public static class DatabaseLayer
    {
        public static string dbloc { get; set; } = "Data Source=test.db";

        public static ObservableCollection<Bills> GetAllBillsFromDB()
        {
            try
            {
                SQLiteConnection m_dbConnection = new SQLiteConnection(dbloc);
                SQLiteCommand sqlCom = new SQLiteCommand("Select * From billdata", m_dbConnection);
                SQLiteDataAdapter sda = new SQLiteDataAdapter(sqlCom);
                DataTable dt = new DataTable();
                sda.Fill(dt);
                var Bill = new ObservableCollection<Bills>();

                foreach (DataRow row in dt.Rows)
                {

                    var p = (row["PaidOn"] == DBNull.Value) ? String.Empty : (string)(row["PaidOn"]);
                    var q = (row["Remarks"] == DBNull.Value) ? String.Empty : (string)(row["Remarks"]);

                    var obj = new Bills()
                    {
                        Id = Convert.ToInt32(row["Id"]),
                        Party = (string)row["Party"],
                        BillNo = (string)row["BillNo"],
                        BillDt = (string)(row["BillDt"]),
                        Amt = (string)(row["Amt"]),
                        DueDt = (string)(row["DueDt"]),
                        PaidOn = p,
                        Remarks =q

                    };
                    Bill.Add(obj);
                    m_dbConnection.Close();

                }

                return Bill;

            }
            catch (Exception ex)
            {
                throw ex;
            }
        }

        public static ObservableCollection<Bills> GetAllAdjBillsFromDB()
        {
            try
            {
                SQLiteConnection m_dbConnection = new SQLiteConnection(dbloc);
                SQLiteCommand sqlCom = new SQLiteCommand("Select * From cndndata", m_dbConnection);
                SQLiteDataAdapter sda = new SQLiteDataAdapter(sqlCom);
                DataTable dt = new DataTable();
                sda.Fill(dt);
                var AdjustmentBill = new ObservableCollection<Bills>();

                foreach (DataRow row in dt.Rows)
                {

                    var p = (row["UsedOn"] == DBNull.Value) ? String.Empty : (string)(row["UsedOn"]);

                    var obj = new Bills()
                    {
                        AId = Convert.ToInt32(row["AId"]),
                        Vendor = (string)row["Vendor"],
                        CndnNo = (string)row["CndnNo"],
                        Date = (string)(row["Date"]),
                        Value = (string)(row["Value"]),
                        UsedOn = p

                    };
                    AdjustmentBill.Add(obj);
                    m_dbConnection.Close();

                }

                return AdjustmentBill;

            }
            catch (Exception ex)
            {
                throw ex;
            }
        }

        //... so on
    }
    

public partial class MainWindow : Window,INotifyPropertyChanged
    {
        public event PropertyChangedEventHandler PropertyChanged;

        protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
        {
            var handler = PropertyChanged;
            if (handler != null)
                handler(this, new PropertyChangedEventArgs(propertyName));
        }

        public MainWindow()
        {
            InitializeComponent();

            this.DataContext = new BillsViewModel();
        }

        private void cb_SelectionChanged(object sender, SelectionChangedEventArgs e)
        {
            var comboboxSelectedItem = ((ComboBoxItem)cb.SelectedValue).Content;
            switch (comboboxSelectedItem)
            {
                case "2020":
                   DatabaseLayer.dbloc = @"Data Source=2020.db";
                    BillsViewModel bvm = new BillsViewModel();
                    OnPropertyChanged("Cnt");
                    OnPropertyChanged("Cnt2");
                    OnPropertyChanged("AllBillss");
                    OnPropertyChanged("AdjBillss");
                    OnPropertyChanged("ComboItems");
                    OnPropertyChanged("ComboItems2");
                    OnPropertyChanged("Bill");
                    OnPropertyChanged("AdjustmentBill");
                    OnPropertyChanged("SelectedCBItem");
                    OnPropertyChanged("SelectedCBItem2");
                    OnPropertyChanged("FilterString");
                    OnPropertyChanged("SelectedInv");
                    OnPropertyChanged("SelectedAdj");
                    OnPropertyChanged("SelectedFilter");
                    OnPropertyChanged("UpdateBindingGroup");
                    OnPropertyChanged("myInfo");
                    OnPropertyChanged("CityName");
                    break;

                case "2021":
                   DatabaseLayer.dbloc = @"Data Source=2021.db";
                    BillsViewModel bvm2 = new BillsViewModel();
                    OnPropertyChanged("Cnt");
                    OnPropertyChanged("Cnt2");
                    OnPropertyChanged("AllBillss");
                    OnPropertyChanged("AdjBillss");
                    OnPropertyChanged("ComboItems");
                    OnPropertyChanged("ComboItems2");
                    OnPropertyChanged("Bill");
                    OnPropertyChanged("AdjustmentBill");
                    OnPropertyChanged("SelectedCBItem");
                    OnPropertyChanged("SelectedCBItem2");
                    OnPropertyChanged("FilterString");
                    OnPropertyChanged("SelectedInv");
                    OnPropertyChanged("SelectedAdj");
                    OnPropertyChanged("SelectedFilter");
                    OnPropertyChanged("UpdateBindingGroup");
                    OnPropertyChanged("myInfo");
                    OnPropertyChanged("CityName");
                    break;

                default:
                   DatabaseLayer.dbloc = @"Data Source=2022.db";
                    BillsViewModel bvm3 = new BillsViewModel();
                    OnPropertyChanged("Cnt");
                    OnPropertyChanged("Cnt2");
                    OnPropertyChanged("AllBillss");
                    OnPropertyChanged("AdjBillss");
                    OnPropertyChanged("ComboItems");
                    OnPropertyChanged("ComboItems2");
                    OnPropertyChanged("Bill");
                    OnPropertyChanged("AdjustmentBill");
                    OnPropertyChanged("SelectedCBItem");
                    OnPropertyChanged("SelectedCBItem2");
                    OnPropertyChanged("FilterString");
                    OnPropertyChanged("SelectedInv");
                    OnPropertyChanged("SelectedAdj");
                    OnPropertyChanged("SelectedFilter");
                    OnPropertyChanged("UpdateBindingGroup");
                    OnPropertyChanged("myInfo");
                    OnPropertyChanged("CityName");
                    break;
            }
        }

        ...
    }