Global.asax界面添加如下方法：

void FormsAuthentication_Authenticate(object sender, FormsAuthenticationEventArgs e)
        {
            string UserName = this.Context.Request.QueryString["UserName"];
            if (!string.IsNullOrEmpty(UserName))
            {  
                FormsAuthentication.SetAuthCookie(UserName, 
 
true);
                e.User = new GenericPrincipal(new GenericIdentity(UserName), null);
                return;
            }
        }

MVC繞過登陸界面驗證時HttpContext.Current.User.Identity.Name取值為空問題解決方法