OmniSharp stops working after Mono is installed.

Environment data

.NET Command Line Tools (2.1.3)

Product Information:
 Version:            2.1.3
 Commit SHA-1 hash:  a0ca411ca5

Runtime Environment:
 OS Name:     gentoo
 OS Version:  
 OS Platform: Linux
 RID:         linux-x64
 Base Path:   /opt/dotnet_core/sdk/2.1.3/

Microsoft .NET Core Shared Framework Host

  Version  : 2.0.4
  Build    : 7f262f453d8c8479b9af91d34c013b3aa05bc1ff

VS Code version: 1.20.0-1516947444 C# Extension version: 1.13.1

Steps to reproduce

Install the .NET Core SDK without Mono and a fresh install of VSCode. Open up a C# project and verify that extension functionality works.

Install Mono alongside the dotnet tool. Try opening the same project in VSCode.

Install MSBuild for Mono since the OmniSharp warning suggests to do so. Try opening the same project in VSCode.

Expected behavior

Intellisense, CodeLens, etc. works in all three scenarios. After installing MSBuild, the warning that it is not installed goes away.

Actual behavior

Intellisense, CodeLens, etc. only work in the case where .NET Core is installed without Mono or Mono MSBuild. The warning that MSBuild is not installed remains after it is installed alongside Mono.

Starting OmniSharp server at 2/2/2018, 12:25:16 AM
    Target: /home/jaccarmac/src/src/jaccarmac.com/junkcode/free-monad-vs-typeclass

OmniSharp server started wth Mono
    Path: /home/jaccarmac/.vscode-insiders/extensions/ms-vscode.csharp-1.13.1/.omnisharp/omnisharp/OmniSharp.exe
    PID: 20413

[info]: OmniSharp.Stdio.Host
        Starting OmniSharp on gentoo 0.0 (x64)
[warn]: OmniSharp.MSBuild.Discovery.Providers.MonoInstanceProvider
        It looks like you have Mono 5.2.0 or greater installed but MSBuild could not be found.
Try installing MSBuild into Mono (e.g. 'sudo apt-get install msbuild') to enable better MSBuild support.
[info]: OmniSharp.MSBuild.Discovery.MSBuildLocator
        Located 1 MSBuild instance(s)
            1: StandAlone 15.0 - "/home/jaccarmac/.vscode-insiders/extensions/ms-vscode.csharp-1.13.1/.omnisharp/omnisharp/msbuild/15.0/Bin"
[info]: OmniSharp.MSBuild.Discovery.MSBuildLocator
        MSBUILD_EXE_PATH environment variable set to '/home/jaccarmac/.vscode-insiders/extensions/ms-vscode.csharp-1.13.1/.omnisharp/omnisharp/msbuild/15.0/Bin/MSBuild.dll'
[info]: OmniSharp.MSBuild.Discovery.MSBuildLocator
        Registered MSBuild instance: StandAlone 15.0 - "/home/jaccarmac/.vscode-insiders/extensions/ms-vscode.csharp-1.13.1/.omnisharp/omnisharp/msbuild/15.0/Bin"
            MSBuildExtensionsPath = /usr/lib/mono/xbuild
            CscToolPath = /home/jaccarmac/.vscode-insiders/extensions/ms-vscode.csharp-1.13.1/.omnisharp/omnisharp/msbuild/15.0/Bin/Roslyn
            CscToolExe = csc.exe
            MSBuildToolsPath = /home/jaccarmac/.vscode-insiders/extensions/ms-vscode.csharp-1.13.1/.omnisharp/omnisharp/msbuild/15.0/Bin
            TargetFrameworkRootPath = /usr/lib/mono/xbuild-frameworks
[info]: OmniSharp.Cake.CakeProjectSystem
        Detecting Cake files in '/home/jaccarmac/src/src/jaccarmac.com/junkcode/free-monad-vs-typeclass'.
[info]: OmniSharp.Cake.CakeProjectSystem
        Could not find any Cake files
[info]: OmniSharp.DotNet.DotNetProjectSystem
        Initializing in /home/jaccarmac/src/src/jaccarmac.com/junkcode/free-monad-vs-typeclass
[info]: OmniSharp.DotNet.DotNetProjectSystem
        Auto package restore: False
[info]: OmniSharp.DotNet.DotNetProjectSystem
        Update workspace context
[info]: OmniSharp.DotNet.DotNetProjectSystem
        Resolving projects references
[info]: OmniSharp.MSBuild.MSBuildProjectSystem
        No solution files found in '/home/jaccarmac/src/src/jaccarmac.com/junkcode/free-monad-vs-typeclass'
[info]: OmniSharp.MSBuild.MSBuildProjectSystem
        Loading project: /home/jaccarmac/src/src/jaccarmac.com/junkcode/free-monad-vs-typeclass/free-monad-vs-typeclass.csproj
[warn]: OmniSharp.MSBuild.MSBuildProjectSystem
        Failed to load project file '/home/jaccarmac/src/src/jaccarmac.com/junkcode/free-monad-vs-typeclass/free-monad-vs-typeclass.csproj'.
/home/jaccarmac/src/src/jaccarmac.com/junkcode/free-monad-vs-typeclass/free-monad-vs-typeclass.csproj(1,1)
Microsoft.Build.Exceptions.InvalidProjectFileException: The imported project "/usr/lib/mono/xbuild/15.0/Microsoft.Common.props" was not found. Confirm that the path in the <Import> declaration is correct, and that the file exists on disk.  /opt/dotnet_core/sdk/2.1.3/Sdks/Microsoft.NET.Sdk/Sdk/Sdk.props
  at Microsoft.Build.Shared.ProjectErrorUtilities.ThrowInvalidProject (System.String errorSubCategoryResourceName, Microsoft.Build.Shared.IElementLocation elementLocation, System.String resourceName, System.Object[] args) [0x00042] in <9ba305957e954df9b0ed46d97ba8f5be>:0 
  at Microsoft.Build.Shared.ProjectErrorUtilities.VerifyThrowInvalidProject[T1] (System.Boolean condition, System.String errorSubCategoryResourceName, Microsoft.Build.Shared.IElementLocation elementLocation, System.String resourceName, T1 arg0) [0x00003] in <9ba305957e954df9b0ed46d97ba8f5be>:0 
  at Microsoft.Build.Shared.ProjectErrorUtilities.ThrowInvalidProject[T1] (Microsoft.Build.Shared.IElementLocation elementLocation, System.String resourceName, T1 arg0) [0x00000] in <9ba305957e954df9b0ed46d97ba8f5be>:0 
  at Microsoft.Build.Evaluation.Evaluator`4[P,I,M,D].ExpandAndLoadImportsFromUnescapedImportExpression (System.String directoryOfImportingFile, Microsoft.Build.Construction.ProjectImportElement importElement, System.String unescapedExpression, System.Boolean throwOnFileNotExistsError, System.Collections.Generic.List`1[Microsoft.Build.Construction.ProjectRootElement]& imports) [0x00517] in <9ba305957e954df9b0ed46d97ba8f5be>:0 
  at Microsoft.Build.Evaluation.Evaluator`4[P,I,M,D].ExpandAndLoadImportsFromUnescapedImportExpressionConditioned (System.String directoryOfImportingFile, Microsoft.Build.Construction.ProjectImportElement importElement, System.Collections.Generic.List`1[Microsoft.Build.Construction.ProjectRootElement]& projects, System.Boolean throwOnFileNotExistsError) [0x001a6] in <9ba305957e954df9b0ed46d97ba8f5be>:0 
  at Microsoft.Build.Evaluation.Evaluator`4[P,I,M,D].ExpandAndLoadImports (System.String directoryOfImportingFile, Microsoft.Build.Construction.ProjectImportElement importElement) [0x00024] in <9ba305957e954df9b0ed46d97ba8f5be>:0 
  at Microsoft.Build.Evaluation.Evaluator`4[P,I,M,D].EvaluateImportElement (System.String directoryOfImportingFile, Microsoft.Build.Construction.ProjectImportElement importElement) [0x00000] in <9ba305957e954df9b0ed46d97ba8f5be>:0 
  at Microsoft.Build.Evaluation.Evaluator`4[P,I,M,D].PerformDepthFirstPass (Microsoft.Build.Construction.ProjectRootElement currentProjectOrImport) [0x001dd] in <9ba305957e954df9b0ed46d97ba8f5be>:0 
  at Microsoft.Build.Evaluation.Evaluator`4[P,I,M,D].EvaluateImportElement (System.String directoryOfImportingFile, Microsoft.Build.Construction.ProjectImportElement importElement) [0x0002d] in <9ba305957e954df9b0ed46d97ba8f5be>:0 
  at Microsoft.Build.Evaluation.Evaluator`4[P,I,M,D].PerformDepthFirstPass (Microsoft.Build.Construction.ProjectRootElement currentProjectOrImport) [0x000c8] in <9ba305957e954df9b0ed46d97ba8f5be>:0 
  at Microsoft.Build.Evaluation.Evaluator`4[P,I,M,D].Evaluate (Microsoft.Build.BackEnd.Logging.ILoggingService loggingService, Microsoft.Build.Framework.BuildEventContext buildEventContext) [0x000df] in <9ba305957e954df9b0ed46d97ba8f5be>:0 
  at Microsoft.Build.Evaluation.Evaluator`4[P,I,M,D].Evaluate (Microsoft.Build.Evaluation.IEvaluatorData`4[P,I,M,D] data, Microsoft.Build.Construction.ProjectRootElement root, Microsoft.Build.Evaluation.ProjectLoadSettings loadSettings, System.Int32 maxNodeCount, Microsoft.Build.Collections.PropertyDictionary`1[T] environmentProperties, Microsoft.Build.BackEnd.Logging.ILoggingService loggingService, Microsoft.Build.Evaluation.IItemFactory`2[S,T] itemFactory, Microsoft.Build.Evaluation.IToolsetProvider toolsetProvider, Microsoft.Build.Evaluation.ProjectRootElementCache projectRootElementCache, Microsoft.Build.Framework.BuildEventContext buildEventContext, Microsoft.Build.Execution.ProjectInstance projectInstanceIfAnyForDebuggerOnly, Microsoft.Build.BackEnd.SdkResolution sdkResolution) [0x00016] in <9ba305957e954df9b0ed46d97ba8f5be>:0 
  at Microsoft.Build.Evaluation.Project.Reevaluate (Microsoft.Build.BackEnd.Logging.ILoggingService loggingServiceForEvaluation, Microsoft.Build.Evaluation.ProjectLoadSettings loadSettings) [0x0004c] in <9ba305957e954df9b0ed46d97ba8f5be>:0 
  at Microsoft.Build.Evaluation.Project.ReevaluateIfNecessary (Microsoft.Build.BackEnd.Logging.ILoggingService loggingServiceForEvaluation, Microsoft.Build.Evaluation.ProjectLoadSettings loadSettings) [0x00034] in <9ba305957e954df9b0ed46d97ba8f5be>:0 
  at Microsoft.Build.Evaluation.Project.ReevaluateIfNecessary (Microsoft.Build.BackEnd.Logging.ILoggingService loggingServiceForEvaluation) [0x00000] in <9ba305957e954df9b0ed46d97ba8f5be>:0 
  at Microsoft.Build.Evaluation.Project.ReevaluateIfNecessary () [0x00007] in <9ba305957e954df9b0ed46d97ba8f5be>:0 
  at Microsoft.Build.Evaluation.Project.Initialize (System.Collections.Generic.IDictionary`2[TKey,TValue] globalProperties, System.String toolsVersion, System.String subToolsetVersion, Microsoft.Build.Evaluation.ProjectLoadSettings loadSettings) [0x000e9] in <9ba305957e954df9b0ed46d97ba8f5be>:0 
  at Microsoft.Build.Evaluation.Project..ctor (System.String projectFile, System.Collections.Generic.IDictionary`2[TKey,TValue] globalProperties, System.String toolsVersion, System.String subToolsetVersion, Microsoft.Build.Evaluation.ProjectCollection projectCollection, Microsoft.Build.Evaluation.ProjectLoadSettings loadSettings) [0x0009c] in <9ba305957e954df9b0ed46d97ba8f5be>:0 
  at Microsoft.Build.Evaluation.Project..ctor (System.String projectFile, System.Collections.Generic.IDictionary`2[TKey,TValue] globalProperties, System.String toolsVersion, Microsoft.Build.Evaluation.ProjectCollection projectCollection, Microsoft.Build.Evaluation.ProjectLoadSettings loadSettings) [0x00000] in <9ba305957e954df9b0ed46d97ba8f5be>:0 
  at Microsoft.Build.Evaluation.Project..ctor (System.String projectFile, System.Collections.Generic.IDictionary`2[TKey,TValue] globalProperties, System.String toolsVersion, Microsoft.Build.Evaluation.ProjectCollection projectCollection) [0x00000] in <9ba305957e954df9b0ed46d97ba8f5be>:0 
  at Microsoft.Build.Evaluation.ProjectCollection.LoadProject (System.String fileName, System.Collections.Generic.IDictionary`2[TKey,TValue] globalProperties, System.String toolsVersion) [0x000f5] in <9ba305957e954df9b0ed46d97ba8f5be>:0 
  at Microsoft.Build.Evaluation.ProjectCollection.LoadProject (System.String fileName, System.String toolsVersion) [0x00000] in <9ba305957e954df9b0ed46d97ba8f5be>:0 
  at OmniSharp.MSBuild.ProjectFile.ProjectFileInfo.LoadProject (System.String filePath, System.String solutionDirectory, Microsoft.Extensions.Logging.ILogger logger, OmniSharp.MSBuild.Discovery.MSBuildInstance msbuildInstance, OmniSharp.MSBuild.SdksPathResolver sdksPathResolver, OmniSharp.Options.MSBuildOptions options, System.Collections.Generic.ICollection`1[T] diagnostics, System.Collections.Immutable.ImmutableArray`1[System.String]& targetFrameworks) [0x00054] in <e904597b17e840d49e5c532b51773be0>:0 
  at OmniSharp.MSBuild.ProjectFile.ProjectFileInfo.Create (System.String filePath, System.String solutionDirectory, Microsoft.Extensions.Logging.ILogger logger, OmniSharp.MSBuild.Discovery.MSBuildInstance msbuildInstance, OmniSharp.MSBuild.SdksPathResolver sdksPathResolver, OmniSharp.Options.MSBuildOptions options, System.Collections.Generic.ICollection`1[T] diagnostics) [0x0000a] in <e904597b17e840d49e5c532b51773be0>:0 
  at OmniSharp.MSBuild.MSBuildProjectSystem.LoadProject (System.String projectFilePath) [0x00038] in <e904597b17e840d49e5c532b51773be0>:0 

[info]: OmniSharp.Script.ScriptProjectSystem
        Detecting CSX files in '/home/jaccarmac/src/src/jaccarmac.com/junkcode/free-monad-vs-typeclass'.
[info]: OmniSharp.Script.ScriptProjectSystem
        Could not find any CSX files
[info]: OmniSharp.Stdio.Host
        Invoking Workspace Options Provider: OmniSharp.Roslyn.CSharp.Services.CSharpWorkspaceOptionsProvider
[info]: OmniSharp.Stdio.Host
        Configuration finished.
[info]: OmniSharp.Stdio.Host
        Omnisharp server running using Stdio at location '/home/jaccarmac/src/src/jaccarmac.com/junkcode/free-monad-vs-typeclass' on host 18205.

[sekseason]

I’m on arch solved this issue by uninstall “msbuild-bin” and install “msbuild-stable” (15.4_xamarinxplat.2017.11.27.11.23_0xamarin1_ubuntu1404b1-3) instead.

[akshita31]

Could you try setting “omnisharp.useGlobalMono”: never and then restarting the omnisharp server.