Thanks for your help. But I have still a problem.
I have problems with Arduino libraries. Sometimes the project is created as follows (Project OK). 

i:\Users\Kucky\Desktop\ProjectOK.png
At the next attempt, it looks different. What am I doing wrong.
i:\Users\Kucky\Desktop\ProjectnotOK.png
Perhaps there is a step by step method for beginners?

Kucky

I have split these questions into a new thread because they did not relate to the cpp thread. Hope that is okay.

1)
Arduino libraries are simply #includes at the top of the main sketch code.

2)
Visual Studio has an additional capability that allows library developers to develop the library source codes. Clicking the menu item called "Project>Show all Arduino files" is for library developers and not for new users. When this menu item is clicked "_Core" and "_Libraries" folders will be displayed in the "Solution Explorer". Click the menu item again to remove these extra folders.

Summary

It sounds like you clicked this menu item ("Project>Show all Arduino files") in one of your projects. Click it again to remove "_Core" and "_Libraries" from the solution. You do not need to see these sources they are confusing you.

After adding a library, to confirm that libraries have been added to your project you need only look at the main sketch (pde/ino) to see if #includes have been added for the libraries you have added. 

Hi kucky,

That doesn't sound right. The arduino ide will also fail to compile that.

Arduino libraries have specific rules. A library consists of one master .h and a series of other cpp/h files. The folder that the library is contained in has to have the same name as the master .h

So when you expend libraries that you find on the internet you must ensure the correct folder name is created/persevered

Does this make sense?

The plugin automatically updates the c++ paths you do not need to change them if you are using things correctly.

If you have changed any library locations and not re-started visual studio then you need to click the "Project>Add Arduino Library>Refresh" option so that the plugin knows about the new locations.

If you still have problems then you should remove all library #includes from your sketch and use the "Project>Add Arduino Library" menu to let the plugin add and handle the libraries for you.

With arduino (and also with the plugin) you do not have to specify the location of libraries anywhere in the code or project.