Dialog as main Window?

Is it usual to use a Dialog as main Windows?

Yes, it is quite common.

So without registering any user class via RegisterClassEx?

A dialog is usually a predefined window class, so there usually no need for registering.

I'd also like to know main difference between a dialog and a windows and why use one the first instead of the second.

Well, two big differences would be that you cannot resize a dialog box and it has no minimize or maximize buttons (by default, but there are workarounds for this). Keep in mind the name, dialog box. In other words they are used for having a dialog with the user (receive input and displays messages to user). In a sense they are just like any other window, underneath CreateWindowxx, etc. is called, etc. However, they are somewhat predefined windows which can be made quickly and there are limitations to what you can do with them.

Also, a dialog uses a dialog procedure rather than a window procedure, which does some default processing for you, such as initializing some controls, etc.

Yes, an application can be dialog-based. There's even a Wizard for that if your'e using VisualStudio and MFC.

In VS2010, Create New Project > MFC Application. In "Application Type" select Dialog Based. Click through the rest of the Wizard, and you're off to the races.

Dialog-based applications are much simpler, architectually, than other designs such as Document/View. As such, simple things are much easier to "bang out" quickly, but the limitations of the design become apparent when you try to do more complex things. You could end up replicating much of the Doc/View architecture in your dialog-based app in order to build a production-quality Dialog-based application. In that case, did you really save yourself anything?

A dialog is a kind of window just as all of the various controls like buttons are really just windows. You can think of a dialog as being a kind of window with a lot of extra functionality to support the kinds of things that dialogs are used for.

There are two types of dialogs, modal which display and expect you to use them and then dismiss them, and non-modal which display but which do not capture and keep the input focus until they are dismissed. You can see these two types used in applications where a modal dialog is used to display an error or require the user to make some setting and a non-modal acts as a kind of tool box that stays displayed and when you need it, you click on it to do something and other times you are using some other window in the application.

Normally a dialog would not have a menu bar but would instead have all of its controls visible or easily accessible via tabs or some other type of presentation. Visual Studio and other IDEs have dialog designers to allow the placement of various controls along with wizards to allow the controls to be tied to classes and class members.

Which brings up a major difference between a dialog and a window. A window is kind of an empty page and to do things with the page requires more work. A dialog has tools that make the design easy however you are also constrained in large part by the toolbox.

If you have an application that is focused on basically allowing a user to specify certain settings and then do some task, a dialog works fairly well. If you have something that requires more complicated user interaction, an application window as the base from which all of your other dialogs and controls will be managed and manipulated will be more necessary.