How to reference an event in C#

Unfortunately there isn't really a way of doing this. Events aren't first class citizens in .NET in general - although F# tries to promote them there.

Either pass in the subscribe/unsubscribe delegate or using a string indicating the name of the event. (The latter is often shorter, but obviously less safe at compile-time.)

Those are the approaches which Reactive Extensions takes - if there were a cleaner way of doing it, I'm sure they would be using that :(

You can create a custom Accessor.

   public event EventHandler NewEvent
            {
                add { Dimension.LengthChanged += value; }
                remove { Dimension.LengthChanged -= value; }
            }

Please See https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/events/how-to-implement-custom-event-accessors

Event is not supposed to be passed into another method. However, you can pass delegate into another method. Perhaps, what you are looking for are just a simple public delegate instead of event.

If you change your event to this

public System.EventHandler LengthChanged; 

You can simply pass the LengthChanged to Observer like this

Foo foo = ...;
Dimension dim = ...;
foo.Observer (dim.LengthChanged);