If an MVC action has detected validation errors in the model, the errors are held in ModelState. If the action returns a view containing the model, and the jQuery Validate plugin is present on the page, the validation errors are displayed. If the MVC action performs a redirect, however, the model validation errors are lost. Use the PreserveModel…

PreserveModelStateOnRedirect.cs

using System;
using System.Linq;
using System.Collections.Generic;
using System.Web.Mvc;

/* Based on the ModelStateToTempDataAttribute class from MVCContrib */
namespace Web.Filters
{
  /// <summary>
	/// When a RedirectResult or RedirectToRouteResult is returned from an action, anything in the ViewData.ModelState dictionary will be copied into TempData.
	/// When a ViewResultBase is returned from an action, any ModelState entries that were previously copied to TempData will be copied back to the ModelState dictionary.
	/// </summary>
	public class PreserveModelStateOnRedirectAttribute : ActionFilterAttribute
	{
		public const string TempDataKey = "ModelState";

		/// <summary>
		/// When a RedirectToRouteResult is returned from an action, anything in the ViewData.ModelState dictionary will be copied into TempData.
		/// When a ViewResultBase is returned from an action, any ModelState entries that were previously copied to TempData will be copied back to the ModelState dictionary.
		/// </summary>
		/// <param name="filterContext"></param>
		public override void OnActionExecuted(ActionExecutedContext filterContext)
		{
        var controller = filterContext.Controller;
			  var modelState = controller.ViewData.ModelState;
			  var tempData = controller.TempData;
        var controllerName = filterContext.ActionDescriptor.ControllerDescriptor.ControllerName;
        var actionName = filterContext.ActionDescriptor.ActionName;
        var result = filterContext.Result;

        if (result is ViewResultBase)
  			{
  				//If there are failures in tempdata, copy them to the modelstate
                  CopyTempDataToModelState(modelState, tempData, controllerName, actionName);
  				return;
  			}

  			//If we're redirecting and there are errors, put them in tempdata instead (so they can later be copied back to modelstate)
        if ((result is RedirectToRouteResult || result is RedirectResult) && !modelState.IsValid)
  			{
            CopyModelStateToTempData(modelState, tempData, controllerName, actionName);
  			}
		}

    private void CopyTempDataToModelState(ModelStateDictionary modelState, TempDataDictionary tempData, string controllerName, string actionName)
		{
        string key = GetKey(controllerName, actionName);

        if (!tempData.ContainsKey(key)) return;

        var fromTempData = tempData[key] as ModelStateDictionary;
			  if(fromTempData == null) return; // No ModelStateDictionary found against the key, so nothing to merge

        modelState.Merge(fromTempData);
		}

		private static void CopyModelStateToTempData(ModelStateDictionary modelState, TempDataDictionary tempData, string controllerName, string actionName)
		{
        string key = GetKey(controllerName, actionName);
        tempData[key] = modelState;
		}

    private static string GetKey(string controllerName, string actionName)
    {
        return controllerName + actionName + TempDataKey;
    }
	}
}