Asp.net Type 'Web User Control" is not defined.

Problem with intellisense in Visual Studio 2017 when adding a Web User Control dynamically.

To solve this problem, just try registering your control at the page you are adding it to.

It will solve the problem with the intellisense, but if your ask what the logic behind it, I don't know. This problem accour long since Visual Studio 2008. But still no fix by Microsoft.

If there still problem, delete the folder "ReflectedSchemas" inside the "C:\Users\<username>\AppData\Roaming\Microsoft\VisualStudio\<vsversion>". Don't forget to close the Visual Studio.

* Delete all if any problem still accour.