These are three different things you can add to a project and I am not quite sure if I understand the difference. They all seem to for example show up in the component toolbox when working with a Form. What are some common usage scenarios for each of them? What is the difference?

The main difference between User Control, Custom Control and Component is that they inherit from different levels in the inheritance tree:

MyComponent
 |-> Component

MyCustomControl
 |-> Control
 |-> Component

MyUserControl
 |-> ContainerControl
 |-> ScrollableControl
 |-> Control
 |-> Component

So, in short you get a different amount of pre-wired functionality with the different options.

When would you use the different options? (these are thoughts and opinions, not truths)

• Create a component if you want to provide functionality without UI (such as Timer components, data sources, ...)


• Create a custom control if you want to make a component where you have full control over its visual appearance, and you don't want any baggage of unnecessary functionality. Typical cases would be simple controls with limited functionality (such as a button)


• Create a user control if you are going to combine existing controls into reusable building blocks (such as two lists with buttons where you can move items between the lists).

This is some difference between a CustomControl and a UserControl :

Custom Control

A loosely coupled control w.r.t code and UI

Derives from Control

Defines UI in a ResourceDictionary

UI is skinable

Has dynamic layout

UI can be changed in different projects

Has full toolbox support

Defines a single control

More flexible

==============

User Control

A tightly coupled control w.r.t code and UI

Derives from UserControl

Defines UI as normal XAML

Child controls are skinable

Has static layout

UI is fixed and can't have different looks in different project

Can't be added to the toolbox

Defines a set of controls

Not very flexible like a Custom Control

Adding to what Fredrik said, generally components and custom controls would be used when you plan on reusing them across projects. Use user controls when you would only be using it in one project.

I believe the last statement is not correct in my opinion .
I create user controls for many different reasons.

The main reason is so that if per say I design an interface of multiple controls
grouped together.

I first create a class library , then I add user controls to it .
Now if i need to change any part of the logic behind how the user control works I can very easily. Also this class library can be used multiple times.

Also within the same classy library I can have multiple classes that can be shared and used for any of my user controls.

This is the main reason I use user controls.
And if you make a change to your user control or class library .
once you build the job .
the dll will dynamically up date in the bin folder .

So if i am referencing this in another project
Those changes will also appear in new project .

Also it doesn't use the same paint routines as the form and anything you have loaded on the form.

So user controls gives us the ability to be very modular
And i Can have multiple user controls that share the basics classes of the class library ...
So a user control purpose is just not for one project . It has no limitations in that respect.
jeff

The main difference between them-
User Control is a page file with extension .ascx which can only be
used within a single application or project But custom controls are assemblies(dll files) that can be used in multiple applications.