How do I put the contents of a list in a single MessageBox?

Basically, I have a list with multiple items in it and I want a single message box to display them all.

The closest I have got is a message box for each item (using foreach).

What I want is the equivalent of:

MessageBox.Show ("List contains:"+

Foreach (string str in list) 
{ str + Environment.Newline + }

                )

But obviously this won't work! What is the correct way of doing this?