C#

namespace ExampleApp.Interfaces
{
    public interface INotebookPlugin
    {
        string LabelName { get; set; }

        Gtk.Widget GetNotebook(); // If you use Subclass of Plugin's class 
    /*    Gtk.Widget Notebook { get; set; } // you use instance of Gtk.Widget in Plugin's content */

        Gtk.Window CurrentWindow { get; set; }
    }
}

public static IEnumerable<INotebookPlugin> LoadNotebookPlugin(string plugin_path)
{
    string root = Path.GetFullPath(Path.Combine(AppDomain.CurrentDomain.BaseDirectory, "Plugins"));
    string pluginLocation = Path.GetFullPath(Path.Combine(root, plugin_path.Replace('/', Path.DirectorySeparatorChar)));
    Assembly asm = Assembly.LoadFile(pluginLocation);
    int count = 0;
    foreach (Type type in asm.GetTypes())
    {
        if (typeof(INotebookPlugin).IsAssignableFrom(type))
        {
            if (Activator.CreateInstance(type) is INotebookPlugin result)
            {
                count++;
                yield return result;
            }
        }
    }

    if (count == 0)
    {
        string availableTypes = string.Join(",", asm.GetTypes().Select(t => t.FullName));
        throw new ApplicationException(
            $"Can't find any type which implements INotebookPlugin in {asm} from {asm.Location}.\n" +
            $"Available types: {availableTypes}");
    }
}

// Advanced loading notebook tabs
foreach (INotebookPlugin notebookPlugin in notebookPlugins)
{
    // Pass to current window if you have parent of window like
    // MessageDialog
    notebookPlugin.CurrentWindow = this;

    // Load plugin from sub-widget
    if (notebookPlugin.GetNotebook() != null)
        notebook.AppendPage(notebookPlugin.GetNotebook(),
            new Gtk.Label(notebookPlugin.LabelName));
}

public class SimpleTab : Gtk.VBox, INotebookPlugin
{
    public SimpleTab() : base(false, 0)
    {
        Gtk.Button btn = new Gtk.Button("Open Dialog!");
        PackStart(btn, false, false, 0);
        btn.Clicked += delegate {
            Gtk.MessageDialog md = new Gtk.MessageDialog(CurrentWindow, Gtk.DialogFlags.Modal, Gtk.MessageType.Question, Gtk.ButtonsType.Ok, "Hello I am loading from plugin.");
            md.Run();
            md.Destroy();
        };
        ShowAll();
    }

    // What is name of Notebook's tablabel?
    public string LabelName { get; set; } = "SimpleTab";

    // Ignore it - I tell you in current MainWindow or applicatioon's window.
    public Gtk.Window CurrentWindow { get; set; }

    // Pass to current widget!
    public Gtk.Widget GetNotebook() => this;
}

/.
./BundleExecutableOfNetCore ( Bundled by Dotnet )
./Plugins
  /SimpleTabNotebook.dll

public IList<InvoiceLine> invoiceLines { get; set; }

InvoiceLine invoiceline = new InvoiceLine();
            string[,] datavalu = new string[dataGridView1.Rows.Count, dataGridView1.Columns.Count];

            foreach (DataGridViewRow row in dataGridView1.Rows)
            {


                foreach (DataGridViewColumn col in dataGridView1.Columns)
                {
                    datavalu[row.Index, col.Index] = dataGridView1.Rows[row.Index].Cells[col.Index].Value.ToString();

                }
            }
            int i ;
          
            for ( i = 0; i < datavalu.GetLength(0); i++)
            {
               
                invoiceline.description = datavalu[i, 0].ToString();
                invoiceline.itemType = datavalu[i, 1].ToString();
                invoiceline.itemCode = datavalu[i, 2].ToString();
                invoiceline.internalCode = datavalu[i, 3].ToString();
                invoiceline.unitType = datavalu[i, 4].ToString();
                invoiceline.quantity = int.Parse(datavalu[i, 5].ToString());

                invoicelines.Add(invoiceline);
                
                
            }

description	itemType	itemCode	internalCode	unitType	quantity
Food	EGS	EG-538486562-1314	1314	EA	1
Food2	EGS	EG-538486562-1315	1500	EA	5

Food2	EGS	EG-538486562-1315	1500	EA	5