Pass ModelState between actions - Allows you to do a redirect to the GET action and retain the ModelState after a failed update (instead of recreating the view model and potentially repeating yourself).

ModelStateTempDataFilters.cs

public abstract class ModelStateTempDataTransfer : ActionFilterAttribute
{
  protected static readonly string Key = typeof(ModelStateTempDataTransfer).FullName;
}
 
public class ExportModelStateToTempData : ModelStateTempDataTransfer
{
	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))
			{
				filterContext.Controller.TempData[Key] = filterContext.Controller.ViewData.ModelState;
			}
		}
 
		base.OnActionExecuted(filterContext);
	}
}
 
public class ImportModelStateFromTempData : ModelStateTempDataTransfer
{
	public override void OnActionExecuted(ActionExecutedContext filterContext)
	{
		ModelStateDictionary modelState = filterContext.Controller.TempData[Key] as ModelStateDictionary;
 
		if (modelState != null)
		{
			//Only Import if we are viewing
			if ((filterContext.Result is ViewResult) || (filterContext.Result is RedirectResult))
			{
				filterContext.Controller.ViewData.ModelState.Merge(modelState);
			}
			else
			{
				//Otherwise remove it.
				filterContext.Controller.TempData.Remove(Key);
			}
		}
 
		base.OnActionExecuted(filterContext);
	}
}