Let's say that there is a solution that contains two projects (Project1 and Project2).

Project1 is set as a StartUp Project (its name is displayed in a bold font). I double-click some file in Project2 to open it. The file opens, but something else happens too - Project2 gets set as a StartUp Project.

I tried to find an option in configuration to change it, but I found none.

Can this feature (though it's more like a bug to me) be disabled?

+3  A: 

The way to select a startup project is described in Sara Ford's blog "Visual Studio Tip of the Day" (highly recommended). She has a post there about setting up StartUp projects. Essentially there are 2 ways, the easiest one being right-clicking on the desired project, and choosing "Set As StartUp Project". That prevents other projects from becoming the StartUp project, even if you click on one their files.

Lea Cohen
+2  A: 

Check your Visual Studio options for the following check box:
Projects and Solutions - Build and Run - For new solutions use the currently selected project as the startup project.

Uncheck that and see if the behavior changes.