using System;

using System.ComponentModel.DataAnnotations;

using System.Collections.Generic;

using System.Linq;

using Microsoft.Extensions.DependencyInjection;

using Newtonsoft.Json;

​

public class Program

{

    public static void Main()

    {

        // configure services

        var services = new ServiceCollection()

            .AddScoped<UserService, UserService>();

​

        // create a service provider from the service collection

        var serviceProvider = services.BuildServiceProvider();

        List<ValidationResult> validationResults = new List<ValidationResult>();

        ValidationContext vc;

        #region Success scenario

        var inputOne = new UpdateUserRequestDto()

        {

            UserName = "user001",

            Email = "user001@email.com"         

        };

        Console.WriteLine($"Input: {JsonConvert.SerializeObject(inputOne)}");

        validationResults = new List<ValidationResult>();

        vc = new ValidationContext(inputOne, null, null);

        // Register/Inject service in vc

        vc.InitializeServiceProvider(type => serviceProvider.GetService(type));

        bool isSuccessOne = Validator.TryValidateObject(inputOne, vc, validationResults, true);

        Console.WriteLine($"Scenario 1: {isSuccessOne}");

        Console.WriteLine(String.Join(",", validationResults.Select(x => x.ErrorMessage).ToList()));

        #endregion  

        Console.WriteLine("====");

        #region Failed scenario

        var inputTwo = new UpdateUserRequestDto()

        {

            UserName = "user001",

            Email = "user002@email.com"         

        };

        Console.WriteLine($"Input: {JsonConvert.SerializeObject(inputTwo)}");

        validationResults = new List<ValidationResult>();

        vc = new ValidationContext(inputTwo, null, null);

        // Register/Inject service in vc

        vc.InitializeServiceProvider(type => serviceProvider.GetService(type));

        bool isSuccessTwo = Validator.TryValidateObject(inputTwo, vc, validationResults, true);

        Console.WriteLine($"Scenario 2: {isSuccessTwo}");

        Console.WriteLine(String.Join(",", validationResults.Select(x => x.ErrorMessage).ToList()));

        #endregion

    }

}

​

public class UpdateUserRequestDto

{

    [Required]

    public string UserName { get; set; }

​

    [Required, DataType(DataType.EmailAddress), EmailUserUnique]

    public string Email { get; set; }

​

    //[Required, MinimumAgeValidator(18)]

    [Required]

    public DateTime DateOfBirth { get; set; }

}

​

​

public class EmailUserUniqueAttribute : ValidationAttribute

{

    #nullable enable

    protected override ValidationResult IsValid(object value, ValidationContext validationContext)

    {

        var usernameProperty = validationContext.ObjectType.GetProperty("UserName");

        var usernameValue = usernameProperty.GetValue(validationContext.ObjectInstance);

        UserService? _context = validationContext.GetService(typeof(UserService)) as UserService;

        //var entity = _context?.Users().Find(x => x.Email == value.ToString()).FirstOrDefault();

        var entity = _context?.Users().FirstOrDefault(x => x.Email == value.ToString()

            && x.UserName != usernameValue);

        if (entity != null)

        {

            return new ValidationResult(GetErrorMessage(value.ToString()));

        }

​

        return ValidationResult.Success;

    }

    #nullable disable

​

    public string GetErrorMessage(string email)

    {

        return $"Email {email} is already in use.";

    }

}

​

​

public class UserService

{

    public List<User> Users()

    {

        return new List<User>()

        {

            new User 

            {

                UserName = "user001",

                Email = "user001@email.com"         

            },

            new User 

            {

                UserName = "user002",

                Email = "user002@email.com"         

            }

        };

    }

}

​

public class User

{

    public string UserName { get; set; }

    public string Email { get; set; }

}