package libmono-system-xml4.0-cil (not installed) failed to install/upgrade: dpkg-deb --control subprocess returned error exit status 2

trying to install gtk-sharp3 2.99.3-2 thriough Synaptic Package Manager.
(actually then also other more packages have failed to install)

ProblemType: Package
DistroRelease: Ubuntu 18.04
Package: libmono-system-xml4.0-cil (not installed)
ProcVersionSignature: Ubuntu 4.15.0-24.26-generic 4.15.18
Uname: Linux 4.15.0-24-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.2
Architecture: amd64
Date: Wed Jul 18 23:41:57 2018
ErrorMessage: dpkg-deb --control subprocess returned error exit status 2
InstallationDate: Installed on 2018-07-02 (16 days ago)
InstallationMedia: Xubuntu 18.04 LTS "Bionic Beaver" - Release amd64 (20180426)
Python3Details: /usr/bin/python3.6, Python 3.6.5, python3-minimal, 3.6.5-3ubuntu1
PythonDetails: /usr/bin/python2.7, Python 2.7.15rc1, python-minimal, 2.7.15~rc1-1
RelatedPackageVersions:
 dpkg 1.19.0.5ubuntu2
 apt 1.6.2
SourcePackage: mono
Title: package libmono-system-xml4.0-cil (not installed) failed to install/upgrade: dpkg-deb --control subprocess returned error exit status 2
UpgradeStatus: No upgrade log present (probably fresh install)