public class ValidateAntiSpamAttribute : ActionFilterAttribute
	{
		public string ErrorMessage { get; set; }

		public ValidateAntiSpamAttribute() { }
		public ValidateAntiSpamAttribute(string errorMessage)
		{
			this.ErrorMessage = errorMessage;
		}

		public override void OnActionExecuting(ActionExecutingContext filterContext)
		{
			string sixtimesnine = (string)filterContext.HttpContext.Request.Form["sixtimesnine"];
			string fourtytwo = (string)filterContext.HttpContext.Request.Form["fourtytwo"];

			if (string.IsNullOrEmpty(sixtimesnine) ||
				string.IsNullOrEmpty(fourtytwo) ||
				sixtimesnine != fourtytwo)
			{
				filterContext.Controller.ViewData.ModelState.AddModelError("Spam", this.ErrorMessage);
			}
			else
			{
				// Merely for unit testing puproses - we need to make sure that the above clause
				// was *not* run.
				filterContext.Controller.ViewData["filter"] = "passed";
			}
			base.OnActionExecuting(filterContext);
		}
	}

ASP.NET MVC - ValidateAntiSpamAtrribute

C#