What I am trying to achieve:

• encapsulate a Login (or any) Form to be reused across site
• post to self
• when Login/validation fails, show original page with Validation Summary
• • (some might argue to just post to Login Page and show Validation Summary there; if what I'm trying to achieve isn't possible, I will just go that route)
• when Login succeeds, redirect to /App/Home/Index
• also, want to:
• • stick to PRG principles
• • avoid ajax
• • keep Login Form (UserController.Login()) as encapsulated as possible; avoid having to implement HomeController.Login() since the Login Form might appear elsewhere

All but the redirect works. My approach thus far has been:

• Home/Index includes Login Form: <%Html.RenderAction("Login","User");%>
• User/Login ViewUserControl<UserLoginViewModel> includes:
• • <%=Html.ValidationSummary("") %>
• • using(Html.BeginForm()){}
• • includes hidden form field "userlogin"="1"

public class UserController : BaseController {
    ...
    [AcceptPostWhenFieldExists(FieldName = "userlogin")]
    public ActionResult Login(UserLoginViewModel model, FormCollection form){
        if (ModelState.IsValid) {
            if(checkUserCredentials()) {
                setUserCredentials()
                return this.RedirectToAction<Areas.App.Controllers.HomeController>(x => x.Index());
            }
        else {
            return View();
        }
    }
    ...
}

Works great when: ModelState or User Credentials fail -- return View() does yield to Home/Index and displays appropriate validation summary.

(I have a Register Form on the same page, using the same structure. Each form's validation summary only shows when that form is submitted.)

Fails when: ModelState and User Credentials valid -- RedirectToAction<>() gives following error:
"Child actions are not allowed to perform redirect actions."

It seems like in the Classic ASP days, this would've been solved with Response.Buffer=True. Is there an equivalent setting or workaround now?

Btw, running: ASP.Net 4, MVC 2, VS 2010, Dev/Debugging Web Server

I hope all of that makes sense.

So, what are my options? Or where am I going wrong in my approach? tia!