绑定MVVM WPF后添加到Combobox

Question

Hi after i bind a combobox observablecollection , i want to add from a textbox to combobx. adn it give me this error :

System.NullReferenceException: 'Object reference not set to an instance of an object.'

WpfApp1.MainWindow.getModel(...) returned null.

Image of the ERROR

Model:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Collections.ObjectModel;

namespace WpfApp1
{
    public class Parts : Changed
    {
        public string name;

        public string Name
        {
            get { return name; }
            set
            {
                if (name != value)
                {
                    name = value;
                    RaisePropertyChanged("Name");
                }
            }            
        }
    }
}

viewModel:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.ComponentModel;
using System.Collections.ObjectModel;

namespace WpfApp1
{
    public class AddViewModel : Changed
    {

        private ObservableCollection<Parts> _persons;
        public string names;
        public AddViewModel()
        {
            Persons = new ObservableCollection<Parts>()
             {
                  new Parts{Name="Nirav"}
                 ,new Parts{Name="Kapil"}
                 ,new Parts{Name="Arvind"}
                 ,new Parts{Name="Rajan"}
             };

        }
        public ObservableCollection<Parts> Persons
        {
            get { return _persons; }
            set {
                if (_persons != value)
                {
                    _persons = value;
                    RaisePropertyChanged("Persons");
                }
            }
        }
        private Parts _sperson;

        public Parts SPerson
        {
            get { return _sperson; }
            set {
                if (_sperson != value)
                {
                    _sperson = value;
                    RaisePropertyChanged("SPerson");
                }
            }
        }

    }
}

MainWindow:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

namespace WpfApp1
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public AddViewModel addviewmodel;

        public MainWindow()
        {
            InitializeComponent();
            DataContext = new AddViewModel();
        }

        public AddViewModel getModel()
        {
            return addviewmodel;
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {

                getModel().Persons.Add(new Parts { Name = cmbtxt.Text});

        }

    }
}

XAML:

<Grid>
        <ComboBox ItemsSource="{Binding Persons}" SelectedItem="{Binding Path=SPersons,Mode=TwoWay}" HorizontalAlignment="Left" Margin="391,17,0,0" VerticalAlignment="Top" Width="314" Height="27">
            <ComboBox.ItemTemplate>
                <DataTemplate>
                    <StackPanel Orientation="Horizontal">
                        <TextBlock Text="{Binding Path=Name}" />
                    </StackPanel>
                </DataTemplate>
            </ComboBox.ItemTemplate>
        </ComboBox>
        <TextBox Name="cmbtxt" HorizontalAlignment="Left" Height="23" Margin="24,21,0,0" TextWrapping="Wrap" VerticalAlignment="Top" Width="172" />
        <Button Content="Add" HorizontalAlignment="Left" Margin="24,88,0,0" VerticalAlignment="Top" Width="156" Height="49" Click="Button_Click"/>
    </Grid>

Answer 1

You could make addviewmodel a private readonly field and initialize it immediately. Then you simply have to set the DataContext to the field in the constructor.

Also, getModel() isn't very C#/.NET friendly. Use a property if you need to expose the field:

public partial class MainWindow : Window
{
    private readonly AddViewModel addviewmodel = new AddViewModel();

    public MainWindow()
    {
        InitializeComponent();
        DataContext = addviewmodel;
    }

    public AddViewModel AddViewModel => addviewmodel;

    private void Button_Click(object sender, RoutedEventArgs e)
    {
        addviewmodel.Persons.Add(new Parts { Name = cmbtxt.Text });
    }

}

Using a property you can actually remove the field altogether:

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
        DataContext = AddViewModel;
    }

    public AddViewModel AddViewModel { get; } = new AddViewModel();

    private void Button_Click(object sender, RoutedEventArgs e)
    {
        AddViewModel.Persons.Add(new Parts { Name = cmbtxt.Text });
    }
}

Answer 2

In MainWindow, you never set a value to addviewmodel , hence it is null. You can fix it by changing your constructor:

public MainWindow()
        {
            InitializeComponent();
            addviewmodel = new AddViewModel()
            DataContext = addviewmodel ;
        }

Here is a posting about NullReferenceException in general: What is a NullReferenceException, and how do I fix it?