I was importing a old project and got a bunch of errors while building. Most errors were similar, saying ‘type or namespace does not exist in the namespace etc’
I took one particular error which was about a missing namespace ‘System.Windows.Forms.Design.Behavior’.
I could see this namespace in Object browser:

But not in the Add Reference window:

I could not find any files with that name under this folder too.
G:\Program Files\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.0\
This namespace exists in msdn.
How do I use this namespace in my code?
You don’t find a file with exactly that name because it is not mandatory for a namespace to have the same name of the assembly containing it.
The one you refer it is probably defined in
System.Design.dll