ViewModel.cs

public class SomeViewModel : INotifyPropertyChanged
{
  public event PropertyChangedEventHandler PropertyChanged;
  
  private string _message;
  public string Message
  {
    get { return _message; }
    set
    {
      if (_message == value) { return; }
      _message = value;
      PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(Message));
    }
  }
}