I know there are a lot of problems like that and you all might think this is a duplicate but NO... this problem is different.
When I add a custom user control to a project and rebuild it, the control is added to the toolbox. If I try to add another one it does not show up in the toolbox. If I close visual studio and reopen it the first control also was removed from the toolbox and will never appear again. From now on the project is broken and no control added, remove + readded will ever show up in the toolbox. So I create a new project and it starts from the beginning.
Before this tip appears I'll say it: Automatically Populate Toolbox is True. Nothing helps against this problem. Build Project/Solution, Rebuild, Clean+Rebuild, Restart of VS, Restart of Computer, Unload and reload the project, set toolbox back to default. Nothing works.
Any ideas??
You have to remember that the designer call your code constructor and some other methods at design time.
If you build the code and the control does not appears and it has not been hidden by an attribute, then probably the code you wrote is not compatible with the designer. For example, the control might throw an exception in design mode because of incorrect code.
The attribute is
[ToolboxItem(false)]. If you have a base class that hide it, it will also be hidden in derived class if not specified otherwise.You will have the same problem if that control is used in another component. You won't see it if it does not compile and run correctly.
The trick to debug that kind of problem, is to attach a debugger from a second instance of Visual Studio to find where in your code you throw an exception.
If the control does not work at run-time, better to fix that first as if it also fix design problem, it is simpler to debug that way.
Otherwise, you have to ensure that your project dependencies are correct. The toolbox will only shows component from DLLs that are referenced by the current project (or from itself I think).