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;