Even though C# and others are listed as a possible language, all of the samples I have found show how to create a task pane AddIn using HTML5/JavaScript only.
I've already gone through all of these:
and others, and they are all HTML5/JavaScript! Even the MSDN samples are HTML5/JavaScript only!
How can I have an ASP.NET Web Application talk to Office 365 so it can provide a task pane control and edit the selected text?
Finally, I've made it! It is not difficult. Firstly you have to use Visual Studio. When you develop directly from browser, download and open that project in Visual Studio. You can follow this way to make very easy example