Methylcyclopropane is dichlorinated on the ring or the side chain. How many dichloro structural isomers of $\mathrm{C}{}_4\mathrm{H}{}_6\mathrm{Cl}{}_2$ are possible (stereoisomers not counted)?

Side chain only: 1,1 on the methyl (dichloromethylcyclopropane) ; one Cl on the side chain and one on the ring at C1 (1-(chloromethyl)-1-chlorocyclopropane), at C2 (1-chloro-2-(chloromethyl)cyclopropane). Ring only: 1,1-dichloro-2-methyl ; 1,2-dichloro-1-methyl (Cl on the methyl-bearing carbon and a neighbour) ; 1,2-dichloro-3-methyl. Total 6. Verified by substituting every H pair by Cl and de-duplicating.