Code Inspection: Class is never instantiated (non-private accessibility)
ReSharper determines if no objects of a class were created in a project — in fact, if no direct instances or usages of the class were found. If the class only contains static members (and/or constant fields), ReSharper suggests making the class static. In other cases ReSharper suggests making the class abstract, if possible.
For the solution-wide inspection to work, you need either or both of the following:
The simplified global usage checking is enabled — Show unused non-private type members when solution-wide analysis is off on the page of ReSharper options (Alt+R, O).
The solution-wide analysis is enabled — Enable solution-wide analysis on the page of ReSharper options (Alt+R, O).
Note that even if a symbol has no direct usages in your solution and ReSharper warns you about it, there could be cases where symbols are used indirectly — for example, via reflection — or they could just be designed as public API. In all those cases you would want to suppress the usage-checking inspection for the symbol, and there are several ways to do so:
The recommended way is to decorate the implicitly used symbols with code annotation attributes. There are two attributes for this purpose: [UsedImplicitly] and [PublicAPI], which are functionally similar, but let you and your teammates understand how the symbol is actually used.
You can also suppress usage-checking inspections with any custom attribute. To do so, mark the definition of that attribute with the [MeansImplicitUse] attribute.
And finally, you can suppress a specific usage-checking inspection as any other code inspection with a suppression comment or a suppression attribute.