Actually, I've checked some other .NET libraries and no any riskware was found inside of them. Also I've downloaded source code and recompiled it by self. Just after that antivirus software found nothing. But after some hours I've repeatedly checked the same library and again Kaspersky found "not-a-virus".
I undestand it is false detection, but the problem is some mail servers don't undestand that, so I have to pack program in the archive with a password in order to send it through the mail.
P.S.: stupid antivirus software :(