Question

I’m trying to make a custom message box with my controls.

public static partial class Msg : Form
{
    public static void show(string content, string description)
    {

    }
}

Actually I need to place some controls (a gridview) in this form and I have to apply my own theme for this window, so I don’t want to use MessageBox. I want to call this from my other forms like

Msg.show(parameters);

I don’t wish to create an object for this form.

I know I can’t inherit from Form class because it isn’t static. But I wonder how MessageBox is implemented, because it is static. It is being called like MessageBox.show("Some message!");

Now I’m getting an error because inheritance is not allowed:

Static class ‘MyFormName’ cannot derive from type ‘System.Windows.Forms.Form’. Static classes must derive from object

How MessageBox is implemented then?

Answer 1

Your form class needs not to be static. In fact, a static class cannot inherit at all.

Instead, create an internal form class that derives from Form and provide a public static helper method to show it.

This static method may be defined in a different class if you don’t want the callers to even “know” about the underlying form.

/// <summary>
/// The form internally used by <see cref="CustomMessageBox"/> class.
/// </summary>
internal partial class CustomMessageForm : Form
{
    /// <summary>
    /// This constructor is required for designer support.
    /// </summary>
    public CustomMessageForm ()
    {
        InitializeComponent(); 
    } 

    public CustomMessageForm (string title, string description)
    {
        InitializeComponent(); 

        this.titleLabel.Text = title;
        this.descriptionLabel.Text = description;
    } 
}

/// <summary>
/// Your custom message box helper.
/// </summary>
public static class CustomMessageBox
{
    public static void Show (string title, string description)
    {
        // using construct ensures the resources are freed when form is closed
        using (var form = new CustomMessageForm (title, description)) {
            form.ShowDialog ();
        }
    }
}

Side note: as Jalal points out, you don’t have to make a class static in order to have static methods in it. But I would still separate the “helper” class from the actual form so the callers cannot create the form with a constructor (unless they’re in the same assembly of course).