UmbracoRazorLogin.cshtml

@using System.Web  
@using System.Web.Security

<style type="text/css">
 p,label {color:black;}
</style>

@{
  // just refactored things a bit
  var flashmessage = Request["flash"];

  // POST
  if (IsPost) 
  {
     var shouldRedirect = false;
     if (Request["submit"]=="login")
     {
        string username=Request["username"];
        string password=Request["password"];
        if (Membership.ValidateUser(username, password))
        {
          FormsAuthentication.SetAuthCookie(username, true);
          flashmessage="Welcome, "+username;
        }
        else
        {
          flashmessage="Login failed: wrong username or password";
        }
        shouldRedirect=true;
     }
     if (Request["submit"]=="logout")
     {
        FormsAuthentication.SignOut();
        flashmessage="Logged off";
        shouldRedirect=true;
     }
     if (shouldRedirect)
     {
        var redirectUrl = Request.Url.PathAndQuery.ToString(); 
        if (!string.IsNullOrEmpty(flashmessage)
        {
          redirectUrl += redirectUrl.Contains("&")?"?":"&";
          redirectUrl += "flash="+UrlEncode(flashmessage); // not sure how to call this urlencode function
        }
       Response.Redirect(redirectUrl);
    }
  }

  // GET
  <p>@flashmessage</p>
  
  var currentUser = Membership.GetUser();

  if (currentUser==null)
  {
    <form method="post">
      <div><label for="name">Username:</label>
      <input type="text" id="username" name="username"/></div>
      <div><label for="password">Password:</label>
      <input type="password" id="password" name="password"/></div>
      <div><input type="submit" id="submit" name="submit" value="login"/></div>
    </form>
  }
  else 
  {
    <form method="post">
      <input type="submit" id="submit" name="submit" value="logout"/>
    </form>
  }
}