Question

How to create create variables/properties in master page, and let sub-pages access them?

So my master will have a string property HomeUrl

How can any page that uses the master page access this property?

Answer 1

You should use a base class for you master page which will define you properties:

public class BaseMasterPage : System.Web.UI.MasterPage
{
    public string HomeUrl {get; set; }
}

And then your master page should inherit from this BaseMasterPage class (as example):

// real master page
public partial class Common_MasterPages_Backend_Default : BaseMasterPage
{
}

After this you can access your property through Page.Master property:

BaseMasterPage baseMaster = (BaseMasterPage)Page.Master;
string homeUrl = baseMaster.HomeUrl;

from any page which uses this master page.