Intercept bad requests before reaching controller in ASP.NET Core

Question

I have a logic to apply in case the request received is a BadRequest, to do this I have created a filter:

public class ValidateModelAttribute : ActionFilterAttribute
{
    public override void OnActionExecuting(ActionExecutingContext context)
    {
        if (!context.ModelState.IsValid)
        {
            // Apply logic
        }
    }
}

In Startup:

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

Controller:

[Route("api/[controller]")]
[ApiController]
public class VerifyController : ControllerBase
{
    [Route("test")]
    [HttpPost]
    [ValidateModel]
    public ActionResult<Guid> validationTest(PersonalInfo personalInfo)
    {
        return null;
    }
}

Model:

public class PersonalInfo
{
    public string FirstName { get; set; }
    [RegularExpression("\\d{4}-?\\d{2}-?\\d{2}", ErrorMessage = "Date must be properly formatted according to ISO 8601")]
    public string BirthDate { get; set; }
}

The thing is when I put a break point on the line:

if (!context.ModelState.IsValid)

execution reaches this line only if the request I send is valid. Why it is not passing the filter if I send a bad request?

Answer 1

As the attribute filter in the life cycle of the .Net Core you can’t handle it. The filter layer with ModelState will run after the model binding.

You can handle it with .Net Core middleware as the following https://docs.microsoft.com/en-us/aspnet/core/fundamentals/middleware/?view=aspnetcore-2.1&tabs=aspnetcore2x

Answer 2

If you want to SuppressModelStateInvalidFilter on individual action, consider to use custom attribute suggested on https://docs.microsoft.com/en-us/answers/questions/297568/how-to-suppress-suppressmodelstateinvalidfilter-at.html. (And similar answer https://github.com/aspnet/Mvc/issues/8575)

public class SuppressModelStateInvalidFilterAttribute : Attribute, IActionModelConvention
     {
    
         private const string FilterTypeName = "ModelStateInvalidFilterFactory";
    
            
         public void Apply(ActionModel action)
         {
    
             for (var i = 0; i < action.Filters.Count; i++)
             {
                 //if (action.Filters[i] is ModelStateInvalidFilter)
                 if (action.Filters[i].GetType().Name == FilterTypeName)
                 {
                     action.Filters.RemoveAt(i);
                     break;
                 }
             }
        }
     }

Example of use

 [ApiController]
 public class PersonController
 {
     [SuppressModelStateInvalidFilter]
     public ActionResult<Person> Get() => new Person();
 }

Answer 3

An example of intersection for logging is describe in Log automatic 400 responses Add configuration in Startup.ConfigureServices.

services.AddControllers()
    .ConfigureApiBehaviorOptions(options =>
    {
        // To preserve the default behavior, capture the original delegate to call later.
        var builtInFactory = options.InvalidModelStateResponseFactory;

        options.InvalidModelStateResponseFactory = context =>
        {
            var logger = context.HttpContext.RequestServices.GetRequiredService<ILogger<Startup>>();

            // Perform logging here.
//E.g. logger.LogError($”{context.ModelState}”);
    logger.LogWarning(context.ModelState.ModelStateErrorsToString());

            // Invoke the default behavior, which produces a ValidationProblemDetails response.
            // To produce a custom response, return a different implementation of IActionResult instead.
            return builtInFactory(context);
        };
    });

public static String ModelStateErrorsToString(this ModelStateDictionary modelState)
        {
            IEnumerable<ModelError> allErrors = modelState.Values.SelectMany(v => v.Errors);
            StringBuilder sb = new StringBuilder();
            foreach (ModelError error in allErrors)
            {
                sb.AppendLine($"error {error.ErrorMessage} {error.Exception}");
            }
            return sb.ToString();
        }