Events in WPF

Background: This is code lifted from an application i made for a company to automate a bunch of calculations and device selections based on a bunch of data regarding a building. I'll be using the surface area of the building as an example.

I've tried to implement the MVVM model as best i could (explanations on it are kinda vague) so we have:


A XAML page with say, a textfield and a button: (Note, this is from a page called Offertetool.XAML, see below)
	<TextBox Text="{Binding surface, UpdateSourceTrigger=PropertyChanged}"/>
	<Button Content="Calculate" Command="{Binding CalculateClick}"/>

A class that uses the data:
    public class Building
    {
        public double surface; //m^2
		public double twiceSurface;

		public void CalculateDoubleSurfaceArea()
		{
			twiceSurface = surface*2;
		}
	}


A class that does nothing but receive input from the UI and do preparations for processing, in this case ProjectHandler:

	internal class ProjectHandler : ObservableObject
	{
		Building thisBuilding = new Building();

		public string surface
        {
            get { return thisBuilding.surface.ToString(); }
            set
            {
                if (Double.TryParse(value, out double surfaceParse))
                {
                    thisBuilding.surface = surfaceParse;
                    OnPropertyChanged("oppervlakte");
                }
            }
        }

        public ICommand CalculateClick
        {
            get { return new DelegateCommand(CalculateStuff); }
        }

		public void CalculateStuff{
		thisBuilding.CalculateDoubleSurfaceArea();
		}

	}

The ProjectHandler class implements ObservableObject, this is where my copy and pasting came in. I've lifted this class straight from an MVVM tutorial, it basically sets a class up to be able to use the events called by the XAML.

public abstract class ObservableObject : INotifyPropertyChanged
    {
        public event PropertyChangedEventHandler PropertyChanged;

        protected void OnPropertyChanged(string propertyName)
        {
            if (PropertyChanged != null)
            {
                PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
            }
        }
    }

The button clicker event uses something called DelegateCommand, again, this is copy pasted:

    class DelegateCommand : ICommand
    {
        private readonly Action action;

        public DelegateCommand(Action _action)
        {
            action = _action;
        }

        public void Execute(object parameter)
        {
            action();
        }

        public bool CanExecute(object parameter) //Dit mogelijk nog aanpassen want nu is geen enkele knop ooit greyed out.
        {
            return true;
        }
        //#pragma warning disable 67
        public event EventHandler CanExecuteChanged;
        //#pragma warning restore 67
    }


In order for the above to work, we need to point Offertetool.XAML.cs to the right datacontext, so:
    public partial class Offertetool : Page
    {

        public Offertetool()
        {
            InitializeComponent();
            App thisApp = (App)Application.Current;
            this.DataContext = thisApp.projectHandler; //<-- in App.XAML.cs the instance of ProjectHandler is created.
        }

    }