ASP.NET Core 3 系列 - 注入多個相同的介面 (Interface)

2019-10-29

-- Pageviews

通常在使用 ASP.NET Core 依賴注入 (Dependency Injection, DI) 都是一個介面對應一個實作類別。
若有多個類別時做相同的介面時，注入的方式就會有點變化。
本篇將介紹 ASP.NET Core 依賴注入多個相同的介面 (Interface)

前置準備

以下介面作為範例：

public enum WalletType
{
    Alipay,
    CreditCard,
    LinePay
}

public interface IWalletService
{
    WalletType WalletType { get; }
    void Debit(decimal amount);
}

public class AlipayService : IWalletService
{
    public WalletType WalletType { get; } = WalletType.Alipay;
    public void Debit(decimal amount)
    {
        // 從支付寶扣錢
    }
}

public class CreditCardService : IWalletService
{
    public WalletType WalletType { get; } = WalletType.CreditCard;
    public void Debit(decimal amount)
    {
        // 從信用卡扣錢
    }
}

public class LinePayService : IWalletService
{
    public WalletType WalletType { get; } = WalletType.LinePay;
    public void Debit(decimal amount)
    {
        // 從 Line Pay 扣錢
    }
}

服務註冊

用相同的介面，註冊不同的實作類別：

public class Startup
{
    public void ConfigureServices(IServiceCollection services)
    {
        services.AddMvc();

        // 註冊 Services
        services.AddSingleton<IWalletService, AlipayService>();
        services.AddSingleton<IWalletService, CreditCardService>();
        services.AddSingleton<IWalletService, LinePayService>();
    }
}

Service Injection

在 Constructor Injection 時用 IEnumerable<T> 注入，便可取得相同介面的全部實例，再依照使用情境選擇要用的實例。範例如下：

using System.Collections.Generic;
using System.Linq;
using Microsoft.AspNetCore.Mvc;

namespace MyWebsite.Controllers
{
    public class ShoppingCartController : Controller
    {
        private readonly IEnumerable<IWalletService> _walletServices;

        public ShoppingCartController(IEnumerable<IWalletService> walletServices)
        {
            _walletServices = walletServices;
        }

        public IActionResult Checkout(WalletType walletType, decimal amount)
        {
            var walletService = _walletServices.Single(x => x.WalletType == walletType);
            walletService.Debit(amount);
            return Ok();
        }
    }
}