Question

I’ve written a couple simple GUI applications but all of the logic is basically written in the default Form1 class that I’m given.

So I thought maybe I’ll rewrite the GUI logic into their own classes. For example, a class for the FileOpenDialog, another class for a ListView, and so on. That way my Form1 class doesn’t have too many unnecessary methods and is just there for handling some basic stuff, possibly not a lot after the other GUI methods have been moved around.

Wrote something like this as a first attempt to get somewhere

namespace WindowsFormsApplication1
{
    class OpenFileDialog1 : OpenFileDialog
    {
    }
}

But then VS tells me I can’t derive from a sealed type.

I haven’t tried it with the other classes, but I’ll probably run into the same issue at some point. Am I not inheriting it correctly? Or do I have to use some sort of workarounds to separate each GUI element into their own classes?

Maybe this is not a good approach?

Answer 1

Based on your clarifying comments, it sounds like you either want some utility classes or factory classes. Perhaps something like below:

public interface IFileOpener
{
    public bool PresentFileOpenDialogToUser();
    public string RequestedFilePath { get; }
}

public class DefaultFileOpener : IFileOpener
{
    private string filePath = default(string);

    public bool PresentFileOpenDialogToUser()
    {
        OpenFileDialog ofd = new OpenFileDialog();
        DialogResult dr = ofd.ShowDialog();
        if (dr == DialogResult.Cancel)
        {
            this.filePath = default(string);
            return false;
        }
        else
        {
            this.filePath = ofd.FileName;
            return true;
        }
    }

    public string RequestedFilePath
    {
        get 
        {
            return this.filePath;
        }
    }
}

public class FileOpenerFactory
{
    public static IFileOpener CreateFileOpener()
    {
        return new DefaultFileOpener();
    }
}

And in your form:

    private void btnOpenFile_Click(object sender, EventArgs e)
    {
        IFileOpener opener = FileOpenerFactory.CreateFileOpener();
        if (opener.PresentFileOpenDialogToUser())
        {
            //do something with opener.RequestedFilePath;
        }
    }

You could even do something with a partial class so in your main form you have something like

    private void btnOpenFile_Click(object sender, EventArgs e)
    {
        this.OpenMyFile();
    }

and in your partial class you have:

public partial class Form1
{
    private void OpenMyFile()
    {
        IFileOpener opener = FileOpenerFactory.CreateFileOpener();
        if (opener.PresentFileOpenDialogToUser())
        {
            //do something with opener.RequestedFilePath;
        }
    }
}

Many times its very useful to use partial classes as implementations of interfaces or focused functionality.