Error: Could not find a part of the path 'E:\sitesroot\0\bin\roslyn\csc.exe'.

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Overview

After publishing the web app to Azure Cloud Services from Visual Studio 2017, it would not run and returned a 500 error.  Turning off custom error messages I was getting this error: Could not find a part of the path 'E:\sitesroot\0\bin\roslyn\csc.exe'. In your solution explorer for your project, right click on the ‘References’ section for your WebRole and select ‘Manage NuGet Packages…’ full stack:

 [DirectoryNotFoundException: Could not find a part of the path 'E:\sitesroot\0\bin\roslyn\csc.exe'.]
   System.IO.__Error.WinIOError(Int32 errorCode, String maybeFullPath) +548
   System.IO.FileStream.Init(String path, FileMode mode, FileAccess access, Int32 rights, Boolean useRights, FileShare share, Int32 bufferSize, FileOptions options, SECURITY_ATTRIBUTES secAttrs, String msgPath, Boolean bFromProxy, Boolean useLongPath, Boolean checkHost) +1159
   System.IO.FileStream..ctor(String path, FileMode mode, FileAccess access, FileShare share) +133
   Microsoft.CodeDom.Providers.DotNetCompilerPlatform.Compiler.get_CompilerName() +127
   Microsoft.CodeDom.Providers.DotNetCompilerPlatform.Compiler.FromFileBatch(CompilerParameters options, String[] fileNames) +832
   Microsoft.CodeDom.Providers.DotNetCompilerPlatform.Compiler.CompileAssemblyFromFileBatch(CompilerParameters options, String[] fileNames) +189
   System.Web.Compilation.AssemblyBuilder.Compile() +2038
   System.Web.Compilation.BuildProvidersCompiler.PerformBuild() +325
   System.Web.Compilation.ApplicationBuildProvider.GetGlobalAsaxBuildResult(Boolean isPrecompiledApp) +406
   System.Web.Compilation.BuildManager.CompileGlobalAsax() +47
   System.Web.Compilation.BuildManager.EnsureTopLevelFilesCompiled() +366

[HttpException (0x80004005): Could not find a part of the path 'E:\sitesroot\0\bin\roslyn\csc.exe'.]
   System.Web.Compilation.BuildManager.ReportTopLevelCompilationException() +76
   System.Web.Compilation.BuildManager.EnsureTopLevelFilesCompiled() +642
   System.Web.Compilation.BuildManager.CallAppInitializeMethod() +34
   System.Web.Hosting.HostingEnvironment.Initialize(ApplicationManager appManager, IApplicationHost appHost, IConfigMapPathFactory configMapPathFactory, HostingEnvironmentParameters hostingParameters, PolicyLevel policyLevel, Exception appDomainCreationException) +927

[HttpException (0x80004005): Could not find a part of the path 'E:\sitesroot\0\bin\roslyn\csc.exe'.]
   System.Web.HttpRuntime.FirstRequestInit(HttpContext context) +534
   System.Web.HttpRuntime.EnsureFirstRequestInit(HttpContext context) +119
   System.Web.HttpRuntime.ProcessRequestNotificationPrivate(IIS7WorkerRequest wr, HttpContext context) +726

Cause

A bug in Microsoft.CodeDom.Providers.DotNetCompilerPlatform package 1.0.7

Fix

Upgrade package to 1.0.8

capture20180222162517321

You will notice Updates will have some updates available and select the Microsoft.CodeDom.Providers.DotNetCompilerPlatform ‘Latest stable 1.0.8’ and hit the Update button.

capture20180222160446837

Publish your application.

Now you may have to wait a bit as the role drains any existing connections, but eventually this will result in getting rid of this pesky error!