Don't understand why we're getting error CS0234
<p>We're working on a WPF project using Visual Studio 2015. We've got a folder in the project named Assets. It shows up fine in VS 2015. The files in it show up fine in the Solution Explorer. But when we build it, VS 2015 complains with the following error:</p>

<p>"Error  CS0234  The type or namespace name 'Assets' does not exist in the namespace 'CoreFramework' (are you missing an assembly reference?)"</p>

<p>(CoreFramework is the name of our solution and the project that Assets is in.)</p>

<p>I don't get how the folder is there in CoreFramework, but when building it, VS 2015 just doesn't see it. I've tried cleaning both the project and the solution in VS 2015, but it doesn't help at all. The same errors keep popping up. And they popup in our nightly builds as well.</p>

<p>So what is causing VS 2015 to simultaneously see a folder within a project and not see that same folder in the project?</p>