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[Query(IsComposable=false)]
public Customer GetCustomersByID(int customerID)
{
    return this.ObjectContext.Customers.SingleOrDefault(
            c => c.CustomerID == customerID);
}

public IQueryable<Customer> GetCustomersByLastNameLetter(
            string startingLastNameLetter)
{
    return this.ObjectContext.Customers.Where
             (c => c.LastName.StartsWith(
                      startingLastNameLetter) == true);
}

<UserControl xmlns:data="clr-namespace:System.Windows.Controls;
                   assembly=System.Windows.Controls.Data"  
    x:Class="RIAServicesExample.MainPage"
    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"
    mc:Ignorable="d"
    d:DesignHeight="300" d:DesignWidth="400">

    <Grid x:Name="LayoutRoot" Background="White">
        <Grid.ColumnDefinitions>
            <ColumnDefinition></ColumnDefinition>
            <ColumnDefinition></ColumnDefinition>
        </Grid.ColumnDefinitions>
        <Grid.RowDefinitions>
            <RowDefinition Height="25"></RowDefinition>
            <RowDefinition></RowDefinition>
        </Grid.RowDefinitions>
        <StackPanel Orientation="Horizontal" Grid.Row="0" Grid.Column="0">
            <TextBlock Text="search by id: " ></TextBlock>
            <TextBox Name="IDValue" Width="50" ></TextBox>
            <Button Name="IDButton" 
               Click="IDButton_Click" Content="Submit">
            </Button>
        </StackPanel>
        <StackPanel Orientation="Horizontal" Grid.Row="0" Grid.Column="1">
            <TextBlock Text="search by name: "></TextBlock>
            <TextBox Name="LetterValue" Width="30"></TextBox>
            <Button Name="LetterButton" Click="LetterButton_Click" 
                                  Content="Submit"></Button>
        </StackPanel>

      <data:DataGrid Grid.Row="1" Grid.Column="0" 
               Grid.ColumnSpan="2" Name="CustomerGrid">
      </data:DataGrid>
    </Grid>
</UserControl>

using System;
using System.Windows;
using System.Windows.Controls;
using RIAServicesExample.Web;
using System.ServiceModel.DomainServices.Client;

namespace RIAServicesExample
{

    public partial class MainPage : UserControl
    {
        private CustomerDomainContext _customerContext =
                          new CustomerDomainContext();

        public MainPage()
        {
            InitializeComponent();
        }

        private void LetterButton_Click(object sender, RoutedEventArgs e)
        {
            IDButton.IsEnabled = false;
            LetterButton.IsEnabled = false;
            LoadOperation<Customer> loadOp = 
             this._customerContext.Load(
             this._customerContext.GetCustomersByLastNameLetterQuery(
             LetterValue.Text), CustomerLoadedCallback, null);
            CustomerGrid.ItemsSource = loadOp.Entities;
        }

        private void IDButton_Click(object sender, RoutedEventArgs e)
        {
            IDButton.IsEnabled = false;
            LetterButton.IsEnabled = false;
            LoadOperation<Customer> loadOp =
               this._customerContext.Load(
               this._customerContext.GetCustomersByIDQuery(
               int.Parse(IDValue.Text)), CustomerLoadedCallback, null);
            CustomerGrid.ItemsSource = loadOp.Entities;
        }

        void CustomerLoadedCallback(
            LoadOperation<Customer> loadOperation)
        {
            IDButton.IsEnabled = true;
            LetterButton.IsEnabled = true;
        }
    }
}