ActionFilterAttribute for model validation

ModelValidationAttribute.cs

public class ModelValidationAttribute : ActionFilterAttribute
{
    public override void OnActionExecuting(HttpActionContext actionContext)
    {
        var arguments = actionContext.ActionArguments;

        var request = actionContext.Request;

        if (HasNullArgument(arguments))
        {
            string message = $"{arguments.First(_ => _.Value == null).Key} cannot be null.";

            actionContext.Response = request.CreateErrorResponse(HttpStatusCode.BadRequest, message);
        }
        else if (!actionContext.ModelState.IsValid)
        {
            actionContext.Response = request.CreateErrorResponse(HttpStatusCode.BadRequest, actionContext.ModelState);
        }
    }

    private static bool HasNullArgument(Dictionary<string, object> arguments) => arguments.Any(_ => _.Value == null);
}