DialogHelper.cs

/// <summary>
/// Helper class for showing message dialogs
/// </summary>
public static class DialogHelper
{
    /// <summary>
    /// Shows a dialog with given message and ok/cancel buttons. 
    /// </summary>
    /// <param name="message">Message</param>
    /// <param name="title">Title</param>
    /// <param name="okText">OK text (optional)</param>
    /// <param name="cancelText">Cancel text (optional)</param>
    /// <returns>True if ok pressed, false otherwise</returns>
    public static async Task<bool> ShowMessageDialog(string message, string title, string okText, string cancelText)
    {
        bool result = false;
        var dialog = new MessageDialog(message, title);

        if (!string.IsNullOrWhiteSpace(okText))
        {
            dialog.Commands.Add(new UICommand(okText, cmd => result = true));
        }

        if (!string.IsNullOrWhiteSpace(cancelText))
        {
            dialog.Commands.Add(new UICommand(cancelText, cmd => result = false));
        }

        await dialog.ShowAsync();            
        return result;
    }
}

DialogHelperService.cs

/// <summary>
/// Helper class for showing message dialogs
/// </summary>
public class DialogHelperService
{
    /// <summary>
    /// Shows a dialog with given message and ok/cancel buttons. 
    /// </summary>
    /// <param name="message">Message</param>
    /// <param name="title">Title</param>
    /// <param name="okText">OK text (optional)</param>
    /// <param name="cancelText">Cancel text (optional)</param>
    /// <returns>True if ok pressed, false otherwise</returns>
    public async Task<bool> ShowMessageDialog(string message, string title, string okText, string cancelText)
    {
        bool result = false;
        var dialog = new MessageDialog(message, title);
 
        if (!string.IsNullOrWhiteSpace(okText))
        {
            dialog.Commands.Add(new UICommand(okText, cmd => result = true));
        }
 
        if (!string.IsNullOrWhiteSpace(cancelText))
        {
            dialog.Commands.Add(new UICommand(cancelText, cmd => result = false));
        }
 
        await dialog.ShowAsync();            
        return result;
    }
}

usage.cs

public async void Message()
{
    var res = await DialogHelper.ShowMessageDialog("Do you really want to do this?","My Title","Hell yeah!","No way");
    if (res)
    {
        Result = "Hell yeah!";
    } else 
    {
       Result = "NOOOO!";
    }
}

usage.xaml

<Page.BottomAppBar>
<CommandBar>
  <AppBarButton Icon="Accept" Label="go ask!"  x:Name="Message" />
</CommandBar>
</Page.BottomAppBar>

usage2.cs

public async void Message()
{
    var res = await _dialogHelperService.ShowMessageDialog("Do you really want to do this?","My Title","Hell yeah!","No way");
    if (res)
    {
        Result = "Hell yeah!";
    } else 
    {
       Result = "NOOOO!";
    }
}