<ComboBox ItemsSource="{Binding Source={StaticResource cbddsEmployee}, Path=Data}" SelectedItem="{Binding Path=Employee, Mode=TwoWay}" DisplayMemberPath="FirstName" ex:ComboBox.Mode="AsyncEager"/>

private NORTHWNDDomainContext domainContext { get { return Application.Current.Resources["DomainContext"] as NORTHWNDDomainContext; } }

private NORTHWNDDomainContext domainContext = new NORTHWNDDomainContext();

<appWeb:NORTHWNDDomainContext x:Key="DomainContext"/> 

private  Customer newRecord { get; set; }

this.domainContext.Customers.Add(newRecord); 

this.domainContext.SubmitChanges(SubmitCompleted, null);

<toolkit:DataForm >
 <toolkit:DataForm.NewItemTemplate>
   <DataTemplate>
   </DataTemplate>
 </toolkit:DataForm.NewItemTemplate>
</toolkit:DataForm>

<toolkit:DataForm >
  <toolkit:DataForm.EditTemplate>
    <DataTemplate>
    </DataTemplate>
 </toolkit:DataForm.EditTemplate>
</toolkit:DataForm>

<toolkit:DataForm.EditTemplate>

<toolkit:DataForm.ReadOnlyTemplate>

 <toolkit:DataForm.NewItemTemplate>

<toolkit:DataForm.HeaderTemplate>

 <toolkit:DataField Label="Employee">
<ComboBox ItemsSource="{Binding Source={StaticResource cbddsEmployee}, Path=Data}" SelectedItem="{Binding Path=Employee, 
Mode=TwoWay}" DisplayMemberPath="FirstName" ex:ComboBox.Mode="AsyncEager"/> </toolkit:DataField>

 <toolkit:DataField Label="Employee" IsRequired="True" >

[Required]
[Include]
public Employee Employee { get; set; }

<toolkit:DataForm x:Name="dfAddRecord" CommandButtonsVisibility="All" EditEnding="dfAddRecord_EditEnding">

private void dfAddRecord_EditEnding(object sender, DataFormEditEndingEventArgs e)
        {
            if (e.EditAction == DataFormEditAction.Commit)
            {
                if (((Order)(((DataForm)sender).CurrentItem)).Employee == null)
                {
                    e.Cancel = true;
                    MessageBox.Show("Поле Employee пустое");
                }
           }
        }

Style TargetType="riaControls:DomainDataSource" x:Key="DDS">
        <Setter Property="LoadSize" Value="{StaticResource DDSLoadSize}"/>
        <Setter Property="PageSize" Value="{StaticResource DDSPageSize}"/>
        <Setter Property="AutoLoad" Value="True"/>
        <Setter Property="DomainContext" Value="{StaticResource DomainContext}"/>

    </Style>

<riaControls:DomainDataSource AutoLoad="True" LoadedData="teamDomainDataSource_LoadedData" Name="teamDomainDataSource" QueryName="GetTeamQuery" 
                                      Style="{StaticResource DDS}" >
            <riaControls:DomainDataSource.FilterDescriptors>
                <riaControls:FilterDescriptor PropertyPath="ProjectId" Operator="IsEqualTo" Value="{Binding projectid}"/>
            </riaControls:DomainDataSource.FilterDescriptors>
        </riaControls:DomainDataSource>

  
publicIQueryable<MyEntity> GetMyEntity(string filter)
  {
   //…
  }