A null reference (Nothing in Visual Basic) if the event was handled and should not trigger a failure; otherwise, returns the localized failure message.
The binary rewriter calls this method to notify listeners about a contract failure. The ContractHelper.RaiseContractFailedEvent(System.Diagnostics.Contracts.ContractFailureKind, string, string, Exception) method does not perform the failure behavior (an assert or throw) itself. If the failure is handled by the listeners, the method returns null. If the failure is not handled by the listeners, a localized failure message is returned.