Appropriate design pattern for the payment modules c#

Question

As i am learning through design pattern concept and also wanted to implement the payment modules in my project using the proper design pattern. So for that I have created some sample code.

Currently I have two concrete implementation for the payment PayPal and Credit Card. But the concrete implementation will be added further on the project.

Payment Service

public interface IPaymentService
{
    void MakePayment<T>(T type) where T : class;
}

Credit Card and Pay Pal Service

public class CreditCardPayment : IPaymentService
{
    public void MakePayment<T>(T type) where T : class
    {
        var creditCardModel = (CreditCardModel)(object)type;
        //Implementation CreditCardPayment
    }
}

class PayPalPayment : IPaymentService
{
    public void MakePayment<T>(T type) where T : class
    {
        var payPalModel = (PayPalModel)(object)type;
        //Further Implementation will goes here
    }
}

Client Code Implementation

var obj = GetPaymentOption(payType);
obj.MakePayment<PayPalModel>(payPalModel);

Get Payment Option

private static IPaymentService GetPaymentOption(PaymentType paymentType)
{
        IPaymentService paymentService = null;

        switch (paymentType)
        {
            case PaymentType.PayPalPayment:
                paymentService = new PayPalPayment();
                break;
            case PaymentType.CreditCardPayment:
                paymentService = new CreditCardPayment();
                break;
            default:
                break;
        }
        return paymentService;
}

I thought of implementing this modules using strategy design pattern, and I got deviated from Strategy and ended up doing this way.

Is this a proper way for creating the payment modules. Is there a more better approach of solving this scenario. Is this a design pattern?

Edited:

Client Code:

static void Main(string[] args)
{
    PaymentStrategy paymentStrategy = null;


    paymentStrategy = new PaymentStrategy(GetPaymentOption((PaymentType)1));
    paymentStrategy.Pay<PayPalModel>(new PayPalModel() { UserName = "", Password = "" });

    paymentStrategy = new PaymentStrategy(GetPaymentOption((PaymentType)2));
    paymentStrategy.Pay<CreditCardModel>(
       new CreditCardModel()
    {
        CardHolderName = "Aakash"
    });

    Console.ReadLine();

}

Strategy:

public class PaymentStrategy
{
    private readonly IPaymentService paymentService;
    public PaymentStrategy(IPaymentService paymentService)
    {
        this.paymentService = paymentService;
    }

    public void Pay<T>(T type) where T : class
    {
        paymentService.MakePayment(type);
    }
}

Does this update inlines with the Strategy Pattern?

Answer 1

One major drawback of using an abstract factory for this is the fact that it contains a switch case statement. That inherently means if you want to add a payment service, you have to update the code in the factory class. This is a violation of the Open-Closed Principal which states that entities should be open for extension but closed for modification.

Note that using an Enum to switch between payment providers is also problematic for the same reason. This means that the list of services would have to change every time a payment service is added or removed. Even worse, a payment service can be removed from the strategy, but still be an Enum symbol for it even though it isn't valid.

On the other hand, using a strategy pattern doesn't require a switch case statement. As a result, there are no changes to existing classes when you add or remove a payment service. This, and the fact that the number of payment options will likely be capped at a small double-digit number makes the strategy pattern a better fit for this scenario.

Interfaces

// Empty interface just to ensure that we get a compile
// error if we pass a model that does not belong to our
// payment system.
public interface IPaymentModel { }

public interface IPaymentService
{
    void MakePayment<T>(T model) where T : IPaymentModel;
    bool AppliesTo(Type provider);
}

public interface IPaymentStrategy
{
    void MakePayment<T>(T model) where T : IPaymentModel;
}

Models

public class CreditCardModel : IPaymentModel
{
    public string CardHolderName { get; set; }
    public string CardNumber { get; set; }
    public int ExpirtationMonth { get; set; }
    public int ExpirationYear { get; set; }
}

public class PayPalModel : IPaymentModel
{
    public string UserName { get; set; }
    public string Password { get; set; }
}

Payment Service Abstraction

Here is an abstract class that is used to hide the ugly details of casting to the concrete model type from the IPaymentService implementations.

public abstract class PaymentService<TModel> : IPaymentService
    where TModel : IPaymentModel
{
    public virtual bool AppliesTo(Type provider)
    {
        return typeof(TModel).Equals(provider);
    }

    public void MakePayment<T>(T model) where T : IPaymentModel
    {
        MakePayment((TModel)(object)model);
    }

    protected abstract void MakePayment(TModel model);
}

Payment Service Implementations

public class CreditCardPayment : PaymentService<CreditCardModel>
{
    protected override void MakePayment(CreditCardModel model)
    {
        //Implementation CreditCardPayment
    }
}

public class PayPalPayment : PaymentService<PayPalModel>
{
    protected override void MakePayment(PayPalModel model)
    {
        //Implementation PayPalPayment
    }
}

Payment Strategy

Here is the class that ties it all together. Its main purpose is to provide the selection functionality of the payment service based on the type of model passed. But unlike other examples here, it loosely couples the IPaymentService implementations so they are not directly referenced here. This means without changing the design, payment providers can be added or removed.

public class PaymentStrategy : IPaymentStrategy
{
    private readonly IEnumerable<IPaymentService> paymentServices;

    public PaymentStrategy(IEnumerable<IPaymentService> paymentServices)
    {  
        this.paymentServices = paymentServices ?? throw new ArgumentNullException(nameof(paymentServices));
    }

    public void MakePayment<T>(T model) where T : IPaymentModel
    {
        GetPaymentService(model).MakePayment(model);
    }

    private IPaymentService GetPaymentService<T>(T model) where T : IPaymentModel
    {
        var result = paymentServices.FirstOrDefault(p => p.AppliesTo(model.GetType()));
        if (result == null)
        {
            throw new InvalidOperationException(
                $"Payment service for {model.GetType().ToString()} not registered.");
        }
        return result;
    }
}

Usage

// I am showing this in code, but you would normally 
// do this with your DI container in your composition 
// root, and the instance would be created by injecting 
// it somewhere.
var paymentStrategy = new PaymentStrategy(
    new IPaymentService[]
    {
        new CreditCardPayment(), // <-- inject any dependencies here
        new PayPalPayment()      // <-- inject any dependencies here
    });


// Then once it is injected, you simply do this...
var cc = new CreditCardModel() { CardHolderName = "Bob" /* Set other properties... */ };
paymentStrategy.MakePayment(cc);

// Or this...
var pp = new PayPalModel() { UserName = "Bob" /* Set other properties... */ };
paymentStrategy.MakePayment(pp);

Additional References:

• Dependency Injection Unity - Conditional Resolving
• Factory method with DI and IoC

Answer 2

This is one approach you could take. There's not a lot to go on from your source, and I'd really reconsider having MakePayment a void instead of something like an IPayResult.

public interface IPayModel { }  // Worth investigating into common shared methods and properties for this 
public interface IPaymentService
{
    void MakePayment(IPayModel  payModel);
}
public interface IPaymentService<T> : IPaymentService where T : IPayModel
{
    void MakePayment(T payModel);  // Void here?  Is the status of the payment saved on the concrete pay model?  Why not an IPayResult?
}

public class CreditCardModel : IPayModel
{
    public string CardHolderName { get; set; }
}
public class PayPalModel : IPayModel
{
    public string UserName { get; set; }
    public string Password { get; set; }
}

public class CreditCardPayment : IPaymentService<CreditCardModel>
{
    public void MakePayment(CreditCardModel payModel)
    {
        //Implmentation CreditCardPayment
    }
    void IPaymentService.MakePayment(IPayModel payModel)
    {
        MakePayment(payModel as CreditCardModel);
    }
}
public class PayPalPayment : IPaymentService<PayPalModel>
{
    public void MakePayment(PayPalModel payModel)
    {
        //Implmentation PayPalPayment
    }
    void IPaymentService.MakePayment(IPayModel payModel)
    {
        MakePayment(payModel as PayPalModel);
    }
}

public enum PaymentType
{
    PayPalPayment = 1,
    CreditCardPayment = 2
}

So following your implementation approach, it could look something like:

static class Program
{
    static void Main(object[] args)
    {
        IPaymentService paymentStrategy = null;
        paymentStrategy = GetPaymentOption((PaymentType)1);
        paymentStrategy.MakePayment(new PayPalModel { UserName = "", Password = "" });

        paymentStrategy = GetPaymentOption((PaymentType)2);
        paymentStrategy.MakePayment(new CreditCardModel { CardHolderName = "Aakash" });

        Console.ReadLine();
    }

    private static IPaymentService GetPaymentOption(PaymentType paymentType) 
    {
        switch (paymentType)
        {
            case PaymentType.PayPalPayment:
                return new PayPalPayment();
            case PaymentType.CreditCardPayment:
                return new CreditCardPayment();
            default:
                throw new NotSupportedException($"Payment Type '{paymentType.ToString()}' Not Supported");
        }
    }
}

I also think for a strategy/factory pattern approach, manually creating an IPayModel type doesn't make much sense. Therefore you could expand the IPaymentService as an IPayModel factory:

public interface IPaymentService
{
    IPayModel CreatePayModel();
    void MakePayment(IPayModel payModel);
}
public interface IPaymentService<T> : IPaymentService where T : IPayModel
{
    new T CreatePayModel();
    void MakePayment(T payModel);
}

public class CreditCardPayment : IPaymentService<CreditCardModel>
{
    public CreditCardModel CreatePayModel()
    {
        return new CreditCardModel();
    }
    public void MakePayment(CreditCardModel payModel)
    {
        //Implmentation CreditCardPayment
    }

    IPayModel IPaymentService.CreatePayModel()
    {
        return CreatePayModel();
    }
    void IPaymentService.MakePayment(IPayModel payModel)
    {
        MakePayment(payModel as CreditCardModel);
    }
} 

Usage would then be:

IPaymentService paymentStrategy = null;
paymentStrategy = GetPaymentOption((PaymentType)1);

var payModel = (PayPalModel)paymentStrategy.CreatePayModel();
payModel.UserName = "";
payModel.Password = "";
paymentStrategy.MakePayment(payModel);