Custom Model Validation Response Globally in DotNetCore

We can validate model globally and can can have custom response globally by using ActionFilter where we can just validate context.ModelState.IsValid or not and based on that we can have custom response there. The advantage of using this filter is that we can set it for few controllers where as if we looking for to apply globally for each controller in dotnet core, now we can change the ApiBehavior using ApiBehaviorOptions. The below code is for creation for extension of IServiceCollection and then simple use inside ConfigureServices();

services.UseCoreAppCustomModelValidation();

Extension:

using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Mvc.ModelBinding;
using Microsoft.Extensions.DependencyInjection;
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Dynamic;
using System.Linq;
 
namespace CoreApp.ModelStateValidation
{
    public static class ServiceCollectionExtensions
    {
        public static IServiceCollection UseCoreAppCustomModelValidation(this IServiceCollection services)
        {
            _ = services.Configure((Action<ApiBehaviorOptions>)(apiBehaviorOptions =>
                    apiBehaviorOptions.InvalidModelStateResponseFactory = actionContext =>
                    {
                        var message = GetModelStateErrorWithException(actionContext);
 
                        return new BadRequestObjectResult(new
                        {
                            Code = 400,
                            Status = 0,
                            Messages = message
                        });
                    }));
            return services;
        }
 
        private static ExpandoObject GetModelStateFriendlyError(ActionContext actionContext)
        {
            var modelType = actionContext.ActionDescriptor.Parameters.FirstOrDefault()?.ParameterType; //Get model type  
                                                                                                       //p => p.BindingInfo.BindingSource.Id.Equals("Body", StringComparison.InvariantCultureIgnoreCase)
 
            var expandoObj = new ExpandoObject();
            var expandoObjCollection = (ICollection<KeyValuePair<string, object>>)expandoObj; //Cannot convert IEnumrable to ExpandoObject  
 
            var dictionary = actionContext.ModelState.ToDictionary(k => k.Key, v => v.Value)
                .Where(v => v.Value.ValidationState == ModelValidationState.Invalid)
                .ToDictionary(
                k =>
                {
                    if (modelType != null)
                    {
                        var property = modelType.GetProperties().FirstOrDefault(p => p.Name.Equals(k.Key, StringComparison.InvariantCultureIgnoreCase));
                        if (property != null)
                        {
                            //Try to get the attribute  
                            var displayName = property.GetCustomAttributes(typeof(DisplayNameAttribute), true).Cast<DisplayNameAttribute>().SingleOrDefault()?.DisplayName;
                            return displayName ?? property.Name;
                        }
                    }
                    return k.Key; //Nothing found, return original validation key  
                },
                v => v.Value.Errors.Select(e => e.ErrorMessage).ToList() as object); //Box String collection
 
            foreach (var keyValuePair in dictionary)
            {
                expandoObjCollection.Add(keyValuePair);
            }
 
            return expandoObj;
        }
 
        private static List<string> GetModelStateErrorWithException(ActionContext actionContext)
        {
            List<string> errorMessages = new List<string>();
            foreach (var error in actionContext.ModelState)
            {
                string message = error.Value.Errors != null && error.Value.Errors.Count() > 0 ?
                                    error.Value.Errors[0].Exception != null ?
                                            error.Key + " : " + error.Value.Errors[0].Exception.Message   //More weight-age is given to exception, so in case of one, use the exception message
                                    : error.Value.Errors[0].ErrorMessage != null ?
                                            error.Key + " : " + error.Value.Errors[0].ErrorMessage //Otherwise use error message
                                    : string.Empty
                            : string.Empty;
 
                if (!string.IsNullOrWhiteSpace(message))
                {
                    errorMessages.Add(message);
                }
            }
            return errorMessages;
        }
 
        private static List<string> GetModelStateError(ActionContext actionContext)
        {
            return actionContext.ModelState.Values.SelectMany(x => x.Errors)
                            .Select(x => x.ErrorMessage).ToList();
        }
    }
}