How to customize the built-in MVC validation response format? - asp.net-mvc

Question

I'm using my own middleware to capture exceptions thrown in my API to format the response to the client. This includes things like checking for the dev env to send additional information and logging. This all works great but the built-in validation middleware responds with a different response format. I want to keep the functionality and just change what data is sent to the client and how it's formatted.
Currently it returns the default
{
"message": "Validation error(s)",
"details": [
"The value '35353535353535353535353535353535353535353535' is not valid."
]
}

Answer 1

You can customize the default response by using a BadResultObject in the InvalidaModelStateResponseFactory of the ApiBehaviorOptions class. As an example:
apiBehaviorOptions.InvalidModelStateResponseFactory = actionContext => {
return new BadRequestObjectResult(new {
Code = 400,
Request_Id = "Someuniqueid",
Messages = actionContext.ModelState.Values.SelectMany(x => x.Errors)
.Select(x => x.ErrorMessage)
});
Configured:
serviceCollection.PostConfigure<ApiBehaviorOptions>(apiBehaviorOptions =>
apiBehaviorOptions.InvalidModelStateResponseFactory = ...
);
Or you can send the response directly from the action you are using as well with your own custom validation error result class. For example:
public class ValidationError
{
[JsonProperty(NullValueHandling=NullValueHandling.Ignore)]
public string Field { get; }
public string Message { get; }
public ValidationError(string field, string message)
{
Field = field != string.Empty ? field : null;
Message = message;
}
}
public class ValidationResultModel
{
public string Message { get; }
public List<ValidationError> Errors { get; }
public ValidationResultModel(ModelStateDictionary modelState)
{
Message = "Validation Failed";
Errors = modelState.Keys
.SelectMany(key => modelState[key].Errors.Select(x => new
ValidationError(key, x.ErrorMessage)))
.ToList();
}
}
Then we can create our own IActionResult. Here:
public class ValidationFailedResult : ObjectResult
{
public ValidationFailedResult(ModelStateDictionary modelState)
: base(new ValidationResultModel(modelState))
{
StatusCode = StatusCodes.Status404...;
}
}
And update our ValidateModelAttribute by overriding the OnActionExecuting to perform actions before they are taken.
public class ValidateModelAttribute : ActionFilterAttribute
{
public override void OnActionExecuting(ActionExecutingContext context)
{
if (!context.ModelState.IsValid)
{
context.Result = new ValidationFailedResult(context.ModelState);
}
}
}
Sources:
Customize automatic response on validation error
https://www.jerriepelser.com/blog/validation-response-aspnet-core-webapi/