The following example demonstrates how to stretch all the columns in a grid equally so that they occupy the full width available in the viewport.

XAML
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<Grid xmlns:xcdg="http://schemas.xceed.com/wpf/xaml/datagrid">
   <Grid.Resources>
      <xcdg:DataGridCollectionViewSource x:Key="cvs_orders"
                                         Source="{Binding Source={x:Static Application.Current}, Path=Orders}"
                                         AutoCreateItemProperties="False">
         <xcdg:DataGridCollectionViewSource.ItemProperties>
            <xcdg:DataGridItemProperty Name="ShipCountry" />
            <xcdg:DataGridItemProperty Name="ShipCity" />
            <xcdg:DataGridItemProperty Name="ShipRegion" />
            <xcdg:DataGridItemProperty Name="ShipVia" />
         </xcdg:DataGridCollectionViewSource.ItemProperties>
      </xcdg:DataGridCollectionViewSource>
   </Grid.Resources>
   <xcdg:DataGridControl x:Name="OrdersGrid"
                         ItemsSource="{Binding Source={StaticResource cvs_orders}}">
      <xcdg:DataGridControl.View>
         <xcdg:TableView ColumnStretchMode="All"
                         ColumnStretchMinWidth="100"/>
      </xcdg:DataGridControl.View>
   </xcdg:DataGridControl>
</Grid>
VB.NET
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Dim collectionView As New DataGridCollectionView( Orders, GetType( System.Data.DataRow ), False, False )
collectionView.ItemProperties.Add( New DataGridItemProperty( "ShipCountry", GetType( String ) ) )
collectionView.ItemProperties.Add( New DataGridItemProperty( "ShipCity", GetType( String ) ) )
collectionView.ItemProperties.Add( New DataGridItemProperty( "ShipRegion", GetType( String ) ) )
collectionView.ItemProperties.Add( New DataGridItemProperty( "ShipVia", GetType( Integer ) ) )
Dim view As New TableView()
view.ColumnStretchMode = ColumnStretchMode.All
view.ColumnStretchMinWidth = 100
dataGridControl.View = view
dataGridControl.ItemsSource = collectionView
C#
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DataGridCollectionView collectionView = new DataGridCollectionView( Orders, typeof( System.Data.DataRow ), false, false );
collectionView.ItemProperties.Add( new DataGridItemProperty( "ShipCountry", typeof( string ) ) );
collectionView.ItemProperties.Add( new DataGridItemProperty( "ShipCity", typeof( string ) ) );
collectionView.ItemProperties.Add( new DataGridItemProperty( "ShipRegion", typeof( string ) ) );
collectionView.ItemProperties.Add( new DataGridItemProperty( "ShipVia", typeof( int) ) );
TableView view = new TableView();
view.ColumnStretchMode = ColumnStretchMode.All;
view.ColumnStretchMinWidth = 100;
dataGridControl.View = view;
dataGridControl.ItemsSource = collectionView;