I am going to program a GUI under windows (will be about 10,000 line code with my estimates) and don't know C# or C++ (QT library) to choose for my needs. Please help me to choose.
If you are developing a Windows application I'd suggest using C# and the .Net Framework as they are the 'native' solution. Using QT is just going to add complexity as it's a cross-platform library and has additional concerns therein. I'd also suspect the support for the .Net Framework is likely to be greater and more widespread.
The overarching question would be, which programming language are you most familiar with. If there is a Windows compiler for it then use that!
Use .NET if your only target is Windows. It is much more productive and you have better IDE support for development of GUI. The disadvantage is that you need to have .NET framework installed on the machine your application will run on.
If you consider going cross platform, or you think you master Qt (and C++) more than .NET, then go on with Qt.
If you have to debate on using C# or C++ then the correct answer is probably C#. I would stay away from a low level language like C++ unless you absolutely have to as the amount of time required to develop/debug with it will be much greater. C# has a lot of GUI functionalilty that it harnesses from the .net framework. There isn't a lot you can't do with right out of the box as opposed to C++ which you'll have to hand code a lot of functionality.
Use C# its has superior GUI builder and is easy to program. Also you can use mono to run on other system which is built by novell and not some open source community ;).
Choose the GUI toolkit based on your requirements, then choose the language.
if QT fits the bill, then C++, if WinForms looks good, C#, if MFC (and the new feature pack) is good enough, C++ ....
Also, depending on your skill (and the skill of your team) of one language or the other, your decision might be easier.
What are the requirements? If the performance is not an issue then I would go with C#.
I'd prefer C#.. In some cases is easier to distribute (windows 7 comes with .NET). Microsoft showed that the future is in WPF, so why wait to adopt it
I think the question should be using c++, .net winforms or wpf.
You will never use c# lonely to build it, you will use the IDE.
If you need something visual I should use wpf, otherwise I would use .net winforms. You can mix them too.
Regards.