If combobox = empty [Quick question]

Hi guys,

I'm quite new to VB. And I'm also new on this forum. I'm very sorry if this is the wrong place for this topic.

But here is my question.
I have made a combobox with for example, 1,2,3,4 as items.
Now I also have a button. I want a messagebox to show up when someone clicks on the button when he hasn't picked a choice within the combobox.

I hope you all understand me and I also hope you have the awnser for me.

Cheers,
Fred