I have question about using .Net 3.5 or 4.0 I know that .Net is more productive then Qt but i am afraid of problems when my client leave my product because he cant run e.g demo of my app with lack of appropriate framework version. I wish to ask experienced persons in that matter. Is that many users that doesn't have .net 3.5? It worth to make app in .Net 3.5 or better use Qt c++ because .net is not that much productive and more important is that every could run app coded in Qt ?
It's not a big issue not to have .Net framework on the machine because the software should be delivered to the customer in installation media with a setup program to install all pre-requisets needed for the application to run properly.
Actually, using native programming languages does not have much advantage in this either because there are usually some libraries to distribute with the application as well. So it's not the point to make you move away from .Net.
You should write the application in the most appropriate language/framework combination.
If you use .NET you can create a ClickOnce installer that will, as M.Sameer points out, install the required version of .NET for you. The user still needs to download the framework but you don't need to include it in your installation program. This could be the source of the confusion as your clients will get asked if they want to install it.
If you use something like Qt you'll have to explicitly include those libraries in your installer - this will result in a bigger installation file, but there won't be any "worrying" extra messages for your clients.
I'm assuming windows windows is the target platform so users will have at least the .NET framework deployed with their OS/Service Pack as outlined here (image is old, but the mail bag snip is more recent).
It's often accepted, and IME it's consistent, that productivity is much higher in a higher level language (ie. C# vs. C++ in this instance).