Errors.cs

public abstract class Error
{
    public abstract ErrorType Type { get; }
}

public class SimpleError : Error
{
    public override ErrorType Type { get { return ErrorType.Simple; } }
    public string Message { get; private set; }
    
    public SimpleError(string message)
    {
        Message = message;
    }
}

public class UserNotFoundError : Error
{
    public override ErrorType Type { get { return ErrorType.UserNotFound; } }
    public int UserId { get; private set; }
    
    public UserNotFoundError(int userId)
    {
        UserId = userId;
    }
}

public enum ErrorType
{
    Simple,
    UserNotFound
}