namespace HelloWorld

{
    [Autodesk.Revit.Attributes.Transaction Autodesk.Revit.Attributes.TransactionMode.Automatic)]
    public class Class1 : IExternalCommand
    {
    public Autodesk.Revit.UI.Result Execute(ExternalCommandData revit,
    ref string message, ElementSet elements)
    {
        ShowMyDialogBox();
    }

    public void ShowMyDialogBox()
    {
        Form1 frm = new Form1();
        frm.ShowDialog();
    }

    }
}