ValidationErrorsHostBehavior Class

Tracks validation errors within a UI container.

Assembly: DevExpress.Xpf.Core.v21.2.dll

Declaration

C#
VB.NET

[TargetType(typeof(FrameworkElement))]
public class ValidationErrorsHostBehavior :
    Behavior<FrameworkElement>

<TargetType(GetType(FrameworkElement))>
Public Class ValidationErrorsHostBehavior
    Inherits Behavior(Of FrameworkElement)

Remarks

You can define the ValidationErrorsHostBehavior for a UI container to track validation errors within it. To do this, set the Binding.NotifyOnValidationError and Binding.ValidatesOnDataErrors properties to true for controls you want to validate.

Use the HasErrors property to check whether a bound UI container includes validation errors.

You can also use Validation Attributes with a ViewModelBase and POCO descendants.

Use the Behavior with ViewModelBase Descendants

To use the ValidationErrorsHostBehavior with ViewModeBase descendants, implement the IDataErrorInfo interface.

The following code sample enables validation for TextEdit controls. If they contain valid text, the Save button is enabled:

MainView.xaml

<UserControl 
    xmlns:ViewModels="clr-namespace:Example.ViewModels"
    xmlns:dxmvvm="http://schemas.devexpress.com/winfx/2008/xaml/mvvm">
    <UserControl.DataContext>
        <ViewModels:MainViewModel/>
    </UserControl.DataContext>
    <UserControl.Resources>
        <dxmvvm:BooleanNegationConverter x:Key="BooleanNegationConverter"/>
    </UserControl.Resources>
    <dxmvvm:Interaction.Behaviors>
        <dxmvvm:ValidationErrorsHostBehavior x:Name="validationErrorsHostBehavior"/>
    </dxmvvm:Interaction.Behaviors>
    <Grid>
        <StackPanel Orientation="Vertical" ...>
            <StackPanel Orientation="Horizontal">
                <TextBlock Text="First Name: " .../>
                <dxe:TextEdit Text="{Binding FirstName, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged, 
                                NotifyOnValidationError=True, ValidatesOnDataErrors=True}" .../>
            </StackPanel>
            <StackPanel Orientation="Horizontal">
                <TextBlock Text="Last Name: " .../>
                <dxe:TextEdit Text="{Binding LastName, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged, 
                                NotifyOnValidationError=True, ValidatesOnDataErrors=True}" .../>
            </StackPanel>
            <Button Content="Login" ... IsEnabled="{Binding ElementName=validationErrorsHostBehavior, 
                                                   Path=HasErrors, Converter={StaticResource BooleanNegationConverter}}"/>
        </StackPanel>
    </Grid>
</UserControl>

MainViewModel.cs
MainViewModel.vb

using DevExpress.Mvvm;
using System.ComponentModel;

namespace Example.ViewModel {
    public class MainViewModel : ViewModelBase, IDataErrorInfo {
        public string FirstName {
            get { return GetValue<string>(); }
            set { SetValue(value); }
        }

        public string LastName {
            get { return GetValue<string>(); }
            set { SetValue(value); }
        }

        public string Error {
            get {
                return this["FirstName"] != null || this["LastName"] != null ? "Invalid values." : null;
            }
        }

        public string this[string columnName] {
            get {
                switch (columnName) {
                    case "FirstName":
                        return string.IsNullOrEmpty(FirstName) ? "First Name cannot be empty." : null;
                    case "LastName":
                        return string.IsNullOrEmpty(LastName) ? "Last Name cannot be empty." : null;
                    default:
                        return null;
                }

            }
        }
    }
}

Imports DevExpress.Mvvm
Imports System.ComponentModel

Namespace Example.ViewModel
    Public Class MainViewModel
        Inherits ViewModelBase
        Implements IDataErrorInfo

        Public Property FirstName As String
            Get
                Return GetValue(Of String)()
            End Get
            Set(ByVal value As String)
                SetValue(value)
            End Set
        End Property

        Public Property LastName As String
            Get
                Return GetValue(Of String)()
            End Get
            Set(ByVal value As String)
                SetValue(value)
            End Set
        End Property

        Public ReadOnly Property [Error] As String
            Get
                Return If(Me("FirstName") IsNot Nothing OrElse Me("LastName") IsNot Nothing, "Invalid values.", Nothing)
            End Get
        End Property

        Default Public ReadOnly Property Item(ByVal columnName As String) As String
            Get

                Select Case columnName
                    Case "FirstName"
                        Return If(String.IsNullOrEmpty(FirstName), "First Name cannot be empty.", Nothing)
                    Case "LastName"
                        Return If(String.IsNullOrEmpty(LastName), "Last Name cannot be empty.", Nothing)
                    Case Else
                        Return Nothing
                End Select
            End Get
        End Property
    End Class
End Namespace

Use the Behavior with POCO Descendants

You can set the POCO ViewModel’s ImplementIDataErrorInfo annotation attibute to trueto implement the IDataErrorInfo interface.

The following code sample enables validation for TextEdit controls. If they contain valid text, the Login button is enabled:

MainView.xaml

<UserControl 
    xmlns:ViewModels="clr-namespace:Example.ViewModels"
    xmlns:dxmvvm="http://schemas.devexpress.com/winfx/2008/xaml/mvvm">
    <UserControl.DataContext>
        <ViewModels:MainViewModel/>
    </UserControl.DataContext>
    <UserControl.Resources>
        <dxmvvm:BooleanNegationConverter x:Key="BooleanNegationConverter"/>
    </UserControl.Resources>
    <dxmvvm:Interaction.Behaviors>
        <dxmvvm:ValidationErrorsHostBehavior x:Name="validationErrorsHostBehavior"/>
    </dxmvvm:Interaction.Behaviors>
    <Grid>
        <StackPanel HorizontalAlignment="Center" VerticalAlignment="Center">
            <StackPanel Orientation="Horizontal" Margin="0,0,0,10">
                <TextBlock Text="First Name: "  VerticalAlignment="Center" Width="71"/>
                <dxe:TextEdit Text="{Binding FirstName, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged, 
                                NotifyOnValidationError=True, ValidatesOnDataErrors=True}"  Width="120"/>
            </StackPanel>
            <StackPanel Orientation="Horizontal" Margin="0,0,0,10">
                <TextBlock Text="Last Name: "  VerticalAlignment="Center" Width="71"/>
                <dxe:TextEdit Text="{Binding LastName, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged, 
                                NotifyOnValidationError=True, ValidatesOnDataErrors=True}"  Width="120"/>
            </StackPanel>
            <Button Content="Login" HorizontalAlignment="Center" VerticalAlignment="Center" IsEnabled="{Binding ElementName=validationErrorsHostBehavior, 
                                                   Path=HasErrors, Converter={StaticResource BooleanNegationConverter}}"/>
        </StackPanel>
    </Grid>
</UserControl>

MainViewModel.cs
MainViewModel.vb

using DevExpress.Mvvm;
using DevExpress.Mvvm.DataAnnotations;
using System.ComponentModel.DataAnnotations;

// ...
    [POCOViewModel(ImplementIDataErrorInfo = true)] 
    public class MainViewModel { 
        [Required] 
        public virtual string FirstName  { get; set; }
        [Required]
        public virtual string LastName { get; set; }
    }
}

Imports DevExpress.Mvvm
Imports DevExpress.Mvvm.DataAnnotations
Imports System.ComponentModel.DataAnnotations

Namespace Example.ViewModel
    <POCOViewModel(ImplementIDataErrorInfo:=True)>
    Public Class MainViewModel
        <Required>
        Public Overridable Property FirstName() As String
        <Required>
        Public Overridable Property LastName() As String
    End Class
End Namespace

Tip

Refer to the following topic for information on how to use validation attributes with POCO: Automatic IDataErrorInfo Implementation.

View Example: WPF MVVM Framework - Validate a View with ValidationErrorsHostBehavior and POCO

Use Validation Attributes

You can specify ValidationAttribute descendants to define ValidationErrorsHostBehavior‘s validation rules.

The following ViewModel checks whether FirstName and LastName are not empty and their values do not contain more that 40 letters and apostrophes:

MainViewModel.cs
MainViewModel.vb

using DevExpress.Mvvm;
using System.ComponentModel;
using System.ComponentModel.DataAnnotations;
// ...
    public class MainViewModel : ViewModelBase, IDataErrorInfo {
        protected string firstname_;
        [Required]
        [RegularExpression(@"^[a-zA-Z''-'\s]{1,40}$", 
         ErrorMessage = "Characters are not allowed.")]
        public string FirstName {
            get { return this.firstname_; }
            set { this.SetProperty(ref this.firstname_, value, "FirstName"); }
        }

        protected string lastname_;
        [Required]
        [RegularExpression(@"^[a-zA-Z''-'\s]{1,40}$", 
         ErrorMessage = "Characters are not allowed.")]
        public string LastName {
            get { return this.lastname_; }
            set { this.SetProperty(ref this.lastname_, value, "LastName"); }
        }
        // ...
    }

Imports DevExpress.Mvvm
Imports System.ComponentModel
Imports System.ComponentModel.DataAnnotations
' ...
    Public Class MainViewModel
        Inherits ViewModelBase
        Implements IDataErrorInfo

        Protected firstname_ As String
        <Required>
        <RegularExpression("^[a-zA-Z''-'\s]{1,40}$", ErrorMessage := "Characters are not allowed.")>
        Public Property FirstName() As String
            Get
                Return Me.firstname_
            End Get
            Set(ByVal value As String)
                Me.SetProperty(Me.firstname_, value, "FirstName")
            End Set
        End Property

        Protected lastname_ As String
        <Required>
        <RegularExpression("^[a-zA-Z''-'\s]{1,40}$", ErrorMessage := "Characters are not allowed.")>
        Public Property LastName() As String
            Get
                Return Me.lastname_
            End Get
            Set(ByVal value As String)
                Me.SetProperty(Me.lastname_, value, "LastName")
            End Set
        End Property
' ...
    End Class

Run Demo: Behaviors Module in the WPF MVVM Demo

View Example: WPF MVVM Framework - Validate a View with ValidationErrorsHostBehavior and ViewModelBase