I have an winforms application that was built using MVC. The controller is subscribing to all of the events from the view (button clicks, etc) and connects multiple views with the model.
The problem is that the controller is now about 3000 lines of (hard to unit test) code.
What is the best practice to avoid getting controllers to do everything and become so big?
Depends on the situation, but assuming you are not at a point where a new controller should be created there are several approaches.
Much depends on your setup. One common approach is to have a service layer or service agent which would do work for the controllers that is not specific. The use of interfaced helpers or even static ones should remove some of the repetition. 300 lines does not sound to bad at all assuming it is broken into testable methods.
I would be interested too to hear of other opinions other than the oft repeated mantra of creating more controllers. We use MVP and have experimented with sub controllers but this does rely on careful usage and is probably a bad idea.
It is common to have one controller per module in MVP, which would relate to a logical part of your application. There should be a few clear ones within your domain and a few that are maybe a bit more difficult to distinguish.
One obvious thing to point out might be that one controller does not have to be implemented as one class. The MVC design pattern simply states that M, V and C are separate components, but not that each must be one, and only one, class.
Controller can be split in various sub-controller without broking the MVC pattern.
At 3k lines, it's for sure that the cohesion is broken somewhere. Try to group together same behavior and create new controller. This way, you will have a "main" controller that will invoke "sub" controller.
For my own experience, I do not have 1 controller for the whole WinForm application. How I created it is that I have mutiple module that are loaded from the menu. When those module are loaded (Form->View) it comes with its own Controller. This way, I only have 1 controller for each module. Those controller aren't over 500 lines of code usually.