Question

I’m trying to create a class like Windows Form, which will have multiple new features. It’s simply going to be a “better” version of Form, making my work easier on this exact program. Here’s what I have so far:

public class SuperForm : Form
{
    protected abstract void OnSizeChanged(object sender, EventArgs e);

    public SuperForm()
    {
        this.SizeChanged += OnSizeChanged;
    }
}

Not much, it only makes sure every Form will have to define OnSizeChanged which will be called automatically when size changes, but there’s going to be more.

What I need next is a method which takes a class/type as it’s parameter and initializes a new object of that type, automatically adds it to Controls and then returns the object. Here’s an example code (not working) on how I would like it to be like:

public cls NewControl(ClassType cls) // This obviously doesn't work.
// I need it to return an object of type "cls" parameter.
{
    cls newControl = new cls();
    this.Controls.Add(newControl);
    return newControl;
}

I know ClassType is not a valid type, that’s why I need help.

And then I could basically call it like this:

Button b = this.NewControl(Button);

Which would return a new button and also add it to this.Controls. I might eventually need to do more of these common tasks when a control is initialized, so that’s why I’d like to have it in it’s own method.

Is this possible in C#? If not, are there any workarounds? One way would be to define a method for each class inheriting from Control like this:

public Button NewControl(Button b);
public TextBox NewControl(TextBox tb);
public ListBox NewControl(ListBox lb);

But it doesn’t seem like a valid option to me.

Answer 1

It sounds like you want to make a generic method, with a couple of constraints on the type parameter:

public T CreateAndAdd<T>() where T : Control, new()
{
    T newControl = new T();
    Controls.Add(newControl);
    return newControl;
}

Here the T : Control constraint makes sure that you’re creating a control, so that you’ll be able to use Controls.Add. The T : new() constraint makes sure the type argument has a public parameterless constructor, so that you can call new T().

To create a TextBox, for example, you would call the function like this:

var tb = CreateAndAdd<TextBox>()

(I’ve renamed the method for what I believe to be clarity, btw.)