publicizes user stuff, changes the IUser.Id to an int - no need for it to be an object and casted everywhere. Updates the login logic to perform the mapping logic for custom membership providers with an extension method.

src/Umbraco.Core/Services/UserServiceExtensions.cs

@@ -0,0 +1,52 @@
+using System;
+using System.Web.Security;
+using Umbraco.Core.Models.Membership;
+
+namespace Umbraco.Core.Services
+{
+    internal static class UserServiceExtensions
+    {
+        /// <summary>
+        /// Maps a custom provider's information to an umbraco user account
+        /// </summary>
+        /// <param name="userService"></param>
+        /// <param name="member"></param>
+        /// <remarks>
+        /// To maintain compatibility we have to check the login name if the provider key lookup fails but otherwise
+        /// we'll store the provider user key in the login column.
+        /// </remarks>
+        public static IUser CreateUserMappingForCustomProvider(this IUserService userService, MembershipUser member)
+        {
+            if (member == null) throw new ArgumentNullException("member");
+
+
+            var valToLookup = member.ProviderUserKey == null ? member.UserName : member.ProviderUserKey.ToString();
+            var found = userService.GetByUsername(valToLookup);
+            if (found == null && member.ProviderUserKey != null)
+            {
+                //try by username
+                found = userService.GetByUsername(member.UserName);
+            }
+
+            if (found == null)
+            {
+                var writer = userService.GetUserTypeByAlias("writer");
+                if (writer == null)
+                {
+                    throw new InvalidOperationException("Could not map the custom user to an Umbraco user, no 'writer' user type could be found");
+                }
+                var user = new User(
+                    member.UserName,
+                    member.Email ?? Guid.NewGuid().ToString("N") + "@example.com", //email cannot be empty
+                    member.ProviderUserKey == null ? member.UserName : member.ProviderUserKey.ToString(),
+                    Guid.NewGuid().ToString("N"), //pass cannot be empty
+                    writer);
+                user.AddAllowedSection(Constants.Applications.Content);
+                userService.Save(user);
+                return user;
+            }
+
+            return found;
+        }
+    }
+}