Exclusion of evidence bars contradiction of credibility-testing answers but permits false-evidence charges; limited exceptions allow contradiction. Exclusion of evidence bars proving contradiction of a witness's answer when the question and response relate only to damaging the witness's character, though a false denial may lead to a charge of giving false evidence. Exceptions permit contradiction where a witness denies a prior conviction or where a question attacking impartiality is denied; contradiction is admissible when the denial negates a substantive fact in issue rather than merely affecting credit.