C# – A project with an output type of class library cannot be started directly (2)

You get this error because the project for the class library is chosen as the startup project in Visual Studio. Right-click your web project and choose "Set as StartUp project".

The project you have downloaded compiles into a dll assembly and provide a set of classes with implemented functionality.

You should add to your solution a new project with Output Type of either Console Application or Windows Application (VS Add Project wizard will offer you different templates of Projects).

In the newly added project, you can implement logic to test your Class Library.

Output type of the project you can find and change by the following steps:

1. Right click on project in Solution Explorer -> Properties.

2. In opened tab with properties select Application and there will be ComboBox marked with Output Type label.