ASP Net Core Web API自定义模型验证

ASP Net Core Web API，我正在尝试为模型验证器返回自定义响应。但是，当必填字段不在请求中时，不会调用ValidateModelFilter。

ValidateModelFilter.cs

public class ValidateModelFilter : ActionFilterAttribute
{
    public override void OnActionExecuting(ActionExecutingContext context)
    {
        if (!context.ModelState.IsValid)
        {
             var response = new Model
             {
                 error = context.ModelState.ToString()
             };
             context.Result = new BadRequestObjectResult(response);
         }
    }
}

启动文件

public void ConfigureServices(IServiceCollection services)
{
    services.AddMvc(options => options.Filters.Add(typeof(ValidateModelFilter)));
}

我得到这样的回应

{
  "errors": {
    "Firstname": [
      "The Firstname field is required."
    ]
  },
  "type": "https://tools.ietf.org/html/rfc7231#section-6.5.1",
  "title": "One or more validation errors occurred.",
  "status": 400,
  "traceId": "|e6752549-4142e91f1074e978."
}

我想返回一个像

{
"error": "The Firstname field is required."
}