This is a short post about using Cake to publish packages from Azure Pipelines to Azure Artifacts that took me the better part of a day to figure out. For completness I'll walk through my entire process but if you just want to know how to do it, skip to the end.

I've been a very happy user of AppVeyor and MyGet for my open source work. At my day job we use an on-premesis Bamboo server which also sends packages to MyGet. In both cases, publishing a package from a Cake build script is relativly straightforward and basically involves getting an API key from MyGet and feeding that to the Cake NuGetPush alias. Now that I'm investigating moving some of the workloads at my day job to Azure DevOps services, I'm finding this simple task isn't so straightforward.

Personal Access Token

My first attempt was to get an Azure DevOps personal access token with package management grants and feed that to the NuGetPush Cake alias, just like I was used to doing with MyGet. That resulted in error messages that look like this:

Unable to load the service index for source https://pkgs.dev.azure.com/xyz/_packaging/xyz/nuget/v3/index.json.
Response status code does not indicate success: 401 (Unauthorized).

After that, I took the most resonable first troubleshooting step...and ranted on Twitter:

Good grief. I always feel so dumb when trying to do anything with Azure. Can't figure out how to push a package from Azure Pipelines to Azure Artifacts using an API key. And no, I don't want to setup and use a special credential provider. This was so easy with AppVeyor/MyGet.
— Dave Glick (@daveaglick) November 6, 2018

Unfortunatly the answer wasn't what I wanted to see:

Thanks for the feedback. There are some under-the-hood reasons why we don't support apikey. But, Azure Pipelines has a "NuGet" task (or ".NET Core" if you prefer) that will automatically authenticate to Azure Artifacts for both push and restore.
— Alex Mullans (@alexmullans) November 6, 2018

It turns out that you can't use a personal access token as an API key to publish packages to Azure Artifacts.

Credential Provider

My next step was to take a look at the VSTS Credential Provider. It's essentially the only documented way of publishing a package. Thankfully the credential provider is on NuGet as Microsoft.VisualStudio.Services.NuGet.CredentialProvider so you can add it as a tool to your Cake script:

#tool "nuget:?package=Microsoft.VisualStudio.Services.NuGet.CredentialProvider&version=0.37.0"

Once you've installed it, you need to tell NuGet where to find it. Fortunatly there's an environment variable called NUGET_CREDENTIALPROVIDERS_PATH that NuGet uses to find credential providers. We can set it from our Cake script like this:

var credentialProviderPath = GetFiles("**/CredentialProvider.VSS.exe").First().FullPath;
Information("Setting NUGET_CREDENTIALPROVIDERS_PATH to " + credentialProviderPath);
System.Environment.SetEnvironmentVariable("NUGET_CREDENTIALPROVIDERS_PATH", credentialProviderPath, EnvironmentVariableTarget.Machine);

Less fortunatly, this doesn't seem to work at all. In fact, I couldn't get NuGet to recognize the NUGET_CREDENTIALPROVIDERS_PATH environment variable no matter how it was set (and I tried everything, including using the NuGetPushSettings.EnvironmentVariables property). That led to just copying the credential provider alongside nuget.exe:

var credentialProviderPath = GetFiles("**/CredentialProvider.VSS.exe").First().FullPath;
var nugetPath = GetFiles("**/nuget.exe").First().GetDirectory();
CopyFiles(new [] { credentialProviderPath }, nugetPath);

This allowed NuGet to find the credential provider, but at that point I couldn't figure out how to automatically get it to authenticate:

CredentialProvider.VSS: Getting new credentials for source:https://pkgs.dev.azure.com/xyz/_packaging/xyz/nuget/v3/index.json, scope:vso.packaging_write vso.drop_write
CredentialProvider.VSS: Couldn't get an authentication token for https://pkgs.dev.azure.com/xyz/_packaging/xyz/nuget/v3/index.json.
Unable to load the service index for source https://pkgs.dev.azure.com/xyz/_packaging/xyz/nuget/v3/index.json.
Response status code does not indicate success: 401 (Unauthorized).

Most of the documentation talks about using the credential provider interactivly, either by displaying a UI or prompting for credentials on the command line. I'm sure there's a way to make this work from a script, but I was getting pretty frustrated with the credential provider at this point.

System.AccessToken

I was tipped off by my Cake buddies to some blog posts from Kevin Smith and Max Vasilyev about using OAuth tokens for publishing to VSTS. It turns out Azure Pipelines exposes a special pipeline variable named System.AccessToken that contains an OAuth key for the VSTS/Azure DevOps REST API. You have to manually activate it from your YAML file:

variables:
  SYSTEM_ACCESSTOKEN: $(System.AccessToken)

That should provide access to a SYSTEM_ACCESSTOKEN environment variable from inside your scripts, but...wait for it...:

Could not resolve SYSTEM_ACCESSTOKEN

Bet you saw that coming. For some reason, I couldn't figure out how to set the enviornment variable globally, but I was able to set it at the script level:

steps:
 - script: build -target Publish
   env:
     SYSTEM_ACCESSTOKEN: $(System.AccessToken)

Once that's done, you can register a NuGet feed from inside your Cake script using the access token and then use it when publishing a package. Here's my working package publishing task inside my Cake script:

Task("Publish")
    .IsDependentOn("Pack")
    .WithCriteria(() => isRunningOnBuildServer)
    .Does(() =>
    {
        // Get the access token
        var accessToken = EnvironmentVariable("SYSTEM_ACCESSTOKEN");
        if (string.IsNullOrEmpty(accessToken))
        {
            throw new InvalidOperationException("Could not resolve SYSTEM_ACCESSTOKEN.");
        }

        // Add the authenticated feed source
        NuGetAddSource(
            "VSTS",
            "https://pkgs.dev.azure.com/xyz/_packaging/xyz/nuget/v3/index.json",
            new NuGetSourcesSettings
            {
                UserName = "VSTS",
                Password = accessToken
            });

        foreach (var nupkg in GetFiles(buildDir.Path.FullPath + "/*.nupkg"))
        {
            NuGetPush(nupkg, new NuGetPushSettings 
            {
                Source = "VSTS",
                ApiKey = "VSTS"
            });
        }
    });

Note the use of "VSTS" for UserName and ApiKey. That's basically a dummy value - NuGet requires something for those properties but it doesn't really care what. The important part is that the SYSTEM_ACCESSTOKEN environment variable is being used as the Password for the NuGetSourcesSettings, and that the name of the new source matches the Source property in the NuGetPushSettings.

Hopefully this post saves you a bit of time. Once it's set up it appears to work well, but discovering the "right way" of doing this took longer than it should have (if this even is the right way).