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

Question

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?

score 1 · Accepted Answer · answered Jun 03 '21 at 11:31

1

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 Jun 03 '21 at 11:31

Jason

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Where exactly in the code – eliHeist Jun 03 '21 at 13:12
"**in your constructor** add this line" – Jason Jun 03 '21 at 13:14
Cleared the error but code does nothing now – eliHeist Jun 03 '21 at 13:40
"Does nothing" is not a helpful description of the problem. Have you attempted to do any debugging? – Jason Jun 03 '21 at 13:43
It was not displaying the input string but now has worked, I put that line at the top in the constructor – eliHeist Jun 04 '21 at 08:00

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

1 Answers1