Improved filters for POST redirect GET model state transfer.

SafeModelStateTempDataTransfer.cs

using System.Web.Mvc;

namespace AspDotNetStorefront.Filters
{
	public class SafeExportModelStateToTempData : ActionFilterAttribute
	{
		public const string Key = "ExportedModelState";

		public override void OnActionExecuted(ActionExecutedContext filterContext)
		{
			// Only export when ModelState is not valid
			if(!filterContext.Controller.ViewData.ModelState.IsValid)
				// Export if we are redirecting
				if((filterContext.Result is RedirectResult) || (filterContext.Result is RedirectToRouteResult))
				{
					var key = GenerateKey(
						filterContext.ActionDescriptor.ControllerDescriptor.ControllerName,
						filterContext.ActionDescriptor.ActionName);

					filterContext.Controller.TempData[key] = filterContext.Controller.ViewData.ModelState;
				}

			base.OnActionExecuted(filterContext);
		}

		public static string GenerateKey(string controllerName, string actionName)
			=> $"{Key}-{controllerName}-{actionName}";
	}

	public class SafeImportModelStateFromTempData : ActionFilterAttribute
	{
		public override void OnActionExecuting(ActionExecutingContext filterContext)
		{
			var key = SafeExportModelStateToTempData.GenerateKey(
				filterContext.ActionDescriptor.ControllerDescriptor.ControllerName,
				filterContext.ActionDescriptor.ActionName);

			var modelState = filterContext.Controller.TempData[key] as ModelStateDictionary;
			if(modelState != null)
				filterContext.Controller.ViewData.ModelState.Merge(modelState);

			base.OnActionExecuting(filterContext);
		}
	}
}