I am here to discuss what are the possible improvement can be made in order to make the following code covering all the principles of programming paradigm ( SOLID , DRY ...etc ). Basically, I am practicing Design Patterns. Example to create multiple types if Cascade Dropdown list of different types ( single select / multiple select items ):public class MyListType{    public string ID { get; set; }    public string ComplexID { get; set; }    public string Name { get; set; }    public bool Selected { get; set; }}public class DropDownItem{    public string Value { get; set; }    public string Name { get; set; }}public class AdminItem{    public string ID { get; set; }    public string AdminName { get; set; }}public class MerchantItem{    public string ID { get; set; }    public string MerchantName { get; set; }}public class CustomerItem{    public string ID { get; set; }    public string CustomerName { get; set; }}public enum DefaultDDItems{    ALL = 0,    All_Selected = 1,    NoDefault,    First_Selected,    CurrentMonth,    CurrentYear}public interface IDropDown{    MultiSelectList GetMultiSelectDropDown();    MultiSelectList GetMultiSelectDropDown(DefaultDDItems defaultDdItem);    List<SelectListItem> GetSingleSelectDropDown();    List<SelectListItem> GetSingleSelectDropDown(DefaultDDItems defaultDdItem);}public class AdminLevelDropDown : IDropDown{    private List<AdminItem> _adminItemes;    private string[] _selectedIds;    public AdminLevelDropDown()    {    }    public AdminLevelDropDown(List<AdminItem> items)    {        this._adminItemes = items;    }    public AdminLevelDropDown(List<AdminItem> items, string[] defaultSelectedItems)    {        this._adminItemes = items;        this._selectedIds = defaultSelectedItems;    }    public MultiSelectList GetMultiSelectDropDown()    {        return GetMultiSelectDropDown(DefaultDDItems.NoDefault);    }    public MultiSelectList GetMultiSelectDropDown(DefaultDDItems defaultDdItem)    {        var mylist = new List<MyListType>();        if (_adminItemes == null)        {            return new MultiSelectList(mylist, ID, Name);        }        if (!_adminItemes.Any())        {            return new MultiSelectList(mylist, ID, Name);        }        switch (defaultDdItem)        {            case DefaultDDItems.ALL:                mylist.Add(new MyListType()                    {                        Name = All,                        ID = 0                    });                break;            case DefaultDDItems.All_Selected:                mylist.Add(new MyListType()                    {                        Name = All,                        ID = String.Join(,, _adminItemes)                    });                break;              // Other switch cases can be implemented here        }        mylist.AddRange(_adminItemes.Select(i => new MyListType()            {                Name = i.AdminName,                ID = i.ID            }).ToList());        return new MultiSelectList(mylist, ID, Name, _selectedIds);    }    public List<SelectListItem> GetSingleSelectDropDown()    {        return GetSingleSelectDropDown(DefaultDDItems.NoDefault);    }    public List<SelectListItem> GetSingleSelectDropDown(DefaultDDItems defaultDdItem)    {        var mylist = new List<SelectListItem>();        if (_adminItemes == null)        {            return new List<SelectListItem>(0);        }        if (!_adminItemes.Any())        {            return new List<SelectListItem>(0);        }        switch (defaultDdItem)        {            case DefaultDDItems.ALL:                mylist.Add(new SelectListItem()                {                    Value = All,                    Text = 0                });                break;            case DefaultDDItems.All_Selected:                mylist.Add(new SelectListItem()                {                    Value = All,                    Text = String.Join(,, _adminItemes)                });                break;        }        mylist.AddRange(_adminItemes.Select(i => new SelectListItem()        {            Text = i.AdminName,            Value = i.ID,            Selected = _selectedIds.FirstOrDefault() == i.ID        }).ToList());        return mylist;    }}public class MerchantLevelDropDown : IDropDown{    private List<MerchantItem> _merchantItemes;    private string[] _selectedIds;    public MerchantLevelDropDown()    {    }    public MerchantLevelDropDown(List<MerchantItem> items)    {        this._merchantItemes = items;    }    public MerchantLevelDropDown(List<MerchantItem> items, string[] defaultSelectedItems)    {        this._merchantItemes = items;        this._selectedIds = defaultSelectedItems;    }    public MultiSelectList GetMultiSelectDropDown()    {        return GetMultiSelectDropDown(DefaultDDItems.NoDefault);    }    public MultiSelectList GetMultiSelectDropDown(DefaultDDItems defaultDdItem)    {        var mylist = new List<MyListType>();        if (_merchantItemes == null)        {            return new MultiSelectList(mylist, ID, Name);        }        if (!_merchantItemes.Any())        {            return new MultiSelectList(mylist, ID, Name);        }        switch (defaultDdItem)        {            case DefaultDDItems.ALL:                mylist.Add(new MyListType()                {                    Name = All,                    ID = 0                });                break;            case DefaultDDItems.All_Selected:                mylist.Add(new MyListType()                {                    Name = All,                    ID = String.Join(,, _merchantItemes)                });                break;        }        mylist.AddRange(_merchantItemes.Select(i => new MyListType()        {            Name = i.MerchantName,            ID = i.ID        }).ToList());        return new MultiSelectList(mylist, ID, Name, _selectedIds);    }    public List<SelectListItem> GetSingleSelectDropDown()    {        return GetSingleSelectDropDown(DefaultDDItems.NoDefault);    }    public List<SelectListItem> GetSingleSelectDropDown(DefaultDDItems defaultDdItem)    {        var mylist = new List<SelectListItem>();        if (_merchantItemes == null)        {            return new List<SelectListItem>(0);        }        if (!_merchantItemes.Any())        {            return new List<SelectListItem>(0);        }        switch (defaultDdItem)        {            case DefaultDDItems.ALL:                mylist.Add(new SelectListItem()                {                    Value = All,                    Text = 0                });                break;            case DefaultDDItems.All_Selected:                mylist.Add(new SelectListItem()                {                    Value = All,                    Text = String.Join(,, _merchantItemes)                });                break;        }        mylist.AddRange(_merchantItemes.Select(i => new SelectListItem()        {            Text = i.MerchantName,            Value = i.ID,            Selected = _selectedIds.FirstOrDefault() == i.ID        }).ToList());        return mylist;    }}public class CustomerLevelDropDown : IDropDown{    private List<CustomerItem> _customerItemes;    private string[] _selectedIds;    public CustomerLevelDropDown()    {    }    public CustomerLevelDropDown(List<CustomerItem> items)    {        this._customerItemes = items;    }    public CustomerLevelDropDown(List<CustomerItem> items, string[] defaultSelectedItems)    {        this._customerItemes = items;        this._selectedIds = defaultSelectedItems;    }    public MultiSelectList GetMultiSelectDropDown()    {        return GetMultiSelectDropDown(DefaultDDItems.NoDefault);    }    public MultiSelectList GetMultiSelectDropDown(DefaultDDItems defaultDdItem)    {        var mylist = new List<MyListType>();        if (_customerItemes == null)        {            return new MultiSelectList(mylist, ID, Name);        }        if (!_customerItemes.Any())        {            return new MultiSelectList(mylist, ID, Name);        }        switch (defaultDdItem)        {            case DefaultDDItems.ALL:                mylist.Add(new MyListType()                {                    Name = All,                    ID = 0                });                break;            case DefaultDDItems.All_Selected:                mylist.Add(new MyListType()                {                    Name = All,                    ID = String.Join(,, _customerItemes)                });                break;        }        mylist.AddRange(_customerItemes.Select(i => new MyListType()        {            Name = i.CustomerName,            ID = i.ID        }).ToList());        return new MultiSelectList(mylist, ID, Name, _selectedIds);    }    public List<SelectListItem> GetSingleSelectDropDown()    {        return GetSingleSelectDropDown(DefaultDDItems.NoDefault);    }    public List<SelectListItem> GetSingleSelectDropDown(DefaultDDItems defaultDdItem)    {        var mylist = new List<SelectListItem>();        if (_customerItemes == null)        {            return new List<SelectListItem>(0);        }        if (!_customerItemes.Any())        {            return new List<SelectListItem>(0);        }        switch (defaultDdItem)        {            case DefaultDDItems.ALL:                mylist.Add(new SelectListItem()                {                    Value = All,                    Text = 0                });                break;            case DefaultDDItems.All_Selected:                mylist.Add(new SelectListItem()                {                    Value = All,                    Text = String.Join(,, _customerItemes)                });                break;        }        mylist.AddRange(_customerItemes.Select(i => new SelectListItem()        {            Text = i.CustomerName,            Value = i.ID,            Selected = _selectedIds.FirstOrDefault() == i.ID        }).ToList());        return mylist;    }}public interface IDropdownFactory{    AdminLevelDropDown GetMultiSelect_AdminDropDown();    AdminLevelDropDown GetMultiSelect_AdminDropDown(List<AdminItem> items);    AdminLevelDropDown GetMultiSelect_AdminDropDown(List<AdminItem> items, string[] selecteditems);    AdminLevelDropDown GetSingleSelect_AdminDropDown();    AdminLevelDropDown GetSingleSelect_AdminDropDown(List<AdminItem> items);    AdminLevelDropDown GetSingleSelect_AdminDropDown(List<AdminItem> items, string[] selectedItems);    MerchantLevelDropDown GetMultiSelect_MerchantDropDown();    MerchantLevelDropDown GetMultiSelect_MerchantDropDown(List<MerchantItem> items);    MerchantLevelDropDown GetMultiSelect_MerchantDropDown(List<MerchantItem> items, string[] selecteditems);    MerchantLevelDropDown GetSingleSelect_MerchantDropDown();    MerchantLevelDropDown GetSingleSelect_MerchantDropDown(List<MerchantItem> items);    MerchantLevelDropDown GetSingleSelect_MerchantDropDown(List<MerchantItem> items, string[] selectedItems);    CustomerLevelDropDown GetMultiSelect_CustomerDropDown();    CustomerLevelDropDown GetMultiSelect_CustomerDropDown(List<CustomerItem> items);    CustomerLevelDropDown GetMultiSelect_CustomerDropDown(List<CustomerItem> items, string[] selecteditems);    CustomerLevelDropDown GetSingleSelect_CustomerDropDown();    CustomerLevelDropDown GetSingleSelect_CustomerDropDown(List<CustomerItem> items);    CustomerLevelDropDown GetSingleSelect_CustomerDropDown(List<CustomerItem> items, string[] selectedItems);}public class DropDownFactory : IDropdownFactory{    public AdminLevelDropDown GetMultiSelect_AdminDropDown()    {        return new AdminLevelDropDown();    }    public AdminLevelDropDown GetMultiSelect_AdminDropDown(List<AdminItem> items)    {        return new AdminLevelDropDown(items);    }    public AdminLevelDropDown GetMultiSelect_AdminDropDown(List<AdminItem> items, string[] selecteditems)    {        return new AdminLevelDropDown(items, selecteditems);    }    public AdminLevelDropDown GetSingleSelect_AdminDropDown()    {        return new AdminLevelDropDown();    }    public AdminLevelDropDown GetSingleSelect_AdminDropDown(List<AdminItem> items)    {        return new AdminLevelDropDown(items);    }    public AdminLevelDropDown GetSingleSelect_AdminDropDown(List<AdminItem> items, string[] selectedItems)    {        return new AdminLevelDropDown(items, selectedItems);    }    public MerchantLevelDropDown GetMultiSelect_MerchantDropDown()    {        return new MerchantLevelDropDown();    }    public MerchantLevelDropDown GetMultiSelect_MerchantDropDown(List<MerchantItem> items)    {        return new MerchantLevelDropDown(items);    }    public MerchantLevelDropDown GetMultiSelect_MerchantDropDown(List<MerchantItem> items, string[] selecteditems)    {        return new MerchantLevelDropDown(items, selecteditems);    }    public MerchantLevelDropDown GetSingleSelect_MerchantDropDown()    {        return new MerchantLevelDropDown();    }    public MerchantLevelDropDown GetSingleSelect_MerchantDropDown(List<MerchantItem> items)    {        return new MerchantLevelDropDown(items);    }    public MerchantLevelDropDown GetSingleSelect_MerchantDropDown(List<MerchantItem> items, string[] selectedItems)    {        return new MerchantLevelDropDown(items, selectedItems);    }    public CustomerLevelDropDown GetMultiSelect_CustomerDropDown()    {        return new CustomerLevelDropDown();    }    public CustomerLevelDropDown GetMultiSelect_CustomerDropDown(List<CustomerItem> items)    {        return new CustomerLevelDropDown(items);    }    public CustomerLevelDropDown GetMultiSelect_CustomerDropDown(List<CustomerItem> items, string[] selecteditems)    {        return new CustomerLevelDropDown(items, selecteditems);    }    public CustomerLevelDropDown GetSingleSelect_CustomerDropDown()    {        return new CustomerLevelDropDown();    }    public CustomerLevelDropDown GetSingleSelect_CustomerDropDown(List<CustomerItem> items)    {        return new CustomerLevelDropDown(items);    }    public CustomerLevelDropDown GetSingleSelect_CustomerDropDown(List<CustomerItem> items, string[] selectedItems)    {        return new CustomerLevelDropDown(items, selectedItems);    }}IoC Using AutoFac: var container = new Container();container.RegisterType<DropDownFactory>().As<IDropdownFactory>().InstancePerHttpRequest();Controller:public class HomeController : Controller{    public IDropdownFactory _dropdownFactory;    public HomeController(IDropdownFactory dropdownFactory)    {        this._dropdownFactory = dropdownFactory;    }    public ViewResult GetMultiSelect_AdminDropDown(string[] selectedids)    {        var adminItems = _adminRepo.GetAdminItems();        var adminDD = _dropdownFactory.GetMultiSelect_AdminDropDown(adminItems, selectedids);        return View(adminDD);    }    public ViewResult GetSingleSelect_AdminDropDown(string[] selectedids)    {        var adminItems = _adminRepo.GetAdminItems();        var adminDD = _dropdownFactory.GetSingleSelect_AdminDropDown(adminItems, selectedids);        return View(adminDD);    }    public ViewResult GetMultiSelect_MerchantDropDown(string[] selectedids)    {        var merchantItems = _merchantRepo.GetMerchantItems();        var merchantDD = _dropdownFactory.GetMultiSelect_MerchantDropDown(merchantItems, selectedids);        return View(merchantDD);    }    public ViewResult GetSingleSelect_MerchantDropDown(string[] selectedids)    {        var merchantItems = _merchantRepo.GetMerchantItems();        var merchantDD = _dropdownFactory.GetSingleSelect_MerchantDropDown(merchantItems, selectedids);        return View(merchantDD);    }    public ViewResult GetMultiSelect_CustomerDropDown(string[] selectedids)    {        var customerItems = _customerRepo.GetCustomerItems();        var merchantDD = _dropdownFactory.GetMultiSelect_CustomerDropDown(customerItems, selectedids);        return View(merchantDD);    }    public ViewResult GetSingleSelect_CustomerDropDown(string[] selectedids)    {        var customerItems = _customerRepo.GetCustomerItems();        var merchantDD = _dropdownFactory.GetSingleSelect_CustomerDropDown(customerItems, selectedids);        return View(merchantDD);    }}
Creating various types of menus using dependency injection
c#;design patterns;unit testing;dependency injection