我正在按照以下官方链接安装 Mono：https: //www.mono-project.com/download/stable/#download-lin-ubuntu
成功运行：
sudo apt install gnupg ca-certificates
然后在执行时收到这个：
sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF
[sudo] password for rduis:
Executing: /tmp/apt-key-gpghome.pxKRgkz7VK/gpg.1.sh --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF
gpg: key A6A19B38D3D831EF: 2 signatures not checked due to missing keys
gpg: key A6A19B38D3D831EF: "Xamarin Public Jenkins (auto-signing) <releng@xamarin.com>" not changed
gpg: Total number processed: 1
gpg: unchanged: 1
然后这个：
echo "deb https://download.mono-project.com/repo/ubuntu stable-bionic main" | sudo tee /etc/apt/sources.list.d/mono-official-stable.list
deb https://download.mono-project.com/repo/ubuntu stable-bionic main
下一个：
sudo apt update
Ign:1 https://download.mono-project.com/repo/ubuntu stable-bionic InRelease
Hit:2 http://us.archive.ubuntu.com/ubuntu bionic InRelease
Get:3 http://security.ubuntu.com/ubuntu bionic-security InRelease [88.7 kB]
Ign:4 https://download.mono-project.com/repo/ubuntu vs-bionic InRelease
Err:5 https://download.mono-project.com/repo/ubuntu stable-bionic Release
Certificate verification failed: The certificate is NOT trusted. The certificate issuer is unknown. Could not handshake: Error in the certificate verification. [IP: 152.xxx.xx.xxx xxx]
Get:6 http://us.archive.ubuntu.com/ubuntu bionic-updates InRelease [88.7 kB]
Err:7 https://download.mono-project.com/repo/ubuntu vs-bionic Release
Certificate verification failed: The certificate is NOT trusted. The certificate issuer is unknown. Could not handshake: Error in the certificate verification. [IP: 152.xxx.xx.xxx xxx]
Get:8 http://security.ubuntu.com/ubuntu bionic-security/main amd64 DEP-11 Metadata [49.0 kB]
Get:9 http://us.archive.ubuntu.com/ubuntu bionic-backports InRelease [74.6 kB]
Get:10 http://security.ubuntu.com/ubuntu bionic-security/universe amd64 DEP-11 Metadata [60.4 kB]
Get:11 http://security.ubuntu.com/ubuntu bionic-security/multiverse amd64 DEP-11 Metadata [2,464 B]
Get:12 http://us.archive.ubuntu.com/ubuntu bionic-updates/main amd64 DEP-11 Metadata [295 kB]
Get:13 http://us.archive.ubuntu.com/ubuntu bionic-updates/universe amd64 DEP-11 Metadata [290 kB]
Get:14 http://us.archive.ubuntu.com/ubuntu bionic-updates/multiverse amd64 DEP-11 Metadata [2,468 B]
Get:15 http://us.archive.ubuntu.com/ubuntu bionic-backports/universe amd64 DEP-11 Metadata [9,292 B]
Reading package lists... Done
E: The repository 'https://download.mono-project.com/repo/ubuntu stable-bionic Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: The repository 'https://download.mono-project.com/repo/ubuntu vs-bionic Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
尽管出现上述错误，继续安装单声道如下：
sudo apt install mono-devel
在经过验证的 mono 安装版本之后：
mono --version
Mono JIT compiler version 4.6.2 (Debian 4.6.2.7+dfsg-1ubuntu1)
Copyright (C) 2002-2014 Novell, Inc, Xamarin Inc and Contributors. www.mono-project.com
TLS: __thread
SIGSEGV: altstack
Notifications: epoll
Architecture: amd64
Disabled: none
Misc: softdebug
LLVM: supported, not enabled.
GC: sgen
但是，对于 Ubuntu 仿生，mono 的最新版本是6.12.0.122-0xamarin1+ubuntu1804b1。我只能获得旧版本的单声道。我的开发需要最高可用版本的单声道。新手到单声道。任何帮助深表感谢。