The following example demonstrates how to create a custom CellValidationRule and add it to a column's CellValidationRules collection to provide UI-level validation.

The implementation for the PeriodVSCompositionCountCellValidationRule and Person class are provided below.

XAML
Copy Code 
<Grid xmlns:xcdg="http://schemas.xceed.com/wpf/xaml/datagrid"
     xmlns:local="clr-namespace:Xceed.Wpf.Documentation">
  <Grid.Resources>
     <xcdg:DataGridCollectionViewSource x:Key="cvs_composers"
                                        Source="{Binding Source={x:Static Application.Current},
                                                         Path=Composers}"/>
     <!--A data provider to bind to the Period enum-->
     <ObjectDataProvider x:Key="periods"
                         MethodName="GetValues"
                         ObjectType="{x:Type local:Period}">
        <ObjectDataProvider.MethodParameters>
           <x:Type TypeName="local:Period"/>
        </ObjectDataProvider.MethodParameters>
     </ObjectDataProvider>
     <!--A cell editor that will be used to edit a Period column with a combo box-->
     <xcdg:CellEditor x:Key="periodEditor">
        <xcdg:CellEditor.EditTemplate>
           <DataTemplate>
              <ComboBox BorderThickness="0"
                        MinHeight="22"
                        VerticalContentAlignment="Top"
                        SelectedValuePath="."
                        ItemsSource="{Binding Source={StaticResource periods}}"
                        SelectedValue="{xcdg:CellEditorBinding}">
                 <ComboBox.Resources>
                    <Style TargetType="Popup">
                       <Setter Property="TextElement.Foreground"
                               Value="{DynamicResource {x:Static SystemColors.WindowTextBrushKey}}" />
                    </Style>
                 </ComboBox.Resources>
              </ComboBox>
           </DataTemplate>
        </xcdg:CellEditor.EditTemplate>
     </xcdg:CellEditor>
  </Grid.Resources>
  <xcdg:DataGridControl ItemsSource="{Binding Source={StaticResource cvs_composers}}"
                        UpdateSourceTrigger="RowEndingEdit">
     <xcdg:DataGridControl.Columns>
       <xcdg:Column FieldName="Period"
                    CellEditor="{StaticResource periodEditor}">                                   
          <xcdg:Column.CellValidationRules>
             <local:PeriodVSCompositionCountCellValidationRule/>
          </xcdg:Column.CellValidationRules>
       </xcdg:Column>
       <xcdg:Column FieldName="CompositionCount">
          <xcdg:Column.CellValidationRules>
             <local:PeriodVSCompositionCountCellValidationRule />
          </xcdg:Column.CellValidationRules>
       </xcdg:Column>
     </xcdg:DataGridControl.Columns>
  </xcdg:DataGridControl>
</Grid>

Implementation of the PeriodVSCompositionCountCellValidationRule validation rule.

VB.NET
Copy Code 
Imports System
Imports Xceed.Wpf.DataGrid.ValidationRules
Imports Xceed.Wpf.DataGrid
Imports System.Globalization
Imports System.Windows.Controls
Namespace Xceed.Wpf.Documentation
  Public Class PeriodVSCompositionCountCellValidationRule
               Inherits CellValidationRule
    Public Overrides Function Validate( ByVal value As Object, ByVal culture As CultureInfo, _
                                        ByVal context As CellValidationContext ) As ValidationResult
      Dim parentRow As Row = context.Cell.ParentRow
      Dim compositionCount As Integer
      Dim period As Period
      If context.Cell.FieldName = "Period" Then
        period = CType( value, Period )
        compositionCount = CInt( parentRow.Cells( "CompositionCount" ).Content )
      Else
        period = CType( parentRow.Cells( "Period" ).Content, Period )
        compositionCount = CInt( value )
      End If
      If( ( period = Period.Modern ) And ( compositionCount > 40 ) ) Then
        Return New ValidationResult( False, "Composition count must be less than 50 when the period is set to Modern." );
      End If
      Return ValidationResult.ValidResult
    End Function
  End Class
End Namespace
C#
Copy Code 
using System;
using Xceed.Wpf.DataGrid.ValidationRules;
using Xceed.Wpf.DataGrid;
using System.Globalization;
using System.Windows.Controls;
namespace Xceed.Wpf.Documentation
{ 
public class PeriodVSCompositionCountCellValidationRule : CellValidationRule
 {
   public override ValidationResult Validate( object value, CultureInfo culture,
                                              CellValidationContext context )
   {
     Row parentRow = context.Cell.ParentRow;
     int compositionCount;
     Period period;
     if( context.Cell.FieldName == "Period" )
     {
       period = ( Period )value;       
       compositionCount = ( int )parentRow.Cells[ "CompositionCount" ].Content;
     }
     else
     {
       period = ( Period )parentRow.Cells[ "Period" ].Content;
       compositionCount = ( int )value;
     }
     if( ( period == Period.Modern ) && compositionCount > 40 )
       return new ValidationResult( false, "Composition count must be less than 50 when the period is set to Modern." );
     return ValidationResult.ValidResult;
   }
 }
}

Implementation of the Person class

VB.NET
Copy Code 
Imports System.ComponentModel
Imports System.Windows.Media
Namespace Xceed.Wpf.Documentation
  Public Class Person
               Implements INotifyPropertyChanged
    Public Sub New( personID As Integer, firstName As String, lastName As String, age As Integer )
      m_personID = personID
      m_firstName = firstName
      m_lastName = lastName
      m_age = age
    End Sub
    public Sub New()
    End sub   
    Public ReadOnly Property PersonID As Integer
      Get
        Return m_personID
      End Get
    End Property
    Public Property FirstName As String
      Get
        Return m_firstName
      End Get
      Set( ByVal value As String )
        If m_firstName <> value Then
          m_firstName = value
          Me.OnPropertyChanged( "FirstName" )
        End If
      End Set
    End Property
    Public Property LastName As String
      Get
        Return m_lastName
      End Get
      Set( ByVal value As String )
        If m_lastName <> value Then
          m_lastName = value
          Me.OnPropertyChanged( "LastName" )
        End If
      End Set
    End Property
    Public Property Age As Integer
      Get
        Return m_age
      End Get
      Set( ByVal value As Integer )
        If m_age = value Then
          Return
        End If
        m_age = value
        Me.OnPropertyChanged( "Age" )
      End Set
    End Property
    ' INotifyPropertyChanged implementation
    Public Event PropertyChanged As PropertyChangedEventHandler Implements INotifyPropertyChanged.PropertyChanged
    Private Sub OnPropertyChanged( ByVal propertyName As String )
      If Not Me.PropertyChanged Is Nothing Then
        Me.PropertyChanged( Me, New PropertyChangedEventArgs( propertyName ) )
      End If
    End Sub
    private string m_firstName = string.Empty;
    private string m_lastName = string.Empty;
    private m_personID As Integer
    Private m_age As Integer = 18
  End Class
End Namespace
C#
Copy Code 
using System.ComponentModel;
using System.Windows.Media;
namespace Xceed.Wpf.Documentation
{
  public class Person: INotifyPropertyChanged, IDataErrorInfo
  {
    public Person( int personID, string firstName, string lastName, int age )
    {
      m_personID = personID;
      m_firstName = firstName;
      m_lastName = lastName;
      m_age = age;
    }
    public Person()
    {
    }
    public int PersonID
    {
      get
      {
        return m_personID;
      }
    }
    public string FirstName
    {
      get
      {
        return m_firstName;
      }
      set
      {
        if( m_firstName != value )
        {
          m_firstName = value;
          this.OnPropertyChanged( "FirstName" );
        }
      }
    }
    public string LastName
    {
      get
      {
        return m_lastName;
      }
      set
      {
        if( m_lastName != value )
        {
          m_lastName = value;
          this.OnPropertyChanged( "LastName" );
        }
      }
    }
    public int Age
    {
      get
      {
        return m_age;
      }
      set
      {
        if( m_age == value )
          return;
        m_age = value;
        this.OnPropertyChanged( "Age" );
      }
    }
    // INotifyPropertyChanged implementation
    public event PropertyChangedEventHandler PropertyChanged;
    private void OnPropertyChanged( string propertyName )
    {
      if( this.PropertyChanged != null )
        this.PropertyChanged( this, new PropertyChangedEventArgs( propertyName ) );
    }
    // IDataErrorInfo implementation
    [EditorBrowsable( EditorBrowsableState.Never ) ]
    [Browsable( false )]
    public string Error
    {
      get
      {
        return "IDataErrorInfo Error Message";
      }
    }
    [EditorBrowsable( EditorBrowsableState.Never )]
    [Browsable( false )]
    public string this[ string propertyName ]
    {
      get
      {
        if( propertyName == "Age" )
        {
          if( m_age < 18 )
            return "An employee must be 18 years or older.";
        }
        return string.Empty;
      }
    }   
    private string m_firstName = string.Empty;
    private string m_lastName = string.Empty;
    private int m_personID;
    private int m_age = 18;
  }
}