  
            <ColumnDefinition Width="100"/>  
        </Grid.ColumnDefinitions>  
  
        <Grid.RowDefinitions>  
            <RowDefinition Height="30"/>  
            <RowDefinition Height="*"/>  
        </Grid.RowDefinitions>  
  
        <TextBox Grid.Row="0" Grid.Column="0" Height="25" Text="{Binding FilterContact, ValidatesOnDataErrors=True, UpdateSourceTrigger=PropertyChanged}" ToolTip="{Binding ErrorCollection[FilterContact]}"/>  
  
        <ListView x:Name="ContactListView" Grid.Row="1" Grid.Column="0" ItemsSource="{Binding View}"  ScrollViewer.HorizontalScrollBarVisibility="Disabled" IsSynchronizedWithCurrentItem="True" SelectedItem="{Binding Contact}">  
            <ListView.ItemTemplate>  
                <DataTemplate>  
                    <StackPanel Orientation="Vertical" HorizontalAlignment="Left">  
                        <TextBlock Width="200" Text="{Binding Firstname}" TextWrapping="Wrap" />  
                        <TextBlock Width="200" Text="{Binding Lastname}" TextWrapping="Wrap" />  
                    </StackPanel>  
                </DataTemplate>  
            </ListView.ItemTemplate>  
        </ListView>  
  
        <StackPanel Grid.Row="1" Grid.Column="1" Orientation="Vertical" VerticalAlignment="Top" HorizontalAlignment="Center" >  
            <Button Height="30" Width="80" Content="Add" Margin="0,5,0,10" Command="{Binding Cmd}" CommandParameter="Add"/>  
            <Button Height="30" Width="80" Content="Edit" Margin="10" Command="{Binding Cmd}" CommandParameter="Edit"/>  
            <Button Height="30" Width="80" Content="Delete" Margin="10" Command="{Binding Cmd}" CommandParameter="Delete"/>  
        </StackPanel>  
  
    </Grid>  
</Window>  
  
******  
  
<Window x:Class="cManagement.Views.AddContactView"  
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"  
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"  
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"  
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"  
        xmlns:local="clr-namespace:cManagement.Views"  
        xmlns:local1="clr-namespace:cManagement.ViewModels"  
        xmlns:Sys="clr-namespace:System;assembly=mscorlib"  
        mc:Ignorable="d"  
        Title="AddContact"  
               Height="200"   
               Width="450"   
               WindowStartupLocation="CenterOwner"  
               local1:AddContactViewModel.AttProp = "True">  
  
  
    <Grid Margin="0,10,0,0">  
        <StackPanel DataContext="{Binding Contact}">  
  
            <StackPanel Orientation="Vertical" VerticalAlignment="Center" HorizontalAlignment="Center" >  
                <Label Content="Firstname:"  FontWeight="SemiBold"/>  
                <TextBox Height="25" Width="290" MaxLength="91" Margin="0,0,5,0" Text="{Binding Firstname, ValidatesOnDataErrors=True, UpdateSourceTrigger=PropertyChanged}" ToolTip="{Binding ErrorCollection[Firstname]}" HorizontalContentAlignment="left" VerticalContentAlignment="Center"/>  
                <Label Content="Lastname:" FontWeight="SemiBold"/>  
                <TextBox Height="25" Width="290" MaxLength="91" Margin="0,0,5,0" Text="{Binding Lastname, ValidatesOnDataErrors=True, UpdateSourceTrigger=PropertyChanged}" ToolTip="{Binding ErrorCollection[Lastname]}" HorizontalContentAlignment="left" VerticalContentAlignment="Center"/>  
            </StackPanel>  
  
              
        </StackPanel>  
  
        <StackPanel Orientation="Horizontal" VerticalAlignment="Bottom" HorizontalAlignment="Right" Margin="10,0,10,10">  
            <Button Height="30" Width="80" Content="Cancel" Margin="10,0,0,0" Command="{Binding CmdCancel}"/>  
            <Button Height="30" Width="80" Content="Save" Margin="10,0,5,0" Command="{Binding CmdSave}"/>  
        </StackPanel>  
    </Grid>  
</Window>  

Code

using cManagement.Commands;  
using cManagement.Models;  
using cManagement.Services;  
using cManagement.Views;  
using Microsoft.EntityFrameworkCore;  
using System;  
using System.Collections.Generic;  
using System.Collections.ObjectModel;  
using System.ComponentModel;  
using System.Windows.Data;  
  
namespace cManagement.ViewModels  
{  
    class ContactListingViewModel : BaseViewModel, IDataErrorInfo  
    {  
        AppDbContext db = new AppDbContext();  
  
        public ContactListingViewModel()  
        {  
            if (ContactRef.Contact == null)  
            {  
                ContactRef.Contact = new ContactModel();  
            }  
            db.tblContact.Load();  
            cvs.Source = db.tblContact.Local.ToObservableCollection();  
            cvs.Source = ContactsList;  
        }  
  
        public ICollectionView View { get => cvs.View; }  
        private CollectionViewSource cvs = new CollectionViewSource();  
        public ObservableCollection<ContactModel> ContactsList { get; set; } = new ObservableCollection<ContactModel>();  
  
        #region Commands  
  
        public DelegateCommand Cmd { get => new DelegateCommand(CanExec , CmdExec); }  
  
        private AddContactView addContactView;  
        private bool? diagResult;  
  
        private void CmdExec(object parameter)  
        {  
            switch (parameter.ToString())  
            {  
                case "Add":  
                    AddContactViewModel vm = new AddContactViewModel();  
                    addContactView = new AddContactView();  
                    addContactView.DataContext = vm;  
                    diagResult = addContactView.ShowDialog();  
                    if(diagResult.HasValue && diagResult.Value)  
                    {  
                        db.tblContact.Add(ContactRef.Contact);  
                        db.SaveChanges();  
                    }  
                    break;  
                case "Edit":  
                    if (ContactRef.Contact != null)  
                    {  
                        AddContactViewModel viewModel = new AddContactViewModel();  
                        addContactView = new AddContactView() ;  
                        addContactView.DataContext = viewModel;  
                        diagResult = addContactView.ShowDialog();  
                        if (diagResult.HasValue && diagResult.Value)  
                        {  
                            db.SaveChanges();  
                        }  
                        else  
                        {  
                            db.Entry(ContactRef.Contact).State = EntityState.Unchanged;  
                            onPropertyChanged(nameof(View));  
                        }  
                    }  
                    break;  
                case "Delete":  
                    if (ContactRef.Contact != null)  
                    {  
                        db.tblContact.Remove(ContactRef.Contact);  
                        db.SaveChanges();  
                    }  
                    break;  
                default:  
                    break;  
            }  
        }  
  
        private bool CanExec(object parameter)  
        {  
            bool res = false;  
            switch (parameter.ToString())  
            {  
                case "Add":  
                    res = true;  
                    break;  
                case "Edit":  
                    res = (ContactRef.Contact != null);  
                    break;  
                case "Delete":  
                    res = (ContactRef.Contact != null);  
                    break;  
                default:  
                    break;  
            }  
            return res;  
        }  
  
        public bool CanCreateContact => HasContact;  
  
        private bool HasContact => (ContactRef.Contact != null);  
  
        #endregion Commands  
  
        #region Filter  
        private string filterContact;  
        public string FilterContact  
        {  
            get  
            {  
                return filterContact;  
            }  
  
            set  
            {  
                filterContact = value;  
                onPropertyChanged();  
                cvs.View.Refresh();  
            }  
        }  
  
        private void ContactFilter(object sender, FilterEventArgs e)  
        {  
            if (string.IsNullOrWhiteSpace(filterContact))  
            {  
                //no filter when no search text is entered  
                e.Accepted = true;  
            }  
            else  
            {  
                ContactModel contact = (ContactModel)e.Item;  
                if (contact.Firstname.StartsWith(filterContact, true, null) ||   
                    contact.Firstname.Contains(filterContact, StringComparison.InvariantCultureIgnoreCase) ||  
                    contact.Lastname.StartsWith(filterContact, true, null) ||  
                    contact.Lastname.Contains(filterContact, StringComparison.InvariantCultureIgnoreCase)  
                    )  
                {  
                    e.Accepted = true;  
                }  
                else  
                {  
                    e.Accepted = false;  
                }  
            }  
        }  
        #endregion Filter  
  
        #region ErrorInfo  
        public Dictionary<string, string> ErrorCollection { get; private set; } = new Dictionary<string, string>();  
        public string Error { get => String.Empty; }  
        public string this[string propertyName]  
        {  
            get  
            {  
                string result = null;  
  
                switch (propertyName)  
                {  
                    case "FilterContact":  
                        if (string.IsNullOrWhiteSpace(ContactRef.Contact.Firstname))  
                            result = "Please enter firstname or lastname";  
  
                        else if (ContactRef.Contact.Firstname.Length < 5)  
                            result = "Firstname must be a minimum of 5 characters.";  
  
                        break;  
                }  
  
                if (ErrorCollection.ContainsKey(propertyName))  
                {  
                    ErrorCollection[propertyName] = result;  
                }  
                else if (result != null)  
                {  
                    ErrorCollection.Add(propertyName, result);  
                }  
  
                onPropertyChanged("ErrorCollection");  
  
                return result;  
            }  
        }  
        #endregion ErrorInfo  
  
    }  
}  
  
*****  
  
using cManagement.Commands;  
using cManagement.Models;  
using System;  
using System.Collections.Generic;  
using System.ComponentModel;  
using System.Windows;  
  
namespace cManagement.ViewModels  
{  
    class AddContactViewModel : BaseViewModel, IDataErrorInfo  
    {  
        public AddContactViewModel()  
        {  
            if (ContactRef.Contact == null)  
            {  
                ContactRef.Contact = new ContactModel();  
            }  
        }  
  
        public DelegateCommand CmdSave { get => new DelegateCommand(CanCreateContact, CmdExecSave); }  
  
        public void CmdExecSave(Object parameter)  
        {  
            WindowRef.DialogResult = true;  
        }  
  
        public DelegateCommand CmdCancel { get => new DelegateCommand(CmdExecCancel); }  
        public void CmdExecCancel(Object parameter)  
        {  
            WindowRef.DialogResult = false;  
        }  
  
        private bool CanCreateContact(Object parameter) =>  
            HasFirstname &&  
            HasLastname &&  
            ContactRef.Contact != null;  
  
        private bool HasFirstname => !string.IsNullOrEmpty(Firstname);  
        private bool HasLastname => !string.IsNullOrEmpty(Lastname);  
  
        #region Propertyname  
        public string Firstname  
        {  
            get { return ContactRef.Contact.Firstname; }  
            set  
            {  
                ContactRef.Contact.Firstname = value;  
                onPropertyChanged();  
                onPropertyChanged(nameof(CanCreateContact));  
                CmdSave.onCanExecuteChanged();  
            }  
        }  
  
        public string Lastname  
        {  
            get { return ContactRef.Contact.Lastname; }  
            set  
            {  
                ContactRef.Contact.Lastname = value;  
                onPropertyChanged();  
                onPropertyChanged(nameof(CanCreateContact));  
                CmdSave.onCanExecuteChanged();  
            }  
        }  
        #endregion Propertyname  
  
        #region ErrorInfo  
        public Dictionary<string, string> ErrorCollection { get; private set; } = new Dictionary<string, string>();  
        public string Error { get => String.Empty; }  
        public string this[string propertyName]  
        {  
            get  
            {  
                string result = null;  
  
                switch (propertyName)  
                {  
                    case "Firstname":  
                        if (string.IsNullOrWhiteSpace(Firstname))  
                            result = "Firstname cannot be empty";  
  
                        else if (Firstname.Length < 5)  
                            result = "Firstname must be a minimum of 5 characters.";  
  
                        break;  
  
                    case "Lastname":  
                        if (string.IsNullOrWhiteSpace(Lastname))  
                            result = "Lastname cannot be empty";  
  
                        else if (Firstname.Length < 5)  
                            result = "Lastname must be a minimum of 5 characters.";  
  
                        break;  
                }  
  
                if (ErrorCollection.ContainsKey(propertyName))  
                {  
                    ErrorCollection[propertyName] = result;  
                }  
                else if (result != null)  
                {  
                    ErrorCollection.Add(propertyName, result);  
                }  
  
                onPropertyChanged("ErrorCollection");  
  
                return result;  
            }  
        }  
        #endregion ErrorInfo  
  
        #region Attached Property  
        public Window WindowRef;  
  
        public static readonly DependencyProperty AttPropProperty =  
            DependencyProperty.RegisterAttached("AttProp", typeof(bool), typeof(AddContactViewModel), new PropertyMetadata(false, OnPropChanged));  
        public static bool GetAttProp(DependencyObject obj) => (bool)obj.GetValue(AttPropProperty);  
        public static void SetAttProp(DependencyObject obj, bool par) => obj.SetValue(AttPropProperty, par);  
        private static void OnPropChanged(DependencyObject depObjet, DependencyPropertyChangedEventArgs e)  
        {  
            Window WindowRef = depObjet as Window;  
            if (WindowRef == null) return;  
            WindowRef.Loaded += (s, e) => ((AddContactViewModel)(WindowRef.DataContext)).WindowRef = WindowRef;  
        }  
        #endregion Attached Property  
    }  
}  
  
*****  
  
namespace cManagement.Models  
{  
    public class ContactModel  
    {  
        public int Id { get; set; }  
        public string Firstname { get; set; }  
        public string Lastname { get; set; }  
    }  
}  
  
*****  
  
namespace cManagement.Models  
{  
    public class ContactRef  
    {  
        public static ContactModel Contact { get; set; }   
    }  
}  
  
*****  
  
using System;  
using System.Windows.Input;  
  
namespace cManagement.Commands  
{  
    public class DelegateCommand : ICommand  
    {  
        readonly Action<object> execute;  
  
        readonly Predicate<object> canExecute;  
  
        public DelegateCommand(Predicate<object> canExecute, Action<object> execute) =>  
            (this.canExecute, this.execute) = (canExecute, execute);  
  
        public DelegateCommand(Action<object> execute) : this(null, execute) { }  
  
        public void onCanExecuteChanged() => this.CanExecuteChanged?.Invoke(this, EventArgs.Empty);  
  
        public event EventHandler CanExecuteChanged;  
  
        public bool CanExecute(object parameter) => this.canExecute?.Invoke(parameter)?? true;   
  
        public void Execute(object parameter) => this.execute?.Invoke(parameter);  
    }  
}  

Answer 2

Hi Sarah,
I didn't unterstand your algorithm:

a) MainWindow:

• in MainWindow you use instance of ContactListingViewModel as DataContext
• ListView use property View with data items
• in ContactListingViewModel is no property View

b) AddContactView

• AddContactView window use instance of AddContactViewModel as DataContext (programmatically)
• TextBoxes use as DataContext Contact property from AddContactViewModel (from StackPanel)
• Text properties of TextBoxes are binded to properties Firstname and Lastname in Contact object
• IDataErrorInfo in AddContactViewModel check properties Firstname and Lastname of static Contact object (via ContacRef)
• static Contact is not loaded when start edit
• because Contact has no IDataErrorInfo TextBoxes cannot show errors detected via IDataErrorInfo
• because you not loaded contact object when startet edit you cannot save values (contact object)

Answer 3

Hi Sarah,
see my answer on Mar 09 2022 at 2:28 PM to your question "How to use the MVVM Pattern correctly?".

If the response is helpful, please click "Accept Answer" and upvote it. I can't tell if my answer is helpful.