//WPF
var dialogResult = WindowService.ShowDialog(content); //внутри свой цикл обработки сообщений

//SL
WindowService.ShowDialog(content, dialogResult => ... );

//БЕЗ ЛЯМБДЫ!!!

var dialogResult = WindowService.ShowDialog(content);

public class DialogService : IDialogService
{
    public bool ShowDialog(object viewModel)
    {
        var dialog = new Dialog() {ViewModel = viewModel,  CloseAction = callback};
        return dialog.ShowDialog() ?? false;
    }
}

public class DialogService : IDialogService
{
    public bool ShowDialog(object viewModel)
    {
        var dialog = new Dialog() { ViewModel = viewModel };
        return dialog.ShowDialog() ?? false;
    }
}

return dialog.ShowDialog() ?? false;

// Теперь в WPF диалоги такие же не удобные как в SL.
public class DialogService : IDialogService
{
    public void ShowDialog(object viewModel, Action<bool> сallback)
    {
        var dialog = new Dialog() { ViewModel = viewModel,  CloseAction = callback};
        dialog.Dispatcher.BeginInvoke(() => dialog.Show());
    }
}

class D2 : Window
{
...
}

class D1 : Window
{
    void btnOkClick(object sender, EventArgs e)
    {
        var d2 = new D2();
        if (d2.ShowDialog() == false)
             return;
        
         //тут чё-то делаем...

         DialogResult = true;
    }
}

///////////////////////////////////////////


var d1 = new D1();
if (d1.ShowDialog())
    // тут чё-то делаем...

class AcceptDialog : Window
{
     public event CancelEventHandler Accepted = delegate { };

     void btnAcceptClick(object sender, EventArgs e)
     {
           var cea = new CancelEventArgs();
           Accepted(this, cea);
           if (e.Cancel == false)
                DialogResult = true;
     }
}

/////////////////////////////

var d = new AcceptDialog();
d.Accepted = (s, e) =>
     e.Cancel = someOtherDialog.ShowDialog() == false;
d.ShowDialog();