System.TypeLoadException on Linux (Linux Mint 19)

Environment data

dotnet --info output:

SDK .NET Core (reflétant tous les global.json) :
 Version:   2.1.402
 Commit:    3599f217f4

Environnement d'exécution :
 OS Name:     linuxmint
 OS Version:  19
 OS Platform: Linux
 RID:         linuxmint.19-x64
 Base Path:   /usr/share/dotnet/sdk/2.1.402/

Host (useful for support):
  Version: 2.1.4
  Commit:  85255dde3e

.NET Core SDKs installed:
  2.1.402 [/usr/share/dotnet/sdk]

.NET Core runtimes installed:
  Microsoft.AspNetCore.All 2.1.4 [/usr/share/dotnet/shared/Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.App 2.1.4 [/usr/share/dotnet/shared/Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 2.1.4 [/usr/share/dotnet/shared/Microsoft.NETCore.App]

To install additional .NET Core runtimes or SDKs:
  https://aka.ms/dotnet-download

VS Code version: (I guess this is code --version?)

1.27.2
f46c4c469d6e6d8c46f268d1553c5dc4b475840f
x64

C# Extension version: 1.16.1

Steps to reproduce

• Launch VSCode in an Unity project (using Unity)
• Watch the console open and immediately crash the OmniSharp server

Expected behavior

I expect to have IntelliSense and everything that comes with the extension.

Actual behavior

As the server crashes, nothing is added to the standard VSCode.

Additional informations

Here is the full log, using trace logging level:

Starting OmniSharp server at 2018-9-16 21:40:01
    Target: /home/$USER/Sources/Unity/Space Shooter/Space Shooter.sln

OmniSharp server started with Mono 5.14.0.
    Path: /home/$USER/.vscode/extensions/ms-vscode.csharp-1.16.1/.omnisharp/1.32.5/omnisharp/OmniSharp.exe
    PID: 25648

System.TypeLoadException: Could not load type of field 'McMaster.Extensions.CommandLineUtils.CommandLineApplication:_validationErrorHandler' (36) due to: Could not load file or assembly 'System.ComponentModel.DataAnnotations, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' or one of its dependencies.
  at OmniSharp.Stdio.StdioCommandLineApplication..ctor () [0x00000] in <e2653a02ab1f423abb805f2353ec9507>:0 
  at OmniSharp.Stdio.Driver.Program+<>c__DisplayClass0_0.<Main>b__0 () [0x00006] in <9671bf457f144c419fb0cdee3ca10088>:0 
  at OmniSharp.HostHelpers.Start (System.Func`1[TResult] action) [0x0001c] in <48aa3fb56cea4bd5ab9bfdf1170dd033>:0 
[ERROR] Error: OmniSharp server load timed out. Use the 'omnisharp.projectLoadTimeout' setting to override the default delay (one minute).

I am using the latest Mono and .NET Core, and everything is up-to-date. I also tried reinstalling the .NET Core. I also tried moving the ~/.vscode folder to an other name and deleting the ~/.omnisharp folder too (as suggested in #2489). I tried looking into the other issues, but no solution worked for me.

[ostuchlik]

I experienced the same issue on Ubuntu 16.04. I had monoruntime package installed. For me, setting omnisharp.useGlobalMono to Never solved the issue.

[TimWillebrands]

For me installing the mono-complete package following this fixed the error for me.