Azure DevOps: Build 1 Project in a Multi-Project Solution - azure-devops

Question

I'm not sure if I'm searching correctly, but I'm hoping I can get some guidance here.
Current Setup
I have one solution with two projects:
Web API project
Node.js project
I'm using Azure DevOps with 2 Builds, each with their own Releases, one for each project. Each build definition only triggers when their respective project is updated/changed.
This works great!
I've noticed that each build actually builds the entire solution. In order to not waste processing power, I'd prefer to have each project only build their own project, with a few caveats.
The Web Api project does not depend on changes to the Node.js project, however, the Node.js can depend on changes to the Web Api. Because of this, if the Web Api fails, I don't want the Node.js to build/release.
Goals
What I'm trying to do is setup my build definitions so that the Web Api build, only builds the Web Api project. Whether it completes or errors out, let the build proceed as it normally does.
However, I want the Node.js project build both the Web Api and the Node.js project, so that if the Web Api build fails, then the whole build fails even if the Node.js would not have failed.
I've tried adding a new Visual Studio Build task and select the project only, but I got the following error:
[warning]C:\Program Files (x86)\Microsoft Visual Studio\2017\Enterprise\MSBuild\15.0\Bin\Microsoft.Common.CurrentVersion.targets(781,5): Warning : The OutputPath property is not set for project 'MyProject.csproj'. Please check to make sure that you have specified a valid combination of Configuration and Platform for this project. Configuration='release' Platform='any cpu'. You may be seeing this message because you are trying to build a project without a solution file, and have specified a non-default Configuration or Platform that doesn't exist for this project.
I'm currently looking on how to fix this, but I want to pose the following questions in case I'm headed in the wrong direction.
Questions
How can I setup the build definition to only build 1 project?
Is there a different configuration I should be creating instead of the one I mentioned?
Build Definition

Answer 1

What I'm trying to do is setup my build definitions so that the Web Api build, only builds the Web Api project. Whether it completes or errors out, let the build proceed as it normally does.
This is the case where you want a build definition that is targeting the project file instead of the .sln. Your error is that building the .csproj requires the OutputPath property to have a value, so just add it to the MSBuild Arguments box: /p:OutputPath="$(build.binariesDirectory)\MyProject". Build.BinariesDirectory is a predefined variable, but is otherwise not a required directory value. You can use what makes sense for you.
I want the Node.js project build both the Web Api and the Node.js project, so that if the Web Api build fails, then the whole build fails even if the Node.js would not have failed.
The simplest and least sophisticated way
From what I understand about your situation, this case doesn't require any additional changes. If you build the solution, then the pipeline will fail if either of the projects are broken. The downsides to this are:
The Node.js project doesn't "get" the newest changes to the Web API "dependency" until the Node.js project is changed and CI triggers a build
If you use a build completion trigger to mitigate downside 1 above, the Web API project gets built twice even though we know it should be successful both times
The more complex and sophisticated (but elegant?) way
Set a Build completion trigger on the Node.js pipeline that will trigger a build when the Web API pipeline is successful. This is similar to what you have now with some differences. With build completion AND a CI triggers on your Node.js pipeline, the Web API build can succeed regardless of the result of the downstream Node.js, but you will be building the Node.js project even when changes are not made to that project explicitly. (This may not be what you want if you're trying to save on agent activity)
Your Node.js pipeline can then have 2 separate build steps, each targeting one of the project files. However, the step for the Web API project build can have a condition to NOT perform if the Build.Reason is BuildCompletion. This allows the Node.js to be a downstream project of the Web API, but doesn't build the Web API if we already know it's successful.
Note: depending on how your references work between these projects in this solution, you may need to add other tasks for downloading the build artifacts and what-not to make sure everything is where it should be for building.