TextBox value not updating on Button click

Question

This is my XAML code:

<TextBox Grid.Row="2" Grid.Column="1" HorizontalAlignment="Left" VerticalAlignment="Center" Width="25" Margin="100,0,0,0" Height="25" Text="{Binding Quantity, UpdateSourceTrigger=PropertyChanged}"></TextBox>
<Button Grid.Row="2" Grid.Column="1" HorizontalAlignment="Center" VerticalAlignment="Center" Width="100" Height="25" Content="+" Command="{Binding AddCommand}"></Button>
<Button Grid.Row="2" Grid.Column="1" HorizontalAlignment="Right" Margin="10" VerticalAlignment="Center" Width="100" Height="25" Content="-" Command="{Binding SubtractCommand}"></Button>

This is my C# code:

public class ViewModel : ObservableObject, INotifyPropertyChanged
{
    private int quantity = 0;
    public int Quantity
    {
        get => quantity;
        set
        {
            if (quantity == value) return;
            quantity = value;
            this.RaisePropertyChangedEvent(nameof(Quantity));
        }
    }
    public ICommand AddCommand => new RelayCommand<string>(
        Add,
        x => true
    );
    public ICommand SubtractCommand => new RelayCommand<string>(
        Sub,
        x => true
    );
    private void Add(string obj)
    {
        quantity += 1;
        Debug.WriteLine(quantity);
    }
    private void Sub(string obj)
    {
        quantity -= 1;
        Debug.WriteLine(quantity);
    }
}

If you press the +/- buttons, the value of quantity changes. The output of Debug.Writeline is the correct result. The problem is that the text of the TextBox is not updated on Button click.

Answer 1

You are changing the value of the backing field which does not call RaisePropertyChangedEvent. Change quantity to Quantity:

private void Add(string obj)
{
    Quantity += 1;
    Debug.WriteLine(quantity);
}
private void Sub(string obj)
{
    Quantity -= 1;
    Debug.WriteLine(quantity);
}