public class UserRegisterDTO
{
    public int Id { get; set; }
    [Required]
    public string Username { get; set; }
    [Required]
    [StringLength(12, MinimumLength = 8, ErrorMessage = "You must specify password between 8 and 12 chars.")]
    public string Password { get; set; }
}

register() {
    this.authService.register(this.model).subscribe(() => {
      this.alertify.success('Registration successful');
    }, error => {
      console.log(error);
      this.alertify.error(error);
    });
  }

{Password: Array(1), Username: Array(1)}
Password: ["You must specify password between 8 and 12 chars."]
Username: ["The Username field is required."]
__proto__:
constructor: ƒ Object()
hasOwnProperty: ƒ hasOwnProperty()
isPrototypeOf: ƒ isPrototypeOf()
propertyIsEnumerable: ƒ propertyIsEnumerable()
toLocaleString: ƒ toLocaleString()
toString: ƒ ()
valueOf: ƒ valueOf()
__defineGetter__: ƒ __defineGetter__()
__defineSetter__: ƒ __defineSetter__()
__lookupGetter__: ƒ __lookupGetter__()
__lookupSetter__: ƒ __lookupSetter__()
get __proto__: ƒ __proto__()
set __proto__: ƒ __proto__()

transformModelStateErrors(modelErrors: { [key:string]: string[] }) {
    const messages: string[] = [];

    for (let key of modelErrors) {
        for (let message of modelErrors[key]) {
            messages.push(`${key}: ${message}`);
        }
    }

    return messages;
}

register() {
    this.authService.register(this.model).subscribe(() => {
        this.alertify.success('Registration successful');
    }, error => {
        const messages = this.transformModelStateErrors(error);
        this.alertify.error(messages);
    });
}