Possible API Gateway Implementation

1_ServiceInterfaces.cs

public interface IOrderService {
  IEnumerable<IOrder> GetOrders(Guid storeId);
  IOrder GetOrder(Guid storeId, Guid id);
  IOrder CreateOrder(Guid storeId);
  void SaveOrder(IOrder order);
  void DeleteOrder(IOrder order);
}

public interface ICurrencyService {
  IEnumerable<ICurrency> GetCurrencies(Guid storeId);
  ICurrency GetCurrency(Guid storeId, Guid id);
  ICurrency CreateCurrency(Guid storeId);
  void SaveCurrency(ICurrency currency);
  void DeleteCurrency(ICurrency currency);
}

2_Services.cs

public class OrderService : IOrderService
{
  public IEnumerable<IOrder> GetOrders(Guid storeId) { ... }
  public IOrder GetOrder(Guid storeId, Guid id) { ... }
  public IOrder CreateOrder(Guid storeId) { ... }
  public void SaveOrder(IOrder order) { ... }
  public void DeleteOrder(IOrder order) { ... }
}

public class CurrencyService : ICurrencyService
{
  public IEnumerable<ICurrency> GetCurrencies(Guid storeId) { ... }
  public ICurrency GetCurrency(Guid storeId, Guid id) { ... }
  public ICurrency CreateCurrency(Guid storeId) { ... }
  public void SaveCurrency(ICurrency currency) { ... }
  public void DeleteCurrency(ICurrency currency) { ... }
}

3_Api.cs

public interface IMyApi : IOrderService, ICurrencyService
{ }

public class MyApi : IMyApi
{
  private readonly Lazy<IOrderService> _orderService;
  private readonly Lazy<ICurrencyService> _currencyService;
  
  public MyApi(Lazy<IOrderService> orderService,
    Lazy<ICurrencyService> currencyService) 
  {
    _orderService = orderService;
    _currencyService = currencyService;
  }
  
  #region IOrderService
  
  public IEnumerable<IOrder> GetOrders(Guid storeId)
    => _orderService.Value.GetOrders(storeId);
    
  public IOrder GetOrder(Guid storeId, Guid id)
    => _orderService.Value.GetOrder(storeId, id);
    
  public IOrder CreateOrder(Guid storeId)
    => _orderService.Value.CreateOrder(storeId);
    
  public void SaveOrder(IOrder order)
    => _orderService.Value.SaveOrder(order);
    
  public void DeleteOrder(IOrder order)
    => _orderService.Value.DeleteOrder(order);
  
  #endregion
  
  #region ICurrencyService
  
  public IEnumerable<ICurrency> GetCurrencies(Guid storeId)
    => _currencyService.Value.GetCurrencies(storeId);
    
  public ICurrency GetCurrency(Guid storeId, Guid id)
    => _currencyService.Value.GetCurrency(storeId, id);
    
  public IOrder CreateCurrency(Guid storeId)
    => _orderService.Value.CreateCurrency(storeId);
    
  public void SaveCurrency(ICurrency currency)
    => _currencyService.Value.SaveCurrency(currency);
    
  public void DeleteCurrency(ICurrency currency)
    => _currencyService.Value.DeleteCurrency(currency);
  
  #endregion
}

4_Example.cs

public class MyClass {

  private readonly IMyApi Api { get; };

  public MyClass(IMyApi myApi)
  {
    Api = myApi;
  }

  public void DoSomething() {
    var storeId = Guid.Parse("bf1b4497-db74-45df-b226-53718349c988");
    var orderId = Guid.Parse("6549499a-b49c-4163-ab22-cf9d3514a28b");
    
    var currency = Api.CreateCurrency(storeId);
    currency.name = "Great British Pounds";
    currency.code = "GBP";
    Api.SaveCurrency(currency);
    
    var order = Api.GetOrder(storeId, orderId);
    order.CurrencyId = currency.Id;
    Api.SaveOrder(order);
  }
}