Validating ComboBox within a DataGridTemplateColumn

Hello!

I am having a problem with validating ComboBox within a DataGridTemplateColumn.  

When I add a new DataGrid record, I need to select a mandatory contact from a Combobox list. If I double click on the Combobox and then select a different record without selecting a value from the list, the validation works and the corresponding TextBlock borders turn red. This is because the contact ID (FK_Contact_ID) is to zero and the criteria is that FK_Contact_ID should have positive value.

However, if after adding a new record, I select another record without double clicking on the combobox, validation does not take place.

I use SQL Server 2012 and Viusal Studion 2012 with Entity Framework for mapping tables. I have implemented MVVM pattern.

I have included below a snippet of my code; please let me know if more information is required.

What I have noticed is that when I double click on the combobox, after adding a new DataGrid record, the string IDataErrorInfo.this[string columnName] indexer in partial class tblContactAddress gets called, the case "FK_Contact_ID" of switch (columnName) gets invoked and then ValidationHasError is set to true for the newly added record because FK_Contact_ID is set to zero which is invalid.  However, if I select another record, without double clicking on the combobox in the new record the string indexer does not get called.

Your assistance is greatly appreciated.

Regards

namespace App.Domain
{
    internal interface IContactAddress
    {
        [Range(1, int.MaxValue, ErrorMessageResourceName = "ContactMandatory", ErrorMessageResourceType = typeof(Resources))]
        Int32 FK_Contact_ID { get; set; }
    }

    [MetadataType(typeof(IContactAddress))]
    public partial class tblContactAddress : IContactAddress, IDataErrorInfo
    {
        [NonSerialized]
        private readonly DataErrorInfoSupport dataErrorInfoSupport;

        public tblContactAddress()
        {
            dataErrorInfoSupport = new DataErrorInfoSupport(this);
        }

        string IDataErrorInfo.Error { get { return dataErrorInfoSupport.Error; } }

        string IDataErrorInfo.this[string columnName]
        {
            get
            {
                switch (columnName)
                {
                    case "FK_Contact_ID":
                        break;

                    default:
                        result = dataErrorInfoSupport[columnName];
                        break;
                }
                if (string.IsNullOrEmpty(result))
                {
                    ValidationHasError = false;
                    result = dataErrorInfoSupport[columnName];
                }
                else
                {
                    ValidationHasError = true;
                }
                return result;
            }
        }
    }
}

XAML:

<UserControl
    x:Class="App.ContactAddressListView"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:c="clr-namespace:App.Converters"
    mc:Ignorable="d"
    d:DesignWidth="500" d:DesignHeight="150" ><UserControl.Resources><Style TargetType="{x:Type DataGridCell}"><EventSetter Event="PreviewMouseLeftButtonDown" Handler="DataGridCell_PreviewMouseLeftButtonDown" /></Style><c:StyleConverter x:Key="xStyleConverter" /></UserControl.Resources><DockPanel><DataGrid.Columns><DataGridTemplateColumn Header="{x:Static p:LocalResources.Contact}" x:Name="xContactCBox"><DataGridTemplateColumn.CellTemplate><DataTemplate ><TextBlock Margin="0"><TextBlock.Style><MultiBinding Converter="{StaticResource xStyleConverter}"><Binding Path="FK_Contact_ID" /></MultiBinding></TextBlock.Style><TextBlock.Text><Binding Path="LinkedToContact.ContactName" Mode="OneWay"
                                        ValidatesOnDataErrors="True" ValidatesOnExceptions="True" NotifyOnSourceUpdated="True" /></TextBlock.Text></TextBlock></DataTemplate></DataGridTemplateColumn.CellTemplate><DataGridTemplateColumn.CellEditingTemplate><DataTemplate><ComboBox Margin="0" IsEditable="True"
                                ItemsSource="{Binding DataContext.ContactList,
                                    RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type UserControl}}}"
                                    SelectedValue="{Binding Path=FK_Contact_ID, UpdateSourceTrigger=PropertyChanged,
                                        ValidatesOnDataErrors=True, ValidatesOnExceptions=True, NotifyOnValidationError=True}"
                                    DisplayMemberPath="ContactName" SelectedValuePath="Contact_ID"></ComboBox></DataTemplate></DataGridTemplateColumn.CellEditingTemplate></DataGridTemplateColumn></DataGrid.Columns></DataGrid></DockPanel></UserControl>

namespace App.Converters
{
    public class StyleConverter: IMultiValueConverter
    {
        public object Convert(object[] value, Type targetType, object parameter, CultureInfo culture)
        {
            int valueAsDecimal = (int)value[0];

            if (valueAsDecimal > 0)
                return Application.Current.FindResource("TextBlockCellTemplateSytle") as Style;

            return Application.Current.FindResource("TextBlockVaidationStyle") as Style;
        }

        public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
        {
            return null;
        }
    }
}

Tables:

tblContactAddress >> ContactAddress_ID (int, PK) , FK_Contact_ID (int), Address, other fields...

tblContact >> Contact_ID (int, PK), ContactName (string).

Navigation Property: 

LinkedToContact >> (From Role: tblContactAddress, To Role: tblContact)

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Abbas