Object reference not set to an instance of an object when running xamarin.forms app

Issue

I was following the xamarin 101 series on youtube and got an error in episode 6 This is how my MainPageViewModel.cs is

    using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.ComponentModel;
using System.Text;
using Xamarin.Forms;

namespace App2.ViewModels
{
    class MainPageViewModel : INotifyPropertyChanged
    {
        public MainPageViewModel()
        {
            EraseCommand  new Command(() >
            {
                TheNote  string.Empty;
            });

            SaveCommand  new Command(() >
            {
                AllNotes.Add(TheNote);
                TheNote  string.Empty;
            });
        }

        public ObservableCollection<string> AllNotes { get; set; }

        public event PropertyChangedEventHandler PropertyChanged;

        string theNote;

        public string TheNote
        {
            get > theNote;
            set
            {
                theNote  value;

                var args  new PropertyChangedEventArgs(nameof(TheNote));

                PropertyChanged?.Invoke(this, args);
            }
        }

        public Command SaveCommand { get; }
        public Command EraseCommand { get; }
    }
}

and this is the MainPage.xaml file

<?xml version"1.0" encoding"utf-8" ?>
<ContentPage xmlns"http://xamarin.com/schemas/2014/forms"
             xmlns:x"http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:local"clr-namespace:App2.ViewModels"
             x:Class"App2.MainPage">

    <ContentPage.BindingContext>
        <local:MainPageViewModel/>
    </ContentPage.BindingContext>

    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height"*"/>
            <RowDefinition Height"2*"/>
            <RowDefinition Height".5*"/>
            <RowDefinition Height"2*"/>
        </Grid.RowDefinitions>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width"*"/>
            <ColumnDefinition Width"*"/>
        </Grid.ColumnDefinitions>

        <Image Source"vscode.png" BackgroundColor"PowderBlue" Grid.Column"0" Grid.Row"0" Grid.ColumnSpan"2"/>

        <Editor Grid.Column"0" Grid.ColumnSpan"2" Grid.Row"1" Placeholder"Enter note here" Text"{Binding TheNote}"/>

        <Button Grid.Row"2" Grid.Column"0" Text"Save" Command"{Binding SaveCommand}" BackgroundColor"Green"/>

        <Button Grid.Row"2" Grid.Column"1" Text"Delete" Command"{Binding EraseCommand}" BackgroundColor"Red"/>

        <CollectionView ItemsSource"{Binding AllNotes}" Grid.Row"3" Grid.ColumnSpan"2">
            <CollectionView.ItemTemplate>
                <DataTemplate>
                    <StackLayout>
                        <Frame>
                            <Label Text"{Binding .}" FontSize"Title"/> 
                        </Frame>
                    </StackLayout>
                </DataTemplate>
            </CollectionView.ItemTemplate>
        </CollectionView>
    </Grid>

</ContentPage>

The EraseCommand works fine but the SaveCommand throws an error System.NullReferenceException: 'Object reference not set to an instance of an object.' on line 21 of MainPageViewModel.cs

In the tutorial everything works fine but i get an error why is that? or how do i fix it?

Solution

AllNotes is null because you have never initialized it, so it throws a NullRef exception when you try to add an item to it

AllNotes.Add(TheNote.ToString());

in your constructor add this line

AllNotes  new ObservableCollection<string>();

Answered By – Jason