Void contract

A void contract cannot be enforced by law. Void contracts are different from voidable contracts, which are contracts that may be (but not necessarily will be) nullified. However, when a contract is being written and signed, there is no automatic mechanism available in every situation that can be utilized to detect the validity or enforceability of that contract. Practically, a contract can be declared to be void by a court of law. So the main question is that under what conditions a contract can be deemed as void?