ubuntu 安装 mono报错 E: Unable to correct problems, you have held broken packages.

系统：ubuntu16.04

由于mono版本较低，需要重新安装mono。当重新安装mono时，出现如下报错

The following packages have unmet dependencies:
 mono-complete : Depends: mono-runtime (= 6.12.0.122-0xamarin1+ubuntu1804b1) but it is not going to be installed
                 Depends: mono-runtime-sgen (= 6.12.0.122-0xamarin1+ubuntu1804b1) but it is not going to be installed
                 Depends: mono-llvm-support (= 6.12.0.122-0xamarin1+ubuntu1804b1) but it is not going to be installed
                 Depends: libmono-2.0-1 (= 6.12.0.122-0xamarin1+ubuntu1804b1) but it is not going to be installed
                 Depends: mono-utils (= 6.12.0.122-0xamarin1+ubuntu1804b1) but it is not going to be installed
                 Depends: mono-devel (= 6.12.0.122-0xamarin1+ubuntu1804b1) but it is not going to be installed
                 Depends: mono-mcs (= 6.12.0.122-0xamarin1+ubuntu1804b1) but it is not going to be installed
                 Depends: mono-roslyn (= 6.12.0.122-0xamarin1+ubuntu1804b1) but it is not going to be installed
                 Depends: mono-csharp-shell (= 6.12.0.122-0xamarin1+ubuntu1804b1) but it is not going to be installed
                 Depends: mono-4.0-gac (= 6.12.0.122-0xamarin1+ubuntu1804b1) but it is not going to be installed
                 Depends: mono-4.0-service (= 6.12.0.122-0xamarin1+ubuntu1804b1) but it is not going to be installed
                 Depends: monodoc-base (= 6.12.0.122-0xamarin1+ubuntu1804b1) but it is not going to be installed
                 Depends: monodoc-manual (= 6.12.0.122-0xamarin1+ubuntu1804b1) but it is not going to be installed
                 Depends: libmono-cil-dev (= 6.12.0.122-0xamarin1+ubuntu1804b1) but it is not going to be installed
                 Depends: referenceassemblies-pcl but it is not going to be installed
                 Depends: msbuild but it is not going to be installed
                 Depends: ca-certificates-mono (= 6.12.0.122-0xamarin1+ubuntu1804b1) but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

出现E: Unable to correct problems, you have held broken packages.时，大概率是镜像源较老，导致有些源无法访问。

为顺利安装mono，我们进行如下步骤。

1. 更新源

参考博客《解决E: Unable to correct problems, you have held broken packages.(亲测有效)》

cd /etc/apt
sudo cp sources.list sources.list.backend
sudo vim sources.list

在其中添加镜像源

#添加阿里源
deb http://mirrors.aliyun.com/ubuntu/ bionic main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ bionic-security main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ bionic-updates main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ bionic-proposed main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ bionic-backports main restricted universe multiverse

#中科大源
deb https://mirrors.ustc.edu.cn/ubuntu/ bionic main restricted universe multiverse
deb https://mirrors.ustc.edu.cn/ubuntu/ bionic-updates main restricted universe multiverse
deb https://mirrors.ustc.edu.cn/ubuntu/ bionic-backports main restricted universe multiverse
deb https://mirrors.ustc.edu.cn/ubuntu/ bionic-security main restricted universe multiverse
deb https://mirrors.ustc.edu.cn/ubuntu/ bionic-proposed main restricted universe multiverse
 
#163源
deb http://mirrors.163.com/ubuntu/ bionic main restricted universe multiverse
deb http://mirrors.163.com/ubuntu/ bionic-security main restricted universe multiverse
deb http://mirrors.163.com/ubuntu/ bionic-updates main restricted universe multiverse
deb http://mirrors.163.com/ubuntu/ bionic-proposed main restricted universe multiverse
deb http://mirrors.163.com/ubuntu/ bionic-backports main restricted universe multiverse
 
#清华源
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-updates main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-backports main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-security main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-proposed main restricted universe multiverse

保存，更新源

sudo apt-get update

2. 安装mono

参考博客《如何在 Ubuntu 20.04 上安装 Mono》

2.1. 安装必要软件包

sudo apt-get install dirmngr gnupg apt-transport-https ca-certificates software-properties-common

2.2. 导入源仓库的 GPG key

sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF

输出如下

gpg: key A6A19B38D3D831EF: public key "Xamarin Public Jenkins (auto-signing) " imported
gpg: Total number processed: 1
gpg:               imported: 1

2.3. 添加 Mono 源仓库到系统源

sudo apt-add-repository 'deb https://download.mono-project.com/repo/ubuntu stable-bionic main'

再次更新源

sudo apt-get update

2.4. 安装mono

sudo apt install mono-complete

安装过程较长，大概10个小时。