title: How to return error from ASP.NET MVC action
body: <p>I am using ASP.NET MVC for developing a web site. I am using jquery for AJAX functionality. In the action methods, I want to return some error to signal that the input is not correct or that the action could not be performed. In such error cases, I expect the jquery ajax error handler to be called and I can take appropriate action in there.  I have not found a way how to do this. Following is my action method.</p>
  
  <p>In error cases, what should I be sending from an Action in order to get the jquery error handler triggered?</p>
  
  <pre><code>public ActionResult AddToFavourites(int entityId, string entityType)
      {
  
          if (!Request.IsAjaxRequest())
              throw new InvalidOperationException("This action can be called only in async style.");
  
          try
          {
              RBParams.EntityType typeOfFavourite = (RBParams.EntityType)Enum.Parse(typeof(RBParams.EntityType), entityType);
              string status = "";
  
              if (typeOfFavourite == RBParams.EntityType.BusinessEntity)
              {
                  status = MarkFavouriteEntity(entityId);
              }
              else if (typeOfFavourite == RBParams.EntityType.Review)
              {
                  status = MarkFavouriteReview(entityId);
              }
              else
              {
                  throw new InvalidOperationException("The type of the entity is not proper");
              }
  
              return Content(status);
  
          }
          catch (Exception ex)
          {
  
              return Content("Error");
          }
      }
  </code></pre>