Inheritance And Events

Considering that C# was developed as an object-oriented language from the ground up, you would expect that events would be inheritable just like properties, fields, and methods. Unfortunately this is not the case. Remember, the C# language is compiled into intermediate language to run on the .NET Runtime, and this Runtime proceeded C# (it is also used to compile Visual Basic), and the way events are implemented in intermediate language does not lend itself to inheritance patterns.

This has some important implications for writing C# events:

  • You cannot invoke events defined in a base class in a derived class
  • The virtual and override keywords used with events do not actually create an overridden event - you instead end up with two separate implementations.

The standard way programmers have adopted to this issue is to:

  1. Define the event normally in the base class
  2. Add a protected helper method to that base class that will invoke the event, taking whatever parameters are needed
  3. Calling that helper method from derived classes.

For example, the PropertyChanged event we discussed previously is often invoked from a helper method named OnPropertyChanged() that is defined like this:

protected virtual void OnPropertyChanged(string propertyName)
    this.PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));

In derived classes, you can indicate a property is changing by calling this event, and passing in the property name, i.e.:

private object _tag = null;

/// <summary>
/// An object to represent whatever you need 
/// </summary>
public object Tag {
  get => _obj;
    if(value != _obj) 
      _obj = value;

Note the call to OnPropertyChanged() - this will trigger the PropertyChanged event handler on the base class.


You might have noticed the use of nameof(this.Tag) in the example code above. The nameof expression returns the name of a property as a string. This approach is preferred over just writing the string, as it makes it less likely a typo will result in your code not working as expected.