Use custom validation responses with fluent validation

Question

Hello I am trying to get custom validation response for my webApi using .NET Core.

Here I want to have response model like

[{
  ErrorCode:
  ErrorFi         


        

Answer 1

In .net core you can use a combination of a IValidatorInterceptor to copy the ValidationResult to HttpContext.Items and then a ActionFilterAttribute to check for the result and return the custom response if it is found.

// If invalid add the ValidationResult to the HttpContext Items.
public class ValidatorInterceptor : IValidatorInterceptor {
    public ValidationResult AfterMvcValidation(ControllerContext controllerContext, ValidationContext validationContext, ValidationResult result) {
        if(!result.IsValid) {
            controllerContext.HttpContext.Items.Add("ValidationResult", result);
        }
        return result;
    }

    public ValidationContext BeforeMvcValidation(ControllerContext controllerContext, ValidationContext validationContext) {
        return validationContext;
    }
}

// Check the HttpContext Items for the ValidationResult and return.
// a custom 400 error if it is found
public class ValidationResultAttribute : ActionFilterAttribute {
    public override void OnActionExecuting(ActionExecutingContext ctx) {
        if(!ctx.HttpContext.Items.TryGetValue("ValidationResult", out var value)) {
            return;
        }
        if(!(value is ValidationResult vldResult)) {
            return;
        }
        var model = vldResult.Errors.Select(err => new ValidationErrorModel(err)).ToArray();
        ctx.Result = new BadRequestObjectResult(model);
    }
}

// The custom error model now with 'ErrorCode'
public class ValidationErrorModel {
     public string PropertyName { get; }
     public string ErrorMessage { get; }
     public object AttemptedValue { get; }
     public string ErrorCode { get; }

     public ValidationErrorModel(ValidationFailure error) {
         PropertyName = error.PropertyName;
         ErrorMessage = error.ErrorMessage; 
         AttemptedValue = error.AttemptedValue; 
         ErrorCode =  error.ErrorCode;
     }
}

Then in Startup.cs you can register the ValidatorInterceptor and ValidationResultAttribute like so:

public class Startup {
    public void ConfigureServices(IServiceCollection services) {
        services.AddTransient();
        services.AddMvc(o => {
            o.Filters.Add()
        });
    }
}