Database

DECLARE @UserID INT
SET @UserID = 1

DECLARE @UtopiaOrderNumber NVARCHAR
SET @UtopiaOrderNumber = (SELECT RIGHT(REPLICATE('0', 3) + CONVERT(NVARCHAR(3), @UserID), 3))

PRINT @UtopiaOrderNumber -- prints 0 instead of 001

DECLARE @UserID INT
SET @UserID = 1

SELECT RIGHT(REPLICATE('0', 3) + CONVERT(NVARCHAR(3), @UserID), 3) -- selects 001 as expected

internal ObservableCollection<INVCategory> GetCategoryList()
{
      DataLoadOptions dataLoadOptions = new DataLoadOptions();
      dataLoadOptions.LoadWith<INVCategory>(t => t.INVSubCategories);
      this.Context.LoadOptions = dataLoadOptions;
             
      var categories = from category in this.Context.INVCategories
                       orderby category.CatgeoryId descending
                       select category;
      return new   ObservableCollection<INVCategory>(categories.ToList());
}

<ComboBox Grid.Column="1" Grid.Row="0" HorizontalAlignment="Stretch"  VerticalAlignment="Stretch" Margin="0,2,10,2" TabIndex="0" x:Name="categoryComboBox" ItemsSource="{Binding CategoryList}" IsEditable="True" DisplayMemberPath="CategoryName"  SelectedValuePath="CatgeoryId" SelectedItem="{Binding CategoryList, Mode=TwoWay}" SelectedValue="{Binding Path=CurrentEntity.CategoryId, Mode=TwoWay}">
</ComboBox>

<ComboBox Grid.Column="1" Grid.Row="1" HorizontalAlignment="Stretch" VerticalAlignment="Stretch" Margin="0,2,10,2" TabIndex="1" ItemsSource="{Binding SelectedItem, ElementName=categoryComboBox, Mode=OneWay}" DisplayMemberPath="SubCategoryName" SelectedValuePath="SubCategoryId" SelectedItem="{Binding INVSubCategories, Mode=TwoWay}" >
</ComboBox>