.NET Core 3 dependency injection in custom validation [duplicate] - asp.net-core

Question

This question already has answers here:
Inject Dependencies into Validation Attribute using ASP.NET Core's WebAPI
(4 answers)
Closed 1 year ago.
I want to make a custom validation attribute in .NET Core called CheckIfEmailExists. I want to make sure the user is not in my database already. So this is my create user view model:
public class CreateUserViewModel
{
public readonly UserManager userManager;
public CreateUserViewModel()
{
}
public ExtendedProfile ExtendedProfile { get; set; }
public User User { get; set; }
public int SchemeId { get; set; }
public SelectList Schemes { get; set; }
[Required]
[EmailAddress(ErrorMessage = "Invalid Email Address")]
[CheckIfEmailExists()]
[Display(Name = "Email Address")]
public string Email { get; set; }
[DataType(DataType.EmailAddress)]
[Display(Name = "Confirm Email Address")]
public string ConfirmEmail { get; set; }
}
Here is my custom validation:
public class CheckIfEmailExists : ValidationAttribute
{
private readonly UserManager _userManager;
public CheckIfEmailExists(UserManager userManager)
{
var _userManager = userManager;
}
protected override ValidationResult IsValid(object value, ValidationContext validationContext)
{
var user = (User)validationContext.ObjectInstance;
var result = _userManager.FindByEmailAsync(user.Email).Result;
//do something
return ValidationResult.Success;
}
}
I get an error when I add my custom validation on my email property, the error is that I must pass in the usermanager object to the custom class constructor.
Why doesn't my app just inject the object itself?
Is there a way I can create a user manager object in my custom class without coupling the classes?
Should I only access my database in my controller?

Answer 1

The comment above from Nikki9696 helped me and now I know how to get my user manager and dbcontext in my custom validation attribute.
public class IsEmailInDatabase : ValidationAttribute
{
protected override ValidationResult IsValid(object value, ValidationContext validationContext)
{
var context = (DatabaseContext)validationContext.GetService(typeof(DatabaseContext));
if (value == null)
{
return null;
}
else
{
var results = context.Users.AnyAsync(x => x.Email == value.ToString());
if (results.Result)
{
return new ValidationResult("This email address already exists in our database");
}
return null;
}
}
}