How to ensure that a method can only be called from a specific dll

I need to prepare my database for unit testing and hence in the set up method I want to get rid of the data.

However, I don't want to be anybody else but the unit test dll to be able to call into my DeleteAllData() method in my Data Access Layer. What can be done?

While others already suggested how to restrict code making calls to your method, they do not create a security boundary and can be spoofed (unless you sign both assemblies and verify the evidence).

Having a method in your business logic called DeleteAllData sounds pretty scary to me. If this method is only used from a unit test, I would probably move it to the unit test assembly. If that is not possible, I would at least put the code in a compiler directive to only compile in debug mode.