Hi Tom,

This got me in a bit of a mess, in the end I started merging some code I have from an old project to see what was happening so I may have changed a few bits!

The error you're getting is a little misleading - it's because the value for Sex can't be found in the GenderList not due to a type issue. As I said I've changed a few bits of your code to get this working (in particular using the IDataTypeService to look up the values, I'm not sure but I think this is the current recommended way of doing it). I've posted the lot here rather than showing what I've changed as there were a couple of other issues I think (or stuff I perhaps introduced!!?).

I've got it working where it will update the member value and post back but it renders just the PartialView and not the view surrounding it! Hopefully this will at least get you back on track though!

As I say this works but is hacky - I think you should change the key - value of the drop down to be both the text and look up the internal ID to set the member property (especially as it results in the hack I've marked and get the GenderList in a Constructor)??

Controller

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Mvc;
using Umbraco.Core;
using Umbraco.Core.Models;
using Umbraco.Core.Services;
using Umbraco.Web;
using Umbraco.Web.Mvc;
using Umbraco.Web.Security;
using UmbracoV7_3.Model;


namespace UmbracoV7_3.Controllers
{
    public class ProfileSurfaceController : SurfaceController
    {

        [Authorize]
        [ActionName("RenderEditProfile")]
        public ActionResult RenderEditProfile()
        {
            ProfileViewModel profileModel = new ProfileViewModel();

            if (@Members.IsLoggedIn())
            {
                var membershipHelper = new MembershipHelper(UmbracoContext);
                var memberId = membershipHelper.GetCurrentMemberId();
                var member = ApplicationContext.Current.Services.MemberService.GetById(memberId);

                profileModel.Sex = member.Properties["sex"].Value.ToString();

                IDataTypeService ds = ApplicationContext.Services.DataTypeService;
                IDataTypeDefinition SizeType = ds.GetAllDataTypeDefinitions().First(d => d.Name == "Sex List");
                profileModel.GenderList = ds.GetPreValuesCollectionByDataTypeId(SizeType.Id).PreValuesAsDictionary.Select(size => new SelectListItem { Text = size.Value.Value, Value = size.Value.Id.ToString(), Selected = false }).ToList();

            }
            else
            {
                //They are not logged in, redirect to home
                return Redirect("/");
            }

            //Pass the model to the view
            return PartialView("EditProfile", profileModel);
        }

        [Authorize]
        [ActionName("HandleEditProfile")]
        public ActionResult HandleEditProfile(ProfileViewModel model)
        {
            if (!ModelState.IsValid)
            {
                return PartialView("EditProfile", model);
            }

           // var member = System.Web.Security.Membership.GetUser();

           // var membershipService = ApplicationContext.Current.Services.MemberService;
            //var updateMember = ApplicationContext.Current.Services.MemberService.GetById(member.IMemberID);

            // bit hacky must be a better way
            MembershipHelper mh = new MembershipHelper(UmbracoContext.Current);
            var membershipService = ApplicationContext.Current.Services.MemberService;
            var updateMember = membershipService.GetById(mh.GetCurrentMember().Id);

            updateMember.Properties["sex"].Value = model.Sex;

            //Save the member
            membershipService.Save(updateMember);
            TempData["Status"] = "Ihr Profil wurde aktualisiert";

            // model contains the ID of the prevalue not the string so let's replace that.
            model.Sex = updateMember.Properties["sex"].Value.ToString();
            // need the gender list - I'd move this to a constructor - but you probably won't return the Partial?
            IDataTypeService ds = ApplicationContext.Services.DataTypeService;
            IDataTypeDefinition SizeType = ds.GetAllDataTypeDefinitions().First(d => d.Name == "Sex List");
            model.GenderList = ds.GetPreValuesCollectionByDataTypeId(SizeType.Id).PreValuesAsDictionary.Select(size => new SelectListItem { Text = size.Value.Value, Value = size.Value.Id.ToString(), Selected = false }).ToList();

            //Return the view
            return PartialView("EditProfile", model);
        }

    }
}

Partial View

@inherits Umbraco.Web.Mvc.UmbracoViewPage<UmbracoV7_3.Model.ProfileViewModel>

@using (Html.BeginUmbracoForm<UmbracoV7_3.Controllers.ProfileSurfaceController>("HandleEditProfile", null, new { @id = "profile-edit-form" }))
{
    @Html.LabelFor(m => m.Sex)
    @Html.DropDownListFor(x => x.Sex, Model.GenderList, "Select...")
    @Html.ValidationMessageFor(x => x.Sex)

    <button id="callback-submit" type="submit" value="Register">Register</button>
}