title: The "CheckFileSystemCaseSensitive" task failed unexpectedly, Could not load file or assembly 'System.IO.FileSystem
body: <p>To my Visual Studio 2017, ASP.NET Core, MVC web project I just added the Microsoft.TypeScript.MSBuild NuGet package v2.3.1, and simultaneously updated my ASP.NET Core assemblies from 1.0.* to 1.1.1.  Now suddenly when I build my project I get the following exception:</p>
  
  <pre><code>Severity    Code    Description Project File    Line    Suppression State
  Error   MSB4018 The "CheckFileSystemCaseSensitive" task failed unexpectedly.
  System.IO.FileNotFoundException: Could not load file or assembly 'System.IO.FileSystem, Version=4.0.1.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' or one of its dependencies. The system cannot find the file specified.
  File name: 'System.IO.FileSystem, Version=4.0.1.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'
     at TypeScript.Tasks.CheckFileSystemCaseSensitive.Execute()
     at Microsoft.Build.BackEnd.TaskExecutionHost.Microsoft.Build.BackEnd.ITaskExecutionHost.Execute()
     at Microsoft.Build.BackEnd.TaskBuilder.&lt;ExecuteInstantiatedTask&gt;d__26.MoveNext()
  </code></pre>
  
  <p>Double-clicking on the exception message takes one to these lines in the file Microsoft.TypeScript.targets:</p>
  
  <pre><code>&lt;Target Name="CheckFileSystemCaseSensitive"&gt;
      &lt;CheckFileSystemCaseSensitive
         MSBuildThisFileFullPath="$(MSBuildThisFileFullPath)" &gt;
        &lt;Output TaskParameter="IsFileSystemCaseSensitive"  PropertyName="IsFileSystemCaseSensitive" /&gt;
      &lt;/CheckFileSystemCaseSensitive&gt;
  &lt;/Target&gt;
  </code></pre>
  
  <p>where it says that "MSBuildThisFileFullPath is not defined".</p>
  
  <p>I've tried adding references to <code>NETStandard.Library</code>, as suggested here:  <a href="https://github.com/dotnet/corefx/issues/16206" rel="noreferrer">https://github.com/dotnet/corefx/issues/16206</a>, but that doesn't make any difference.</p>
  
  <p>The diagnostic MSBuild output looks like this:</p>
  
  <pre><code>1&gt;Target "CheckFileSystemCaseSensitive" in file "C:\Users\dkent\.nuget\packages\microsoft.typescript.msbuild\2.3.1\tools\netstandard1.3\Microsoft.TypeScript.targets":
  1&gt;  Using "CheckFileSystemCaseSensitive" task from assembly "C:\Users\dkent\.nuget\packages\microsoft.typescript.msbuild\2.3.1\tools\netstandard1.3\TypeScript.Tasks.dll".
  1&gt;  Task "CheckFileSystemCaseSensitive"
  1&gt;    Task Parameter:MSBuildThisFileFullPath=C:\Users\dkent\.nuget\packages\microsoft.typescript.msbuild\2.3.1\tools\netstandard1.3\Microsoft.TypeScript.targets
  1&gt;    [...]\.nuget\packages\microsoft.typescript.msbuild\2.3.1\tools\netstandard1.3\Microsoft.TypeScript.targets(172,5): error MSB4018: The "CheckFileSystemCaseSensitive" task failed unexpectedly.
  1&gt;    [...]\.nuget\packages\microsoft.typescript.msbuild\2.3.1\tools\netstandard1.3\Microsoft.TypeScript.targets(172,5): error MSB4018: System.IO.FileNotFoundException: Could not load file or assembly 'System.IO.FileSystem, Version=4.0.1.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' or one of its dependencies. The system cannot find the file specified.
  1&gt;    [...]\.nuget\packages\microsoft.typescript.msbuild\2.3.1\tools\netstandard1.3\Microsoft.TypeScript.targets(172,5): error MSB4018: File name: 'System.IO.FileSystem, Version=4.0.1.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'
  1&gt;    [...]\.nuget\packages\microsoft.typescript.msbuild\2.3.1\tools\netstandard1.3\Microsoft.TypeScript.targets(172,5): error MSB4018:    at TypeScript.Tasks.CheckFileSystemCaseSensitive.Execute()
  1&gt;    [...]\.nuget\packages\microsoft.typescript.msbuild\2.3.1\tools\netstandard1.3\Microsoft.TypeScript.targets(172,5): error MSB4018:    at Microsoft.Build.BackEnd.TaskExecutionHost.Microsoft.Build.BackEnd.ITaskExecutionHost.Execute()
  1&gt;    [...]\.nuget\packages\microsoft.typescript.msbuild\2.3.1\tools\netstandard1.3\Microsoft.TypeScript.targets(172,5): error MSB4018:    at Microsoft.Build.BackEnd.TaskBuilder.&lt;ExecuteInstantiatedTask&gt;d__26.MoveNext()
  1&gt;  Done executing task "CheckFileSystemCaseSensitive" -- FAILED.
  </code></pre>
  
  <p>Thanks for any suggestions.</p>