我试过Install Older Version of Mono 但它没有帮助 这是我跑的
sudo apt-get purge mono-devel
echo "deb http://download.mono-project.com/repo/debian wheezy/snapshots/4.2.4.4 main" | sudo tee /etc/apt/sources.list.d/mono-xamarin.list
sudo apt-get update
sudo apt-get install -f mono-devel=4.2.4.4-0wheezy1
但是当我运行最后一个命令时，我得到了：
Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created or
been moved out of Incoming.
The following information may help to resolve the situation:
The following packages have unmet dependencies:
mono-devel : Depends: libmono-cecil-private-cil (< 4.2.4.5) but 5.0.0.100-0xamarin4+raspbian8b1 is to be installed
Depends: mono-mcs (= 4.2.4.4-0wheezy1) but 5.0.0.100-0xamarin4+raspbian8b1 is to be installed
Depends: mono-gac (= 4.2.4.4-0wheezy1) but 5.0.0.100-0xamarin4+raspbian8b1 is to be installed
Depends: mono-xbuild (= 4.2.4.4-0wheezy1) but 5.0.0.100-0xamarin4+raspbian8b1 is to be installed
Depends: libmono-cil-dev (= 4.2.4.4-0wheezy1) but 5.0.0.100-0xamarin4+raspbian8b1 is to be installed
Depends: libmono-2.0-dev (< 4.2.4.4-0wheezy1.1~) but 5.0.0.100-0xamarin4+raspbian8b1 is to be installed
Recommends: mono-csharp-shell but it is not going to be installed
E: Unable to correct problems, you have held broken packages.
我认为它与我有关，然后我运行sudo apt-update我得到以下作为最后两行：
W: GPG error: http://debian.yeasoft.net wheezy InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40FC0CD26BF18B15
W: Conflicting distribution: http://download.mono-project.com wheezy/snapshots/4.2.4.4 InRelease (expected wheezy/snapshots but got wheezy)