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public class AllowJsonGetAttribute : ActionFilterAttribute
{
    public override void OnResultExecuting(ResultExecutingContext filterContext)
    {
        var jsonResult = filterContext.Result as JsonResult;

        if (jsonResult == null)
            throw new ArgumentException("Action does not return a JsonResult, attribute AllowJsonGet is not allowed");

        jsonResult.JsonRequestBehavior = JsonRequestBehavior.AllowGet;            

        base.OnResultExecuting(filterContext);
    }
}