I usally add a reference to the startpage on the basepage used for all pagetypes in the prodject. That way you can always get the settings added to the startpage.

/// <summary>
/// Base class for all page types
/// </summary>
public abstract class BasePageData : PageData
{

/// <summary>
/// Returns the Start page
/// </summary>
public StartPage StartPage
{
get
{
return DataFactory.Instance.Get<StartPage>(ContentReference.StartPage);
}
}

....

}

I'm sorry, but I don't quite understand.
Would you mind elaborating your explanation a little?

What is this basepage you're talking about?
Do I have to create it, and if so - how?

The basepage is a class I create so yes you will have to add your own. 

Just add a code file to your prodject and let it inheret from PageData. Then on this you could add all property definitions that will be used on all pages. Then let the pagetypes that you create inheret from the created basepage.

I would recomend you to install the alloy template site and have a look at the setup in there. They use the SitePageData as their base class for all pagetypes.

I think what Petter is referring to is the assumption that the properties live solely on the StartPage, which in my opinion, is the correct way to handle this, assuming that the property values do not change per each page instance.

If that is the case, keep the properties on the StartPage, and don't let any other pages inherit from the StartPage, then you can access those properties using the code that Petter provided (e.g. CurrentPage.StartPage.InvitationMessageTitle)

If the property values need to be different per each page instance, then you could inherit from the StartPage, but I think it would be better to inherit from a different base class, or even just create an interface, since the StartPage is typically a unique page type. In my projects, I never inherit from the StartPage.

Okay, I'm still not quite sure how to do this, I'll try to explain what I want to do better.

This is one of the ViewModels of a different PageType I want to populate with values inherited from the StartPage / SitePageData:

public AdsPageViewModel(AdsPage currentPage)
        {

        }

        public string InvitationMessage { get; set; }
        public string InvitationMessageTitle { get; set; }

And my AdsPage also inherits from SitePageData:

    [ContentType(DisplayName = "AdsPage", GUID = "93f82d57-f1b8-473f-9ba1-14843f10b8c5", Description = "")]
    public class AdsPage : SitePageData

This is my newly created SitePageData based on the code above, not sure if it's correct:

public class SitePageData : PageData
    {

        public StartPage StartPage
        {
            get
            {
                return DataFactory.Instance.Get<StartPage>(ContentReference.StartPage);
            }
        }


    }

My StartPage with the properties I want:

[ContentType(DisplayName = "StartPage", GUID = "061a1365-7fd6-4f72-ad76-4e8f349029ba", Description = "")]
    public class StartPage : SitePageData
    {
        [CultureSpecific]
        [Display(
            Name = "Content Area",
            GroupName = SystemTabNames.Content,
            Order = 100)]
        public virtual ContentArea ContentArea { get; set; }

        [CultureSpecific]
        [Display(
            Name = "Main Body",
            GroupName = SystemTabNames.Content,
            Order = 200)]
        public virtual XhtmlString MainBody { get; set; }

        [Required]
        [Display(
            Name = "Login redirect URL",
            Description = "Specify which page you want your user to be redirected to after signing in.",
            Order = 300
            )]
        public virtual PageReference LoginRedirect { get; set; }

        [Required]
        [Display(
            Name = "Facebookbutton text",
            Description = "The text that will be displayed on the 'Sign in with facebook'-button.",
            Order = 400
            )]
        public virtual string FacebookButtonText { get; set; }

        [Required]
        [CultureSpecific]
        [Display(
            Name = "Invitation Message Title",
            Description = "Title of the message that will be sent to invited users. Use [EventName] or [User] tag if you want them to be a part of the title.",
            GroupName = SystemTabNames.Content,
            Order = 500)]
        public virtual string InvitationMessageTitle { get; set; }

        [Required]
        [CultureSpecific]
        [Display(
            Name = "Invitation Message",
            Description = "The message that will be sent when a user invites others to their event. Always specify a part in the message which contains these tags '[EventName]' or '[User]' if you want to link to the specific event and user in the message.",
            GroupName = SystemTabNames.Content,
            Order = 600)]
        public virtual string InvitationMessage { get; set; }

        public override void SetDefaultValues(ContentType contentType)
        {

            base.SetDefaultValues(contentType);

            LoginRedirect = PageReference.StartPage;
            FacebookButtonText = "Logga in med";
        }

    }

I want the InvitationMessage and InvitationTitle properties to be available on all/multiple PageTypes with inherited values which the editor sets in the StartPage, this is because I want the same value on several Pages which without having the editor type them in on each PageType as this could end with different values etc.

public AdsPageViewModel(AdsPage currentPage)
{
 
 
       public string InvitationMessage 
       { 
         get
         {
           currentPage.StartPage.InvitationMessage
         }
       }
....
}

This should give you the value from the startPage.

So in your controller, where you create your instance of the AdsPageViewModel, populate the view model's properties with those set on the StartPage:

public ActionResult Index(AdsPage currentPage)
{
	var model = new AdsPageViewModel 
	{
		InvitationMessage = currentPage.StartPage.InvitationMessage,
		InvitationMessageTitle = currentPage.StartPage.InvitationMessageTitle
	};

	return View(model);
}

Not sure why but seems my code got stripped.

Thanks a lot you guys, now it works perfectly!

Regards,
Ludvig Flemmich