Question

What is exactly the += ( s, e ) in the code?

example:

this.currentOperation.Completed += ( s, e ) => this.CurrentOperationChanged();

Answer 1

This is the way to attach an event handler using Lambda expression.

For example:

button.Click += new EventHandler(delegate (Object s, EventArgs e) {
            //some code
        });

Can be rewritten using lambda as follows:

button.Click += (s,e) => {
            //some code
        };

One thing to note here. It is not necessary to write ‘s’ and ‘e’. You can use any two letters, e.g.

button.Click += (o,r) => {};

The first parameter would represent the object that fired the event and the second would hold data that can be used in the eventhandler.