NullReferenceException with incorrect package source in NuGet.Config - local package source and protocolversion attribute

[dtivel]

Repro steps:

1. Create a C# class library project.
2. Add a package reference (e.g.: Newtonsoft.Json).
3. Save the solution.
4. Exit Visual Studio.
5. Delete the project's .\packages directory.
6. Create a NuGet.Config file with the following contents:

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <packageSources>
    <clear />
    <add key="CSV sources" value="file://C:\packages,https://api.nuget.org/v3/index.json" protocolVersion="3" />
  </packageSources>
</configuration>

7. Open a command prompt and path to the directory which contains the .sln file.
8. Execute:

nuget.exe restore -ConfigFile <PathToNuGetConfigFile>

Result:

MSBuild auto-detection: using msbuild version '14.0' from 'C:\Program Files (x86)\MSBuild\14.0\bin'.
Restoring NuGet package Newtonsoft.Json.9.0.1.
WARNING: Unable to find version '9.0.1' of package 'Newtonsoft.Json'.
  file://C:\packages,https://api.nuget.org/v3/index.json: Object reference not set to an instance of an object.

Errors in packages.config projects
    Unable to find version '9.0.1' of package 'Newtonsoft.Json'.
      file://C:\packages,https://api.nuget.org/v3/index.json: Object reference not set to an instance of an object.

NuGet Config files used:
    C:\repro\NuGet.Config
    C:\Users\dtivel\AppData\Roaming\NuGet\NuGet.Config
    C:\ProgramData\nuget\Config\Microsoft.VisualStudio.Offline.config

Feeds used:
    file://C:\packages,https://api.nuget.org/v3/index.json

The visible NullReferenceException exception reproes on 3.6.0 beta 1, 3.5.0, 3.4.4, but not 3.3.0.

Observation: The -Source command line option without the project's NuGet.Config file handles the input more gracefully.

nuget.exe restore -Source file://C:\packages,https://api.nuget.org/v3/index.json

MSBuild auto-detection: using msbuild version '14.0' from 'C:\Program Files (x86)\MSBuild\14.0\bin'.
Restoring NuGet package Newtonsoft.Json.9.0.1.
WARNING: Unable to find version '9.0.1' of package 'Newtonsoft.Json'.
  file://C:\packages,https://api.nuget.org/v3/index.json: Failed to retrieve information from remote source 'file://C:\packages,https://api.nuget.org/v3/index.json'.
  The given path's format is not supported.

Errors in packages.config projects
    Unable to find version '9.0.1' of package 'Newtonsoft.Json'.
      file://C:\packages,https://api.nuget.org/v3/index.json: Failed to retrieve information from remote source 'file://C:\packages,https://api.nuget.org/v3/index.json'.
      The given path's format is not supported.

NuGet Config files used:
    C:\Users\dtivel\AppData\Roaming\NuGet\NuGet.Config
    C:\ProgramData\nuget\Config\Microsoft.VisualStudio.Offline.config

Feeds used:
    file://C:\packages,https://api.nuget.org/v3/index.json

[AArnott]

Just hit this with nuget.exe 4.9.2.5706 after adding this local package source that accidentally retained protocolVersion="3" from another line:

    <add key="local" value="D:\git\DevCore\bin\Debug\Packages" protocolVersion="3" />

nuget restore C:\Users\andarno\source\repos\SampleServiceHubUser MSBuild auto-detection: using msbuild version '16.1.68.42340' from 'C:\Program Files (x86)\Microsoft Visual Studio\2019\Preview\MSBuild\Current\bin'. Restoring packages for C:\Users\andarno\source\repos\SampleServiceHubUser\SampleServiceHubUser\SampleServiceHubUser.csproj... Restoring packages for C:\Users\andarno\source\repos\SampleServiceHubUser\EchoService\EchoService.csproj... Object reference not set to an instance of an object.

Note the unhelpful error message. I was only able to determine the root cause being the inappropriate xml attribute after attaching a debugger to nuget.exe and observing first chance exceptions.