Managing Multiple Payment Integrations in C#: Strategies for Unified Interfaces and Scalable Code

Modern applications often integrate with multiple payment providers (e.g., Stripe, PayPal, or Adyen), each with unique APIs, request formats, and response structures. As a C# developer, managing these differences can become complex. This article explores strategies to unify payment integrations into a maintainable code structure while allowing flexibility for future changes.

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1. Why Use a Unified Interface?

A unified interface simplifies interactions with payment providers by abstracting their differences. For example, regardless of the provider, your application needs to:

• Process payments.
• Handle refunds.
• Check transaction statuses.

By defining a common IPaymentService interface, you enforce consistency:

public interface IPaymentService
{
    Task<PaymentResult> ProcessPaymentAsync(PaymentRequest request);
    Task<RefundResult> ProcessRefundAsync(RefundRequest request);
    Task<TransactionStatus> GetTransactionStatusAsync(string transactionId);
}

Each provider (e.g., StripePaymentService, PayPalPaymentService) implements this interface, hiding provider-specific logic.

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2. Code Structure Strategies

Strategy 1: Adapter Pattern for Request/Response Mapping

Payment providers require different payloads and return varied responses. Use adapter classes to convert your application’s standardized models into provider-specific formats.

Example:

public class StripePaymentAdapter : IPaymentService
{
    public async Task<PaymentResult> ProcessPaymentAsync(PaymentRequest request)
    {
        // Convert PaymentRequest to Stripe's API model
        var stripeRequest = new StripePaymentRequest 
        { 
            Amount = request.Amount, 
            Currency = request.Currency 
        };

        // Call Stripe API
        var stripeResponse = await _stripeClient.Process(stripeRequest);

        // Convert Stripe response to your app's PaymentResult
        return new PaymentResult 
        { 
            Success = stripeResponse.IsSuccessful, 
            TransactionId = stripeResponse.Id 
        };
    }
}

Strategy 2: Factory Pattern for Provider Selection

Use a factory to instantiate the correct payment service based on configuration or user choice:

public class PaymentServiceFactory
{
    public IPaymentService Create(string providerName)
    {
        return providerName switch
        {
            "Stripe" => new StripePaymentAdapter(),
            "PayPal" => new PayPalPaymentAdapter(),
            _ => throw new NotSupportedException()
        };
    }
}

Strategy 3: Centralized Error Handling

Wrap provider-specific exceptions into a common error type:

public class StripePaymentAdapter : IPaymentService
{
    public async Task<PaymentResult> ProcessPaymentAsync(PaymentRequest request)
    {
        try 
        {
            // Call Stripe API
        }
        catch (StripeException ex)
        {
            throw new PaymentException("Stripe payment failed", ex);
        }
    }
}

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3. Adding New Payment Methods or Features

Approach 1: Extend the Interface

If a new feature (e.g., recurring payments) is supported by most providers, extend the interface:

public interface IPaymentService
{
    // Existing methods...
    Task<SubscriptionResult> CreateSubscriptionAsync(SubscriptionRequest request);
}

Providers that don’t support the feature can throw NotImplementedException, but this violates the Interface Segregation Principle.

Approach 2: Segregate Interfaces

Split the interface into smaller, focused contracts:

public interface IRecurringPaymentService
{
    Task<SubscriptionResult> CreateSubscriptionAsync(SubscriptionRequest request);
}

Only providers supporting subscriptions implement this interface.

Approach 3: Use Optional Parameters or Middleware

For minor differences (e.g., a provider requires an extra field), add optional parameters to methods:

public Task<PaymentResult> ProcessPaymentAsync(PaymentRequest request, string providerSpecificField = null);

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4. Key Takeaways

• Unified Interface: Use a base interface to standardize core operations.
• Adapters: Isolate provider-specific logic in adapter classes.
• Dependency Injection: Inject the correct adapter at runtime using a factory.
• Modular Design: Follow the Open/Closed Principle—your code should be open for extension (new providers) but closed for modification.

By adopting these strategies, you reduce technical debt and ensure that adding a new payment channel (e.g., Bitcoin) requires minimal changes to existing code—just a new adapter and factory registration. This approach keeps your codebase clean, testable, and scalable.