DialogService

public class MainViewModel : ViewModelBase {
    protected IDialogService DialogService { get { return this.GetService<IDialogService>(); } }
    // ...
}

Public Class MainViewModel
    Protected ReadOnly Property DialogService() As IDialogService
        Get
            Return Me.GetService(Of IDialogService)()
        End Get
    End Property
    ' ...
End Class

<UserControl x:Class="MVVMDemo.Services.DialogServiceView" 
            ...
            DataContext="{dxmvvm:ViewModelSource local:DialogServiceViewModel}">
    <UserControl.Resources>
        <DataTemplate x:Key="dialogTemplate">
            <dxlc:LayoutControl Orientation="Vertical" VerticalAlignment="Top" MinWidth="300">
                <dxlc:LayoutItem Label="Customer Name:">
                    <dxe:TextEdit Text="{Binding CustomerName, UpdateSourceTrigger=PropertyChanged}"/>
                </dxlc:LayoutItem>
            </dxlc:LayoutControl>
        </DataTemplate>
    </UserControl.Resources>
    <dxmvvm:Interaction.Behaviors>
        <!-- The ViewTemplate property specifies the dialog template -->
        <dx:DialogService x:Name="DialogService" ViewTemplate="{StaticResource dialogTemplate}">
            <dx:DialogService.DialogStyle>
                <Style TargetType="Window">
                    <Setter Property="SizeToContent" Value="WidthAndHeight" />
                    <Setter Property="WindowStyle" Value="ToolWindow" />
                </Style>
            </dx:DialogService.DialogStyle>
        </dx:DialogService>
    </dxmvvm:Interaction.Behaviors>
    ...
</UserControl>

public static class DialogServiceExtensions {
    // Use these methods when the a child View is defined though the DialogService.ViewTemplate or DialogService.ViewTemplateSelector property. 
    //The child View should not contain a View Model, because it is passed through the service.
    public static UICommand ShowDialog(this IDialogService service, IEnumerable<UICommand> dialogCommands, string title, object viewModel);
    public static MessageResult ShowDialog(this IDialogService service, MessageButton dialogButtons, string title, object viewModel);

    // Use these methods to create a child View and pass the specified View Model to the created child View.
    public static UICommand ShowDialog(this IDialogService service, IEnumerable<UICommand> dialogCommands, string title, string documentType, object viewModel);
    public static MessageResult ShowDialog(this IDialogService service, MessageButton dialogButtons, string title, string documentType, object viewModel);

    // Use these methods to create a child View that already contains a View Model. 
    // The Parameter and ParentViewModel are passed to the child View Model.
    public static UICommand ShowDialog(this IDialogService service, IEnumerable<UICommand> dialogCommands, string title, string documentType, object parameter, object parentViewModel);
    public static MessageResult ShowDialog(this IDialogService service, MessageButton dialogButtons, string title, string documentType, object parameter, object parentViewModel);
}

Public NotInheritable Class DialogServiceExtensions
    ' Use these methods when the a child View is defined though the DialogService.ViewTemplate or DialogService.ViewTemplateSelector property. 
    ' The child View should not contain a View Model, because it is passed through the service.
    <System.Runtime.CompilerServices.Extension> _
    Public Shared Function ShowDialog(service As IDialogService, dialogCommands As IEnumerable(Of UICommand), title As String, viewModel As Object) As UICommand
    <System.Runtime.CompilerServices.Extension> _
    Public Shared Function ShowDialog(service As IDialogService, dialogButtons As MessageButton, title As String, viewModel As Object) As MessageResult

    ' Use these methods to create a child View and pass the specified View Model to the created child View.
    <System.Runtime.CompilerServices.Extension> _
    Public Shared Function ShowDialog(service As IDialogService, dialogCommands As IEnumerable(Of UICommand), title As String, documentType As String, viewModel As Object) As UICommand
    System.Runtime.CompilerServices.Extension> _
    Public Shared Function ShowDialog(service As IDialogService, dialogButtons As MessageButton, title As String, documentType As String, viewModel As Object) As MessageResult

    ' Use these methods to create a child View that already contains a View Model. 
    ' The Parameter and ParentViewModel are passed to the child View Model.
    <System.Runtime.CompilerServices.Extension> _
    Public Shared Function ShowDialog(service As IDialogService, dialogCommands As IEnumerable(Of UICommand), title As String, documentType As String, parameter As Object, parentViewModel As Object) As UICommand
    <System.Runtime.CompilerServices.Extension> _
    Public Shared Function ShowDialog(service As IDialogService, dialogButtons As MessageButton, title As String, documentType As String, parameter As Object, parentViewModel As Object) As MessageResult
End Class

using DevExpress.Mvvm;
using DevExpress.Mvvm.POCO;

namespace MVVMDemo.Services {
    public class DialogServiceViewModel {
        public void ShowDetail(string serviceName) {
            DialogServiceDetailViewModel detailViewModel = DialogServiceDetailViewModel.Create();

            UICommand registerCommand = new UICommand(
                id: null,
                caption: "Register",
                command: new DelegateCommand<CancelEventArgs>(
                    cancelArgs => {
                        try {
                            myExecuteMethod();
                        }
                        catch (Exception e) {
                            this.GetService<IMessageBoxService>().ShowMessage(e.Message, "Error", MessageButton.OK);
                            cancelArgs.Cancel = true;
                        }
                    },
                    cancelArgs => !string.IsNullOrEmpty(registrationViewModel.UserName)
                ),
                isDefault: true,
                isCancel: false
            );

            UICommand cancelCommand = new UICommand(
                id: MessageBoxResult.Cancel,
                caption: "Cancel",
                command: null,
                isDefault: false,
                isCancel: true
            );

            IDialogService service = this.GetService<IDialogService>(serviceName);
            UICommand result = service.ShowDialog(
                dialogCommands: new[] { registerCommand, cancelCommand }, 
                title: "Registration Dialog", 
                viewModel: detailViewModel
            );

            if(result == registerCommand)
                MessageBox.Show("Registered: " + detailViewModel.CustomerName);
        }
    }
}

Imports DevExpress.Mvvm
Imports DevExpress.Mvvm.POCO

Namespace MVVMDemo.Services
    Public Class DialogServiceViewModel
        Public Sub ShowDetail(ByVal serviceName As String)
            Dim detailViewModel As DialogServiceDetailViewModel = DialogServiceDetailViewModel.Create()

            Dim registerCommand As UICommand = New UICommand(
                id:=Nothing, 
                caption:="Register", 
                command:=New DelegateCommand(Of CancelEventArgs)(
                    Function(cancelArgs)
                        Try
                            myExecuteMethod()
                        Catch e As Exception
                            Me.GetService(Of IMessageBoxService)().ShowMessage(e.Message, "Error", MessageButton.OK)
                            cancelArgs.Cancel = True
                        End Try
                    End Function, 
                    Function(cancelArgs) 
                        Not String.IsNullOrEmpty(registrationViewModel.UserName)), isDefault:=True, isCancel:=False)

            Dim cancelCommand As UICommand = New UICommand(
                id:=MessageBoxResult.Cancel, 
                caption:="Cancel", 
                command:=Nothing, 
                isDefault:=False, 
                isCancel:=True)

            Dim service As IDialogService = Me.GetService(Of IDialogService)(serviceName)
            Dim result As UICommand = service.ShowDialog(
                dialogCommands:={registerCommand, cancelCommand},
                title:="Registration Dialog",
                viewModel:=detailViewModel
            )

            If result Is registerCommand Then
                MessageBox.Show("Registered: " & detailViewModel.CustomerName)
            End If
        End Sub
    End Class
End Namespace

<UserControl ...
             xmlns:dx="http://schemas.devexpress.com/winfx/2008/xaml/core"
             xmlns:dxmvvm="http://schemas.devexpress.com/winfx/2008/xaml/mvvm">
    <Grid>
        <dxmvvm:Interaction.Behaviors>
            <dxmvvm:DialogService SetWindowOwner="True"
                                  WindowStartupLocation="CenterOwner">
                <dxmvvm:DialogService.DialogStyle>
                    <Style TargetType="dx:ThemedWindow">
                        <Setter Property="Width" Value="500" />
                        <Setter Property="Height" Value="300" />
                    </Style>
                </dxmvvm:DialogService.DialogStyle>
            </dxmvvm:DialogService>
        </dxmvvm:Interaction.Behaviors>
        ...
    </Grid>
</UserControl>