Web Api - how to stop the web pipeline directly from an OnActionExecuting Filter

Question

I have a pre-action web api hook that will check ModelState.IsValid. If the ModelState is not valid I do not want to execute the action and just return my message immediately. How exactly do I do this?

public class ValidateModelStateAttribute : ActionFilterAttribute
{
    public override void OnActionExecuting(System.Web.Http.Controllers.HttpActionContext actionContext) {
        if (!actionContext.ModelState.IsValid)
        {
            var msg = actionContext.Request.CreateErrorResponse(HttpStatusCode.BadRequest, actionContext.ModelState);
            // Now What?
        }
        base.OnActionExecuting(actionContext);
    }
}

Answer 1

set the Response.Result. If the result is not null it will not execute the action. the exact syntax is escaping me right now, but it's as simple as

if(actionContext.ModelState.IsValid == false)
{
       var response = actionContext.Request.CreateErrorResponse(...);
       actionContext.Response = response;
}