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public enum UserTypes
{
    STANDARD,
    PREMIUM,
    DIAMOND
}

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public class User
{
    public string Name { get; set; }
    public UserTypes Type { get; set; }
}

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public abstract class DiscountRate
{
    protected abstract double PercentOfDiscountRate { get; }
    public abstract double CalculateDiscount(double value);
}

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public sealed class Standard : DiscountRate
{
    protected override double PercentOfDiscountRate { get =>  0.0; }

    public override double CalculateDiscount(double value)
    {
        return value * PercentOfDiscountRate;
    }
}
public sealed class Premium : DiscountRate

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public class PurchaseOrder
{
    public double Value { get; set; }
    public User User { get; set; }

    private DiscountRate _discountRate { get; set; }

    public double CalculateOrderPrice()
    {
        if (User.Type == UserTypes.STANDARD)

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public class User
{
  ....
    public DiscountRate GetDiscountRate()
    {
        if (Type == UserTypes.STANDARD)
            return new Standard();
        else if (Type == UserTypes.PREMIUM)
            return new Premium();
        else if (Type == UserTypes.DIAMOND)

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public class UserTypeAttribute : Attribute
{
    private Type _implementation { get; }

    public DiscountRate CreateInstance => (DiscountRate) Activator.CreateInstance(_implementation);

    public UserTypeAttribute(Type implementation)
    {
        _implementation = implementation;
    }

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  public enum UserTypes
  {
      [UserTypeAttribute(typeof(Standard))]
      STANDARD,
      [UserTypeAttribute(typeof(Premium))]
      PREMIUM,
      [UserTypeAttribute(typeof(Diamond))]
      DIAMOND
  }

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public static class UserTypesExtensionMethods
{
    public static DiscountRate GetDiscountRateImplementation(this Enum userType)
    {
         ....
    }
}

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public static class UserTypesExtensionMethods
{
    public static DiscountRate GetDiscountRateImplementation(this Enum userType)
    {
        var attribute = typeof(UserTypes).GetFields()
            .Where(f => f.Name == userType.ToString())
            .FirstOrDefault()
            ?.GetCustomAttributes(false).First() as UserTypeAttribute;

        if (attribute != null)