In reply to Have_A_Doubt:
Section 15.2 explains that these can be used a base classes. Note that you cannot extend an untyped mailbox as this feature is an oddity not supported in the rest of SystemVerilog. It was leftover from another language that did not have parameterization.
In reply to mitesh.patel:
I also say that just because your testebnch is not based on UVM, doesn’t you can’t introduce pieces of it. But your bigger problem is not being able to change your driver code. get() is not virtual for the reason explained above. The only solution is changing your driver code.