Why default value of datagridcomboboxcolumn does not appear in the datagrid in C# WPF?

See more:

C#

XAML

WPF

DataGrid

Binding

Project subject: I had an MS Access 2003 adp VBA project that would connect to SQL Server Database, Now I'm converting that project to C# WPF, and I'm going to save data directly into SQL Server database from DataGrid in C# WPF

Detail: I have DataGrid that ItemsSource binds to an 'ObservableCollection' named: 'INVO_DATA_RASID_KHARID' and also 'DataGridComboBoxColumn' named: 'ANBAR_COLUMN' the 'ItemSource' of this 'DataGridComboBoxColumn' is filled through SQL Server.

I want this 'DataGridComboBoxColumn' to be displayed with a default value so that the user knows that there is no need to manually select it from the 'DataGridComboBoxColumn' list.

I set this default value through a static variable in code behind in this method named: 'FillALLComboboxes'.

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The Problem: Considering that 'DataGridComboBoxColumn' in the DataGrid, the 'ANBAR_COL' 'DataGridComboBoxColumn' has a default value, the user does not need to select that item manually from the 'DataGridComboBoxColumn' list, but as long as they double-click on the 'ANBAR_COL' combobox cell, the default value It will appear! The point is that the default value set for 'DataGridComboBoxColumn' will not be displayed until the user enters the cell!

https://i.stack.imgur.com/UL1gU.png

What I have tried:

XAML :

        <DataGrid   x:Name="INVO_LST_RASID_SUB"
                        EnableColumnVirtualization="True"
                        EnableRowVirtualization="True"
                        VirtualizingPanel.IsVirtualizing="True"
                        VirtualizingPanel.VirtualizationMode="Recycling"
                        ScrollViewer.CanContentScroll="False"
                    
                        KeyboardNavigation.TabNavigation="Contained"
                        ItemsSource="{Binding INVO_DATA_RASID_KHARID}"
                        Grid.Row="5"
                        Margin="5,10"
                        AutoGenerateColumns="False"  FlowDirection="RightToLeft"  SelectionUnit="FullRow"   BorderThickness="1" >
       
            <DataGrid.Columns>

                <DataGridComboBoxColumn x:Name="ANBAR_COLUMN" MinWidth="80" Width="auto" Header=" انبار " 
                                        SelectedValueBinding="{Binding ANBAR,UpdateSourceTrigger=PropertyChanged}"
                                        DisplayMemberPath="NAMES" SelectedValuePath="CODE">
                    <DataGridComboBoxColumn.EditingElementStyle>
                        <Style TargetType="{x:Type ComboBox}">
                            <Setter Property="IsEditable" Value="True"/>
                        </Style>
                    </DataGridComboBoxColumn.EditingElementStyle>

                </DataGridComboBoxColumn>
       
            </DataGrid.Columns>
        </DataGrid>


Code behind of my Window (C#):

//MainWindow:
    public partial class HEAD_LST_RASID : Window
    {
        public ObservableCollection<INVO_LST_RASID_KHARID_CSHARP> INVO_DATA_RASID_KHARID { get; set; } = new ObservableCollection<INVO_LST_RASID_KHARID_CSHARP>();

        public HEAD_LST_RASID()
        {
            InitializeComponent();
            this.DataContext = this;
        }
        public void FillALLComboboxes()
        {
            string RowSource_ANBAR = "SELECT    CODE, NAMES FROM  dbo.TCOD_ANBAR";
            //Search for the default value if it exists
            var rst = dbms.Database.SqlQuery<int?>("SELECT     ANBCO FROM dbo.OPANBACCESS WHERE     (USERCO = " + Baseknow.USERCOD + " ) ORDER BY dbo.OPANBACCESS.RDF").ToList();
            if (rst.Count > 0)
            {
                Baseknow.anbardef = (int)rst.FirstOrDefault();
            }
            var ARST = dbms.Database.SqlQuery<Custom_TCODANBAR>(RowSource_ANBAR).ToList();
            ANBAR_COLUMN.ItemsSource = ARST;
        }
        private void Window_Loaded(object sender, RoutedEventArgs e)
        {
            FillALLComboboxes();
            
              var QRE_LST = dbms.Database.SqlQuery<INVO_LST_RASID_KHARID_CSHARP>($@"SELECT * FROM INVOICES").ToList();
              INVO_DATA_RASID_KHARID?.Clear();
              
                foreach (var item in QRE_LST)
                    INVO_DATA_RASID_KHARID.Add(item);
        }
    }
Model Class :

//Model:
    public class CTABLES
    {
        public class INVO_LST_RASID_KHARID_CSHARP : INotifyPropertyChanged, ICloneable
        {
            public object Clone()
            {
                return this.MemberwiseClone();
            }
        
            public INVO_LST_RASID_KHARID_CSHARP()
            {
                ANBAR = Baseknow.anbardef;
            }

            private int? _anbar;
            public int? ANBAR
            {
                get
                {
                    return _anbar;
                }
                set
                {
                    if (_anbar == value)
                        return;
                    _anbar = value;
                    OnPropertyChanged("ANBAR");
                }
            }

            public event PropertyChangedEventHandler PropertyChanged;
            protected virtual void OnPropertyChanged(string propertyName = "")
            {
                PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
            }
        }
    }

What I want is to show that a default value has been selected for this combobox inside the 'DataGrid' when the 'DataGrid' data is loaded (without the need to double-click and go to CellEditing mode).