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How to: Project Data Service Query Results (WCF Data Services/Silverlight)

Projection is defined in the Open Data Protocol (OData) as a way to reduce the amount of data returned by a query by specifying that only certain properties of an entity are returned in the response. This enables you to project the data from an entity type in the data service into a different entity type at the client. You can perform projections on the results of a LINQ query by using the select clause (Select in Visual Basic). You can also call the AddQueryOption() method to add a $select clause to the query URI. For more information, see Querying the Data Service (WCF Data Services).

The Northwind sample data service accessed by the application is created when you complete the procedures in the topic How to: Create the Northwind Data Service (WCF Data Services/Silverlight). You can also use the Northwind sample data service that is published on the OData Web site; this sample data service is read-only and attempting to save changes returns an error.

The following example shows a LINQ query that projects Customer entities into the CustomerAddress type that has already been defined in the client application. The CustomerAddress class is defined on the client and is attributed so that the client library can recognize it as an entity type. This new type contains only address-specific properties plus the identity property.


using System;
using System.Linq;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using ClientProjection.Northwind;
using System.Data.Services.Client;

namespace ClientProjection
{
    public partial class MainPage : UserControl
    {
        // Create the binding collections and the data service context.
        private DataServiceCollection<CustomerAddress> binding;
        NorthwindEntities context;
        CollectionViewSource customerAddressViewSource;

        public MainPage()
        {
            InitializeComponent();
        }

        private void MainPage_Loaded(object sender, RoutedEventArgs e)
        {
            // Instantiate the binding collection.
            binding = new DataServiceCollection<CustomerAddress>();

            // Instantiate the context.
            context = 
                new NorthwindEntities(new Uri("http://localhost:12345/Northwind.svc/"));

            // Register the LoadCompleted event for the binding collection.
            binding.LoadCompleted += new EventHandler<LoadCompletedEventArgs>(binding_LoadCompleted);

            // Define an anonymous LINQ query that projects the Customers type into 
            // a CustomerAddress type that contains only address properties.
            var query = from c in context.Customers
                        where c.Country == "Germany"
                        select new CustomerAddress
                        {
                            CustomerID = c.CustomerID,
                            Address = c.Address,
                            City = c.City,
                            PostalCode = c.PostalCode,
                            Country = c.Country
                        };

            // Execute the query asynchronously.
            binding.LoadAsync(query);
        }

        void binding_LoadCompleted(object sender, LoadCompletedEventArgs e)
        {
            if (e.Error == null)
            {
                // Load all pages of Customers before binding.
                if (binding.Continuation != null)
                {
                    binding.LoadNextPartialSetAsync();
                }
                else
                {
                    //Load your data here and assign the result to the CollectionViewSource.

                    customerAddressViewSource = 
                        (CollectionViewSource)this.Resources["customerAddressViewSource"];
                    customerAddressViewSource.Source = binding;
                }
            }
        }
        private void saveChangesButton_Click(object sender, RoutedEventArgs e)
        {
            try
            {
                // Start the saving changes operation. 
                context.BeginSaveChanges(OnChangesSaved, context);
            }
            catch (Exception ex)
            {
                MessageBox.Show(string.Format("The changes could not be saved to the data service.\n"
                    + "The following error occurred: {0}", ex.Message));
            }
        }

        private void OnChangesSaved(IAsyncResult result)
        {
            bool errorOccured = false;

            // Use the Dispatcher to ensure that the 
            // asynchronous call returns in the correct thread.
            Dispatcher.BeginInvoke(() =>
            {
                context = result.AsyncState as NorthwindEntities;

                try
                {
                    // Complete the save changes operation and display the response.
                    DataServiceResponse response = context.EndSaveChanges(result);

                    foreach (ChangeOperationResponse changeResponse in response)
                    {
                        if (changeResponse.Error != null) errorOccured = true;
                    }
                    if (!errorOccured)
                    {
                        MessageBox.Show("The changes have been saved to the data service.");
                    }
                    else
                    {
                        MessageBox.Show("An error occured. One or more changes could not be saved.");
                    }
                }
                catch (Exception ex)
                {
                    // Display the error from the response.
                    MessageBox.Show(string.Format("The following error occured: {0}", ex.Message));
                }
            }
            );
        }
    }
}


The following example shows the definition of the CustomerAddress type that is used in the previous example.


[DataServiceKey("CustomerID")]
public partial class CustomerAddress : INotifyPropertyChanged
{
    private string _customerID;
    private string _address;
    private string _city;
    private string _postalCode;
    private string _country;

    public string CustomerID
    {
        get { return this._customerID; }
        set
        {
            this.OnCustomerIDChanging(value);
            this._customerID = value;
            this.OnCustomerIDChanged();
            this.OnPropertyChanged("CustomerID");
        }
    }
    public string Address
    {
        get { return this._address; }
        set
        {
            this.OnAddressChanging(value);
            this._address = value;
            this.OnAddressChanged();
            this.OnPropertyChanged("Address");
        }
    }
    public string City
    {
        get { return this._city; }
        set
        {
            this.OnCityChanging(value);
            this._city = value;
            this.OnCityChanged();
            this.OnPropertyChanged("City");
        }
    }
    public string PostalCode
    {
        get { return this._postalCode; }
        set
        {
            this.OnPostalCodeChanging(value);
            this._postalCode = value;
            this.OnPostalCodeChanged();
            this.OnPropertyChanged("PostalCode");
        }
    }
    public string Country
    {
        get { return this._country; }
        set
        {
            this.OnCountryChanging(value);
            this._country = value;
            this.OnCountryChanged();
            this.OnPropertyChanged("Country");
        }
    }
    #region INotifyPropertyChanged implementation
    // Members required by the INotifyPropertyChanged implementation.
    public event PropertyChangedEventHandler PropertyChanged;
    protected virtual void OnPropertyChanged(string property)
    {
        if ((this.PropertyChanged != null))
        {
            this.PropertyChanged(this, new PropertyChangedEventArgs(property));
        }
    }
    partial void OnCustomerIDChanging(string value);
    partial void OnCustomerIDChanged();
    partial void OnAddressChanging(string value);
    partial void OnAddressChanged();
    partial void OnCityChanging(string value);
    partial void OnCityChanged();
    partial void OnRegionChanging(string value);
    partial void OnRegionChanged();
    partial void OnPostalCodeChanging(string value);
    partial void OnPostalCodeChanged();
    partial void OnCountryChanging(string value);
    partial void OnCountryChanged();
    #endregion
}


The following XAML defines the main page of the Silverlight application.


    <UserControl x:Class="ClientProjection.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="312" d:DesignWidth="577" 
             xmlns:sdk="http://schemas.microsoft.com/winfx/2006/xaml/presentation/sdk" 
             xmlns:my="clr-namespace:ClientProjection" Loaded="MainPage_Loaded">
    <UserControl.Resources>
        <CollectionViewSource x:Key="customerAddressViewSource" 
                              d:DesignSource="{d:DesignInstance my:CustomerAddress, CreateList=True}" />
    </UserControl.Resources>
    <Grid x:Name="LayoutRoot" Background="White" DataContext="" Height="312" Width="577"
          VerticalAlignment="Top" HorizontalAlignment="Left">
        <Grid.RowDefinitions>
            <RowDefinition Height="203*" />
            <RowDefinition Height="119*" />
        </Grid.RowDefinitions>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="17*" />
            <ColumnDefinition Width="336*" />
        </Grid.ColumnDefinitions>
        <sdk:DataGrid AutoGenerateColumns="False" Height="233" HorizontalAlignment="Left" 
                      ItemsSource="{Binding Source={StaticResource customerAddressViewSource}}" 
                      Name="customerAddressDataGrid" RowDetailsVisibilityMode="VisibleWhenSelected" 
                      VerticalAlignment="Top" Width="553" Margin="12,24,0,0" 
                      Grid.RowSpan="2" Grid.ColumnSpan="2">
            <sdk:DataGrid.Columns>
                <sdk:DataGridTextColumn x:Name="customerIDColumn" Binding="{Binding Path=CustomerID}" 
                                        Header="Customer" Width="80" />
                <sdk:DataGridTextColumn x:Name="addressColumn" Binding="{Binding Path=Address}" 
                                        Header="Address" Width="180" />
                <sdk:DataGridTextColumn x:Name="cityColumn" Binding="{Binding Path=City}" 
                                        Header="City" Width="120" />
                <sdk:DataGridTextColumn x:Name="countryColumn" Binding="{Binding Path=Country}" 
                                        Header="Country" Width="80" />
                <sdk:DataGridTextColumn x:Name="postalCodeColumn" Binding="{Binding Path=PostalCode}" 
                                        Header="Postal Code" Width="90" />
            </sdk:DataGrid.Columns>
        </sdk:DataGrid>
        <Button Content="Save" Grid.Column="1" HorizontalAlignment="Left" 
                Margin="462,78,0,0" Name="saveChangesButton" Width="75" Grid.Row="1" 
                Click="saveChangesButton_Click" Height="25" VerticalAlignment="Top" />
    </Grid>
</UserControl>


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