exam_prepare/ViewModels/MainWindowViewModel.cs

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

namespace examPrepare.ViewModels
{
  internal class MainWindowViewModel : INotifyPropertyChanged
  {

    public event PropertyChangedEventHandler PropertyChanged;
    protected void OnPropertyChanged(string propertyName)
    {
      PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }
  }
}