public static int combo_index

    public void InsertZ_speciality(z_speciality z_speciality)
        {
            if ((z_speciality.EntityState != EntityState.Detached))
            {
                this.ObjectContext.ObjectStateManager.ChangeObjectState(z_speciality, EntityState.Added);
            }
            else
            {
                
                z_speciality.edit_date = DateTime.Parse(DateTime.Now.ToString
                                        ("yyyy.MM.dd HH:mm", new CultureInfo("ru-RU")) + "\n");
                z_speciality.combo_index=combo_index;//для заполнения поля combo_index в таблице z_speciality расположенной на MSSQL-Server
                z_speciality.delete = "notdelete";
                this.ObjectContext.z_speciality.AddObject(z_speciality);
            }
        }

<toolkit:DataForm x:Name="DFAdd_Inst"
                           Style="{StaticResource DataFormStyle_inst}" >
            <toolkit:DataForm.NewItemTemplate>
                <DataTemplate>
                    <StackPanel>
                        <toolkit:DataField Label="Наименование" FontWeight="Bold" Description="Введите наименование института">
                            <TextBox Text="{Binding Path=name, Mode=TwoWay}" FontWeight="Normal" />
                        </toolkit:DataField>
                       <toolkit:DataField Label="Сокащение" FontWeight="Bold" Description="Сокращение наименования института">
                            <TextBox Text="{Binding Path=s_name, Mode=TwoWay}" FontWeight="Normal" />
                        </toolkit:DataField>
                    </StackPanel>
                </DataTemplate>
            </toolkit:DataForm.NewItemTemplate>
        </toolkit:DataForm>

            this.newRecord = new rb_struc();
            this.DFAdd_Inst.CurrentItem = this.newRecord;
            this.newRecord_i = new z_institute();
            this.DFAdd_Inst.CurrentItem = this.newRecord_i;

<toolkit:DataField Grid.Column="2" Label="City:">
                                            <ComboBox x:Name="cbCity" ItemsSource="{Binding ElementName=cbddsCity, Path=Data}"  DisplayMemberPath="Name" ex:ComboBox.Mode="AsyncEager"/>
                                        </toolkit:DataField>

 <toolkit:DataField Label="Last name">
       <TextBox Text="{Binding Path=City.Name, Mode=TwoWay}"/>
 </toolkit:DataField>

var tbVolume = DFAdd_Inst.FindNameInContent("Boxshort");//Boxshort-его имя

 var tbVolume = DFAdd_Inst.FindNameInContent("Boxshort") as TextBox;
 var e = tbVolume.Text;