public class UICommand :
    BindableBase,
    IUICommand

Public Class UICommand
    Inherits BindableBase
    Implements IUICommand

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
);

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)