XAML TwoWay Binding

CS file

public class ViewModel : INotifyPropertyChanged
{
  public event PropertyChangedEventHandler PropertyChanged;

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

  private string _firstProperty;
  public string FirstProperty
  {
    get{ return _firstProperty; }
    set
    {
      if (_firstProperty == value)
        return
      else
        _firstProperty = value;
        OnPropertyChanged("FirstProperty");
        //propertyName in parentheses must be the same as property name
    }
  }
}

XAML file

<TextBox Text="{Binding Path=FirstName, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"/>