Answer by Henke - Нава́льныйПсм for Could not find a part of the path ... bin\roslyn\csc.exe

0. The quick fix

As already noted in the currently highest voted answer,the quick fix is to use the package manager, Tools> Nuget PackageManager> Package Manager Console, to run

Update-Package Microsoft.CodeDom.Providers.DotNetCompilerPlatform -r

1. Code to reproduce the error

Here is code that reproduces the error:
https://user.it.uu.se/%7Ehesc0353/SrvrErr-reproduce.zip
(Originally fromhttps://github.com/aspnet/AspNetDocs/tree/master/aspnet/web-api/overview/advanced/calling-a-web-api-from-a-net-client/sample/server/ProductsApp)

Consider trying the example code provided in the zip file above.
If no changes are made, Ctrl+F5 will reproducethe error.

2. A more robust solution

An alternative solution is to remove an attribute from the project'sWeb.config file.
(Web.config is in the same directory as the .csproj file.)
This will automatically and silently recreate your packages if theyare missing.

Open the Web.config file in a text editor or in Visual Studio.

<?xml version="1.0" encoding="utf-8"?><configuration><appSettings></appSettings>  ...<system.codedom><compilers><compiler language="c#;cs;csharp" extension=".cs"        type="Microsoft.CodeDom.Providers.DotNetCompilerPlatform.CSharpCodeProvider, Microsoft.CodeDom.Providers.DotNetCompilerPlatform, Version=1.0.5.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"        warningLevel="4" compilerOptions="/langversion:default /nowarn:1659;1699;1701"/><compiler language="vb;vbs;visualbasic;vbscript" extension=".vb"        type="Microsoft.CodeDom.Providers.DotNetCompilerPlatform.VBCodeProvider, Microsoft.CodeDom.Providers.DotNetCompilerPlatform, Version=1.0.5.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"        warningLevel="4" compilerOptions="/langversion:default /nowarn:41008 /define:_MYTYPE=\&quot;Web\&quot; /optionInfer+"/></compilers></system.codedom></configuration>

In the tag configuration> system.codedom> compilers>compiler language="c#;cs;csharp", completely removethe type attribute.– In short, remove the line that starts withtype="Microsoft.CodeDom.Providers.DotNetCompilerPlatform.CSharpCodeProvider, .¹

Visual Studio will take care of the rest.– No more Server Error in '/' Application.

3. HTTP Error 403

In the example provided above, hitting Ctrl+F5will now result in an HTTP Error 403.

Try replacing http://localhost:64195 in your web browser withhttp://localhost:64195/api/products.The web API now displays correctly:

As a provocation, I tried removing the whole package directory fromthe Visual Studio project.
It was automatically and silently recreated as soon as the project wasrebuilt.

References

• Project files that reproduce the error

• The original project