The currently selected startup project is a Class Library.
You need an entry point for your application. Does your solution contain multiple projects?
Are you sure you want to 'run' the Class Library?
Possibly you've downloaded a Class Library solution only. In that case there might be unit tests that you can 'debug' to step-trough the code.
Update
In response to your comment, these are possibilities to run the class library:
- Create a console application and reference the class library. Then 'use' its contents
- Debug an unit-test project
- start existing client project
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