ValidationSummaryMultipleForms

AccountsController.cs

namespace project 
{
    // ...
    
    public class AccountsController : Controller {
    
        // ...
      
        [HttpGet, Route("accounts/manage/change-passwords", Name="change-password")]
        [Authorize(Roles="Admin")]
        public ActionResult ManagePasswords() {

            var users = this.usersStore.GetAllUsers();
            var passwordEditorsList = new List<PasswordEditorViewModel>();

            foreach(var user in users) {
                passwordEditorsList.Add(new PasswordEditorViewModel { Username = user.Username });
            }

            var passwordEditorsListViewModel = new PasswordEditorsListViewModel(passwordEditorsList);

            return View(viewName: "~/views/accounts/manage-passwords.cshtml", model: passwordEditorsListViewModel);
        }
        
        [HttpPost]
        [Route("accounts/manage/change-password", Name="change-password-request")]
        [Authorize(Roles="Admin"), ValidateAntiForgeryToken]
        public ActionResult ChangePassword(PasswordEditorViewModel model) {

            if(this.ModelState.IsValid) {

                var passwordChangeResult = this.passwordManagementService.ChangePassword(model.Username, model.OldPassword, model.NewPassword);
                if(passwordChangeResult.ResultCode != PasswordChangeResultCode.Success) {

                    var modelPrefix = this.ModelState.Keys.First().Split(new char[]{'.'})[0];

                    this.ModelState.AddModelError(
                        key: modelPrefix + ".PasswordChangeError." + passwordChangeResult.ResultCode.ToString(), 
                        errorMessage: passwordChangeResult.ErrorMessage
                    );

                }
                else { 
                    
                    this.TempData["PasswordChangeMessage"] = string.Format("Password for user '{0}' was successfully changed", model.Username);
                    
                    if(model.Username == "Admin") {
                        this.OwinAuthenticationManager.SignOut(DefaultAuthenticationTypes.ApplicationCookie);
                        return RedirectToAction("login");
                    }
                }
            }

            return ManagePasswords();
        }
    }
}

CollectionEditorModelBinder.cs

namespace project 
{
    using System.Web.Mvc;

    public class CollectionEditorModelBinder : DefaultModelBinder {
        public override object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext) {
            bindingContext.ModelName = controllerContext.HttpContext.Request.Form["model.prefix"];
            return base.BindModel(controllerContext, bindingContext);
        }
    }
}

PasswordEditorsListViewModel.cs

namespace project 
{
    public class PasswordEditorsListViewModel {
    
        public PasswordEditorsListViewModel(IEnumerable<PasswordEditorViewModel> passwordEditors) {
            this.PasswordEditors = passwordEditors;
        }
    
        public IEnumerable<PasswordEditorViewModel> PasswordEditors { get; set; }
    }
}

PasswordEditorViewModel.cs

namespace project 
{
    using System.ComponentModel.DataAnnotations;

    public class PasswordEditorViewModel {
        [Required]
        public string Username { get; set; }
        [Required, DataType(DataType.Password)]
        public string OldPassword { get; set; }
        [Required, DataType(DataType.Password)]
        public string NewPassword { get; set; }
    }
}

PasswordEditorViewModel.cshtml

@model PasswordEditorViewModel

@* 
   as it is suggested in the http://stackoverflow.com/questions/18102918/multiple-forms-in-mvc-view-modelstate-applied-to-all-forms 
   post the editor template is located at ~/views/shared/EditorTemplates and has the same name as the corresponding view model 
   (PasswordEditorViewModel)
*@

@using(Html.BeginRouteForm(routeName: "change-password-request")) {
        
    @Html.AntiForgeryToken()
    
    <input type="hidden" name="model.prefix" value="@ViewData.TemplateInfo.HtmlFieldPrefix" />
    <h4>@Model.Username</h4>
    
    @Html.HiddenFor(i => i.Username)
    
    <div class="control-group">
        <label>old password</label>
        @Html.PasswordFor(i => i.OldPassword, new { placeholder = "enter old password" })
    </div>
    
    <div class="control-group">
        <label>new password</label>
        @Html.PasswordFor(i => i.NewPassword, new { placeholder = "enter new password" })
    </div>
    
    @if(!ViewData.ModelState.IsValid) {
    
        <ul class="validation-errors">
            @* the below thing works *@
            
            @*
            <li class="validation-error">@Html.ValidationMessageFor(i => i.OldPassword)</li>
            <li class="validation-error">@Html.ValidationMessageFor(i => i.NewPassword)</li>
            <li class="validation-error">@Html.ValidationMessage("PasswordChangeError.WrongPassword")</li>
            *@
            
            @* and this one is broken *@
            @Html.ValidationSummary()
            
        </ul>
        
    }
    
    <button type="submit">Submit Changes</button>
}