IDirect3DDevice9Ex::TestCooperativeLevel

DX9 vtable[3] → HRESULT

IDirect3DDevice9Ex::TestCooperativeLevel is a method of the COM interface IDirect3DDevice9Ex in Direct3D 9. Its function pointer sits at vtable index 3 — the zero-based slot in that interface's virtual method table, counted from QueryInterface and including every entry inherited from a base interface. It takes no parameters and returns HRESULT.

COM methods are reached through that table rather than by an exported name, so the slot index is what a wrapper, an interface hook or a disassembly listing has to resolve against.