.NET: Inherit from MyControl for Windows.Forms

Question

Hi, I want to add some custom functionality to all my Controls AND my Forms. I have created a class MyControl, and there's no problem just doing:

MyControl : Control

and then letting my controls inherit from MyControl.

However, I want my forms to have the same functionality, and because Form inherits from ContainerControl, Scrollable Container, and THEN Control, I would have to create my own versions of all of these to make it use MyControl, which seems inefficient.

Is there any way around this? I can't use an interface, and I don't want to create a copy of the class called MyForm with the same functionality as MyControl

Answer 1

I think the only way to do it is with

MyForm : Form

Maybe use an interface plus extension methods to reduce code duplication as much as possible.

Answer 2

What you're looking for is either multiple inheritance (which doesn't exist in .net, with no plans to change that), or Default Interface Implementation, aka DII (which might exist in a future version of .net)

As of today, there's unfortunately no clean solution to this problem, and you best bet is to create two base classes. You can share the code in a third helper class, though, so that only the glue code is duplicated, not the actual implementation.

Depending on what functionality you're trying to achieve, another useful technique is to create a single class getting passed an object (either a Form or a Control) in its constructor, and adding the extra functionality you're wanting. However, you won't be able to access the control/form protected members in that way.

Here is a link discussing your issue.

Answer 3

If all you want to do is add extra methods, you can create extension methods which are methods that appear to be part of a certain object, but aren't.

You can declare one like this:

void MyMethod(this Control obj, string foobar);

You can then call this method as any other method of the Control class:

myButton.MyMethod("baz");

However, if you wish to override virtual methods, play with the internals of the class, etc. then there isn't much that I could help you with.

Answer 4

You don't have many choices here since multiple inheritance is not supported.

1. Use extensions methods on Control

2. Reimplement in MyForm what you did in MyControl.

For 2. you can make an interface, say IMyControl, and an helper class that implements it, like MyHelperControl : IMyControl. Then in MyControl and MyForm implements IMyControl but use a private MyHelperControl and maps all interface members to call of this helper, that way you only have one real code to manage.

interface IMyControl {
    void A();
}

class MyHelperControl : IMyControl {
    void A() {
        // your implementation here
    }
}

class MyControl : Control, IMyControl {
    private MyHelperControl _myHelper;
    void A() {
        _myHelper.A();
    }
}

class MyForm : Form, IMyControl {
    private MyHelperControl _myHelper;
    void A() {
        _myHelper.A();
    }
}

Answer 5

One thing you can do is to have your forms inherit from a custom "master" form that contains an instance of MyControl, like this:

public partial class MasterForm : Form
{
    private MyControl _MyControl = new MyControl();

    public MasterForm()
    {
        InitializeComponent();
    }

    public void Method1()
    {
        _MyControl.Method1();
    }
}

This means duplicating all of MyControl's method signatures in MasterForm (or all the ones you need in your form, at least), but at least the method code itself would not be duplicated.

public partial class Form1 : MasterForm
{
    public Form1()
    {
        InitializeComponent();
    }

    private void button1_Click(object sender, EventArgs e)
    {
        Method1();
    }
}

This approach avoids using an interface, although I'm curious as to why exactly you can't use an interface here.