#region INotifyPropertyChanged event

///<summary>
///Occurs when a property value changes.
///</summary>
public event PropertyChangedEventHandler PropertyChanged;


/// <summary>
/// Raises the <see cref="PropertyChanged"/> event for
/// a given property.
/// </summary>
/// <param name="propertyName">The name of the changed property.</param>
protected void OnPropertyChanged(string propertyName)
{
  //validate the property name in debug builds
  VerifyProperty(propertyName);

  if (PropertyChanged != null)
  {
    PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
  }
}


/// <summary>
/// Verifies whether the current class provides a property with a given
/// name. This method is only invoked in debug builds, and results in
/// a runtime exception if the <see cref="OnPropertyChanged"/> method
/// is being invoked with an invalid property name. This may happen if
/// a property's name was changed but not the parameter of the property's
/// invocation of <see cref="OnPropertyChanged"/>.
/// </summary>
/// <param name="propertyName">The name of the changed property.</param>
[Conditional("DEBUG")]
private void VerifyProperty(string propertyName)
{
  Type type = this.GetType();

  //look for a *public* property with the specified name
  PropertyInfo pi = type.GetProperty(propertyName);
  if (pi == null)
  {
    //there is no matching property - notify the developer
    string msg = "OnPropertyChanged was invoked with invalid property name {0}: ";
    msg += "{0} is not a public property of {1}.";
    msg = String.Format(msg, propertyName, type.FullName);
    Debug.Fail(msg);
  }
}

#endregion

public class Class1 : INotifyPropertyChanged

#region UserId

/// <summary>
/// The numeric ID of the user. Defaults to null.
/// </summary>
private int? userId = null;


/// <summary>
/// The numeric ID of the user. Defaults to null.
/// </summary>
public int? UserId
{
  get { return userId; }
  set
  {
    //ignore if values are equal
    if (value == userId) return;

    userId = value;
    OnPropertyChanged("UserId");
  }
}

#endregion