Asp.net Web api 6: Use ValidationAttribute for a unique userName - validation

Question

I do not have much code here
But I want to create my own validation for username that will not have duplicates.
Model:
[Table("User")]
public partial class User
{
[Key]
public Guid Id { get; set; } = Guid.NewGuid();
[Column("userName")]
[StringLength(200)]
[AllValidation(ErrorMessage = "foo")]
[Required(ErrorMessage = "Username is a required field")]
public string UserName { get; set; } = null!;
[StringLength(50, MinimumLength = 3, ErrorMessage = "The password should be between 3 characters to 50")]
[Required(ErrorMessage = "Password is a required field")]
public string Password { get; set; } = null!;
//[Column(TypeName = "date")]
[DisplayFormat(ApplyFormatInEditMode = true, DataFormatString = "{0:yyyy/MM/dd}")]
public DateTime? LastLogin { get; set; }
public string? Token { get; set; }
}
ValidationAttribute:
public class AllValidationAttribute : ValidationAttribute
{
private readonly TalkBackDbContext _context;
public AllValidationAttribute(TalkBackDbContext context)
{
_context = context;
}
public override string FormatErrorMessage(string name)
{
return _context.Users.SingleOrDefault(x => x.UserName == name)!.ToString()!;
}
}
I get an error when I try to insert ErrorMessage into an attribute
this is the error:

Answer 1

you can do this.
remove it from constructor and override IsValid method.
public class AllValidationAttribute : ValidationAttribute
{
private string username;
protected override ValidationResult IsValid(object value,
ValidationContext validationContext)
{
var _context = (TalkBackDbContext )validationContext
.GetService(typeof(TalkBackDbContext ));
username = value.ToString();
if(!_context.Users.Any(cus => cus.UserName == value.ToString()))
{
return ValidationResult.Success;
}
else
{
return new ValidationResult
("Unique Name expected" + value.ToString());
}
}
public override string FormatErrorMessage(string name)
{
return "Expected Uniquename" + username;
}
}