why does methyl chloride gives methyl cyanide on treatment with KCN and methyl isocyanide on treatment with AgCN

Because KCN is predominantly has ionic characterand provides cyanide ions in the solution. Although both carbon and nitrogen atoms are in position to donate electron pairs, the attack takes place mainly through carbon atom and not through nitrogen atom since C-C bond is more stable than C-N bond. However, AgCN is mainly covalent in nature and nitrogen is free to donate electron pair and thus forms isocyanide as main product