What is the difference between MinusEqual ( =) and PlusEqual (+=) as used in event declaration c#

Please I want to know the difference between these two. I often use this += for events like 

this.btnExport.Click += new System.EventHandler(this.btnExport_Click);

From time to time, I do come across sometimes I do come across some declarations like below.

this.cmbClient.SelectedIndexChanged -= new System.EventHandler(this.cmbClient_SelectedIndexChanged);

I try checking for what the differences are on msdn yet couldn't find a source. Please any help or clarification would be appreciated.

Jon Skeet
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Simply put, += subscribe a handler to the event, and -= unsubscribes a handler from the event. (If the specified handler isn't an existing subscriber, the attempt is ignored.)

Note that you can use significantly simpler syntax as of C# 2:

this.btnExport.Click += this.btnExport_Click;

This uses a method group conversion to convert this.btnExport_Click into an EventHandler.

How the event implements subscription and unsubscription is up to the implementation. Often it's just a matter of using Delegate.Combine and Delegate.Remove, but it doesn't have to be. Fundamentally, an event is a bit like a property - except instead of get and set functionality, it has add and remove; using += on an event calls the add part, and using -= calls the remove part.

See my article on delegates and events for more details.

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