I recently upgraded my UI for ASP.NET AJAX using the Telerik control panel. After doing this, my project would no longer compile. My treeview control was "Not found in current context." I was able to view the control in design view and see that it was correctly recognized. Nothing had changed, and yet the error. I've seen this before where Visual Studio somehow gets confused linking the designer and the code behind, and to solve this I've always done the following: I browse Solution Explorer to the ASPX designer page. To be clear, I delete the [My Page].aspx.designer.cs file. Then using Visual Studio, I convert the page to a web application. In Visual studio 2012, you did this by simply right clicking the ASPX file in Solution Explorer and "Convert to Web Application." In Visual Studio 2013, this option moved under the Project menu. But in Visual Studio 2015, although it remains in the same location as VS 2013, it doesn't do anything.
To resolve my "not found in current context" issue, I opened Visual Studio 2012, deleted the aspx.designer.cs file and converted the page to a web application. The new designer file popped right up. I compiled. The project built fine. Next, I opened it in VS 2015. Again I compiled it. And again it worked fine. There is a bug in VS 2015. It won't correctly build the designer file. Hopefully you won't waste hours on this like I did!