How does addition assignment operator behave

Question

How does addition assignment operator behaves here -

btn.Click += delegate(object sender, EventArgs e)

Answer 1

It adds an event handler to the event Click. When Click event is raised all the handlers method added to it are called.

For example:

void BtnClickHandler1(object sender, EventArgs e)
{
    MessageBox.Show("BtnClickHandler1");
}

void BtnClickHandler2(object sender, EventArgs e)
{
    MessageBox.Show("BtnClickHandler2");
}

And you add these methods to Click event like this:

btn.Click += BtnClickHandler1
btn.Click += BtnClickHandler2

When button is clicked the methods will be called in the order you added them, so the message box will be:

BtnClickHandler1
BtnClickHandler2

If you want specific info about += operator, MSDN says:

The += operator is also used to specify a method that will be called in response to an event; such methods are called event handlers. The use of the += operator in this context is referred to as subscribing to an event.

For more info look at:

https://msdn.microsoft.com/en-us/library/edzehd2t%28v=vs.110%29.aspx

http://www.dotnetperls.com/event