NullReferenceException when I try to add a new ApiController in .NET Core

Question

I see this error message:

When I try to scaffold a controller for and empty API Controller by following these steps:

Right-click on folder

> Add

> Controller...

> Add Scaffold -> API Controller - Empty

> Click Add

Specifications:

Windows 10 Enterprise
Visual Studio Professional 2017 15.6.4
Microsoft .NET Framework 4.7.02556
.NET Core 2.0
Web API ASP.NET Core Web Application

---Edit---

I can't replicate the problem now. One of the suggestions given by @chriss-pratt must have worked (Thank You!) While doing a tutorial, I came across the section to add scaffold tooling and perform initial migration. In case you experience this and the below doesn't work, this part of the tutorial my be promising (if you decide to stick with scaffolding that is).

So this happens when you run it in debug mode on your local machine? What does the stack trace say? — Marcus Höglund
– Marcus Höglund, Commented Mar 29, 2018 at 6:20
It's not not during runtime and unfortunately there is no stacktrace. Simple steps are to Right click > Add > controller > API Controller - Empty — Clarice Bouwer
– Clarice Bouwer, Commented Mar 29, 2018 at 13:40
Same issue on vs 2019 v 16.3.8 .Net Framework V4.8.03761 OS: Win 10 — navule
– navule, Commented Nov 20, 2019 at 16:12

Chris Pratt · Accepted Answer · 2018-03-29 13:00:30Z

2

ASP.NET Core doesn't strictly have an ApiController class. ASP.NET Core doesn't have separate MVC and Web Api components. Everything is just Controller. ApiController is available through a separate "shim" NuGet package, but that's only to make migrating old ASP.NET Web Api code easier. You shouldn't be using it for any new development.

That said, the problem you're having here is with the scaffold. It might be due to the fact that you're trying to scaffold ApiController rather than just Controller. You haven't give any information about what version of Visual Studio you're using or what version of ASP.NET Core.

Try the simple stuff like closing and restarting Visual Studio. If it persists, you can try repairing Visual Studio. Hit the windows key and begin typing "Visual Studio Installer". Open that when it shows up. Below your installed copy of Visual Studio in the list, there will be a drop down menu that looks like three dots stacked on top of each other. There's an option there to "Repair". Start the process and go grab a cup of coffee. It essentially reinstalls Visual Studio, so it'll likely take a while.

All that said, honestly, your best bet is to just don't worry about it. Scaffolding is all but useless anyways. Especially with ASP.NET Core, all you get is a class, which you can easily create yourself. A controller is merely a class that inherits from Controller. Add a new class in your Controllers directory, name it WhateverController and then add : Controller after the name in the code. Then, just start adding your actions and such as normal.

answered Mar 29, 2018 at 13:00

Chris Pratt

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1 Comment

Clarice Bouwer Over a year ago

Thanks Chris. I initially just created my class and inherited from Controller. I was just intrigued when the scaffolding didn't work on Visual Studio 2018. It's interesting that the options will be there if they don't work. I'll try your suggestion and see what happens... but yes I don't need it to work - just intrigued.

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