How might one go about creating a Modeless MessageBox? Do I have to just create my own Windows Form class and use that? If so, is there an easy way of adding a warning icon (rather than inserting my own image of one) and resizing based on text volume?
You'll have to create a Form and use Show() to display it Modeless. MessageBox.Show(...) behaved Modal as seen in the example by ghiboz; "Description of the message" is displayed until the user presses a button.
With MessageBox.Show(...) you get the result as soon as the messagebox is closed; with a modeless message box, your code will have to have a mechanism such as an event to react when the user eventually selects something on your message box.