I'm trying to set two startup projects in my solution, and I have three projects. So I tried to exclude the Main method in my third project that I do not set as a startup project, and somewhy when I run the solution, it tells me that I don't have a main method. But I don't need a main method, so why? I have two main methods in my startup projects...
Because a project of a certain type expects certain things, otherwise as a project it is invalid. If you configure the solution to not start that project that is all you need to do.
To set startup projects, in the solution explorer right click the solution node, and then 'Set startup projects', then select 'Multiple Startup Projects' and choose which ones you want.
If you have no need for a Main method in a project, but the project requires a Main method, then you probably have chosen the wrong project type. Typically project templates with the name Library in them are the ones that do not have a Main method, such as Class Library or WPF Custom Control Library.