software for drawing my application forms design

This is called Rapid Application Development (RAD), see http://en.wikipedia.org/wiki/Rapid_application_development[^]. See also http://en.wikipedia.org/wiki/Very_Rapid_application_development[^].

First of all, Visual Studio already provides RAD feature for Forms development. It's not clear what do you expect from other software which is not available with Visual Studio. Perhaps, the cost of Visual Studio? Or the level of the skills required by Visual Studio?

It looks like what you want is something that could be called Rapid Application Prototyping; see http://en.wikipedia.org/wiki/Prototype[^].

I'm personally pretty skeptical about your idea. Here is why. If your customers are professional in software, they could more or less rapidly use Visual Studio, create a UI prototype in pure layout and order you to complete it with functionality. If you don't like this idea, it may mean that your clients are not so professional. But in fact the UI design requires no less professionalism then the coding of the functionality, but may be more. If somebody pretends it is not so, you may end up struggling putting professional code in non-professional UI design. Making a software design based on the set and layout of controls were attempted many times; from what I know, the results were quite sorrowful.

I would advise you re-think it all very thoroughly. UI designers and application developers should work in one highly collaborative team, UI design should be considered as a part of software development, not a prerequisite for it, such as functional requirements; the development should involve both aspect in multiple tight iterations.

Also, If you haven't done it so far, I would suggest you learn and analyze applicability of the following architectural patterns (http://en.wikipedia.org/wiki/Architectural_pattern_(computer_science)[^]):

MVVM — Model View View Model,
http://en.wikipedia.org/wiki/Model_View_ViewModel[^],

MVC — Model-View-Controller,
http://en.wikipedia.org/wiki/Model-view-controller[^]),

MVA — Model-View-Adapter,
http://en.wikipedia.org/wiki/Model–view–adapter[^],

MVP — Model-View-Presenter,
http://en.wikipedia.org/wiki/Model-view-presenter[^].

Pay attention for the motivation of those architectures. If you understand it, you would be able to create better design ideas.

—SA