Error: OmniSharp fails to load

Environment data

dotnet --info output:

.NET Core SDK (reflecting any global.json):
 Version:   2.1.400
 Commit:    8642e60a0f

Runtime Environment:
 OS Name:     Mac OS X
 OS Version:  10.14
 OS Platform: Darwin
 RID:         osx-x64
 Base Path:   /usr/local/share/dotnet/sdk/2.1.400/

Host (useful for support):
  Version: 2.1.2
  Commit:  811c3ce6c0

.NET Core SDKs installed:
  2.1.3 [/usr/local/share/dotnet/sdk]
  2.1.4 [/usr/local/share/dotnet/sdk]
  2.1.200 [/usr/local/share/dotnet/sdk]
  2.1.300 [/usr/local/share/dotnet/sdk]
  2.1.301 [/usr/local/share/dotnet/sdk]
  2.1.400 [/usr/local/share/dotnet/sdk]

.NET Core runtimes installed:
  Microsoft.AspNetCore.All 2.1.0 [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.All 2.1.1 [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.All 2.1.2 [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.App 2.1.0 [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 2.1.1 [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 2.1.2 [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 2.0.4 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.0.5 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.0.7 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.1.0 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.1.1 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.1.2 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]

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

VS Code version: 1.29.1 C# Extension version: 1.17.1

Steps to reproduce

Open C# project

Expected behavior

OmniSharp server starts up normally.

Actual behavior

Omnisharp fails during startup

Starting OmniSharp server at 12/8/2018, 3:25:30 PM

OmniSharp server started with Mono 5.16.0.
    Path: /Users/alekseyusatov/.vscode/extensions/ms-vscode.csharp-1.17.1/.omnisharp/1.32.8/omnisharp/OmniSharp.exe
    PID: 45088

[info]: OmniSharp.Stdio.Host
        Starting OmniSharp on MacOS 10.14.1 (x64)
[info]: OmniSharp.Services.DotNetCliService
        DotNetPath set to dotnet
Failed to launch 'mono' with args, '--version'
[info]: OmniSharp.MSBuild.Discovery.MSBuildLocator
        Located 1 MSBuild instance(s)
            1: StandAlone 15.0 - "/Users/alekseyusatov/.vscode/extensions/ms-vscode.csharp-1.17.1/.omnisharp/1.32.8/omnisharp/msbuild/15.0/Bin"
[info]: OmniSharp.MSBuild.Discovery.MSBuildLocator
        MSBUILD_EXE_PATH environment variable set to '/Users/alekseyusatov/.vscode/extensions/ms-vscode.csharp-1.17.1/.omnisharp/1.32.8/omnisharp/msbuild/15.0/Bin/MSBuild.dll'
[info]: OmniSharp.MSBuild.Discovery.MSBuildLocator
        Registered MSBuild instance: StandAlone 15.0 - "/Users/alekseyusatov/.vscode/extensions/ms-vscode.csharp-1.17.1/.omnisharp/1.32.8/omnisharp/msbuild/15.0/Bin"
            MSBuildExtensionsPath = /Library/Frameworks/Mono.framework/Versions/5.16.0/lib/mono/xbuild
            BypassFrameworkInstallChecks = true
            CscToolPath = /Users/alekseyusatov/.vscode/extensions/ms-vscode.csharp-1.17.1/.omnisharp/1.32.8/omnisharp/msbuild/15.0/Bin/Roslyn
            CscToolExe = csc.exe
            MSBuildToolsPath = /Users/alekseyusatov/.vscode/extensions/ms-vscode.csharp-1.17.1/.omnisharp/1.32.8/omnisharp/msbuild/15.0/Bin
            TargetFrameworkRootPath = /Library/Frameworks/Mono.framework/Versions/5.16.0/lib/mono/xbuild-frameworks
System.TypeLoadException: Could not load type of field 'OmniSharp.MSBuild.ProjectManager:_queue' (11) due to: Could not load file or assembly 'System.Threading.Tasks.Dataflow, Version=4.5.24.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' or one of its dependencies.
  at (wrapper managed-to-native) System.RuntimeType.GetPropertiesByName_native(System.RuntimeType,intptr,System.Reflection.BindingFlags,bool)
  at System.RuntimeType.GetPropertiesByName (System.String name, System.Reflection.BindingFlags bindingAttr, System.Boolean icase, System.RuntimeType reflectedType) [0x0001b] in <98fac219bd4e453693d76fda7bd96ab0>:0 
  at System.RuntimeType.GetPropertyCandidates (System.String name, System.Reflection.BindingFlags bindingAttr, System.Type[] types, System.Boolean allowPrefixLookup) [0x00010] in <98fac219bd4e453693d76fda7bd96ab0>:0 
  at System.RuntimeType.GetProperties (System.Reflection.BindingFlags bindingAttr) [0x00000] in <98fac219bd4e453693d76fda7bd96ab0>:0 
  at System.Reflection.RuntimeReflectionExtensions.GetRuntimeProperties (System.Type type) [0x00006] in <98fac219bd4e453693d76fda7bd96ab0>:0 
  at System.Composition.TypedParts.Discovery.TypeInspector+<DiscoverPropertyExports>d__7.MoveNext () [0x00030] in <9aac21a4a3f24063b2b8b59143acddfc>:0 
  at System.Composition.TypedParts.Discovery.TypeInspector+<DiscoverExports>d__5.MoveNext () [0x000dc] in <9aac21a4a3f24063b2b8b59143acddfc>:0 
  at System.Composition.TypedParts.Discovery.TypeInspector.InspectTypeForPart (System.Reflection.TypeInfo type, System.Composition.TypedParts.Discovery.DiscoveredPart& part) [0x00061] in <9aac21a4a3f24063b2b8b59143acddfc>:0 
  at System.Composition.TypedParts.TypedPartExportDescriptorProvider..ctor (System.Collections.Generic.IEnumerable`1[T] types, System.Composition.Convention.AttributedModelProvider attributeContext) [0x00037] in <9aac21a4a3f24063b2b8b59143acddfc>:0 
  at System.Composition.Hosting.ContainerConfiguration.CreateContainer () [0x00042] in <9aac21a4a3f24063b2b8b59143acddfc>:0 
  at OmniSharp.CompositionHostBuilder.Build () [0x001f6] in <b0a7a1010bfe4b01af0da4ccbeac0c7a>:0 
  at OmniSharp.Stdio.Host..ctor (System.IO.TextReader input, OmniSharp.Stdio.Services.ISharedTextWriter writer, OmniSharp.IOmniSharpEnvironment environment, System.IServiceProvider serviceProvider, OmniSharp.CompositionHostBuilder compositionHostBuilder, Microsoft.Extensions.Logging.ILoggerFactory loggerFactory, System.Threading.CancellationTokenSource cancellationTokenSource) [0x00057] in <5e8e2ad26ef5465098aa876c7f625e58>:0 
  at OmniSharp.Stdio.Driver.Program+<>c__DisplayClass0_1.<Main>b__1 () [0x00138] in <7c70c184a3894fbab0d699bc9b4ceac6>:0 
  at OmniSharp.CommandLineApplication+<>c__DisplayClass11_0.<OnExecute>b__0 () [0x0000b] in <b0a7a1010bfe4b01af0da4ccbeac0c7a>:0 
  at McMaster.Extensions.CommandLineUtils.CommandLineApplication.Execute (System.String[] args) [0x00039] in <344a1e8cd6594b17b1e01f8df6ae8827>:0 
  at OmniSharp.CommandLineApplication.Execute (System.Collections.Generic.IEnumerable`1[T] args) [0x00042] in <b0a7a1010bfe4b01af0da4ccbeac0c7a>:0 
  at OmniSharp.Stdio.Driver.Program+<>c__DisplayClass0_0.<Main>b__0 () [0x00028] in <7c70c184a3894fbab0d699bc9b4ceac6>:0 
  at OmniSharp.HostHelpers.Start (System.Func`1[TResult] action) [0x0001c] in <b0a7a1010bfe4b01af0da4ccbeac0c7a>:0 

[djkiwe]

In my case the problem was solved by installing the msbuild package.

[tuscen]

Yes, useGlobalMono set to never solves the problem.

I don’t know what’s the exact problem with my global mono. I don’t use it myself for anything but I can run it using terminal. I’ve tried wiping it out and reinstall the latest one. Unfortunately, the problem hasn’t gone if I set useGlobalMono for auto or always.