問題緣由

最近在寫 .NET Core 的時候遇到了一個問題

A circular dependency was detected for the service of type 'WebApplication1.Service.A1Service'

A1Service

public class A1Service
{
    private readonly string guid = null;
    private readonly A2Service _a2Service = null;

    public A1Service(A2Service a2Service)
    {
        _a2Service = a2Service;

        if (guid == null)
        {
            guid = Guid.NewGuid().ToString();
        }
    }

    public void Run()
    {
        Console.WriteLine("A1Service: " + guid);
    }

    public void RunOther()
    {
        _a2Service.Run();
    }
}

這是網路教學文章上常見的從建構子中依賴注入 A2Service 的程式
接著我們再來看看另外一支程式

A2Service

public class A2Service
{
    private readonly string guid = null;
    private readonly A1Service _a1Service = null;

    public A2Service(A1Service a1Service)
    {
        _a1Service = a1Service;

        if (guid == null)
        {
            guid = Guid.NewGuid().ToString();
        }
    }

    public void Run()
    {
        Console.WriteLine("A2Service: " + guid);
    }

    public void RunOther()
    {
        _a1Service.Run();
    }
}

這裡就發生了一個循環依賴的問題 A1Service 在建構子中注入了 A2Service，而 A2Service 也在建構子中注入了 A1Service，造成兩個物件互相依賴造成 Exception

解決方法

改成不直接在建構子中注入 Service ，改注入 IServiceProvider，再利用 GetService 這個方法來取得物件，這樣就可以順利解決這個問題囉。

A3Service

public class A3Service
{
    private readonly string guid = null;
    private readonly IServiceProvider _serviceProvider = null;

    public A3Service(IServiceProvider serviceProvider)
    {
        _serviceProvider = serviceProvider;

        if (guid == null)
        {
            guid = Guid.NewGuid().ToString();
        }
    }

    public void Run()
    {
        Console.WriteLine("A3Service: " + guid);
    }

    public void RunOther()
    {
        _serviceProvider.GetService<A4Service>().Run();
    }
}

A4Service

public class A4Service
{
    private readonly string guid = null;
    private readonly IServiceProvider _serviceProvider = null;

    public A4Service(IServiceProvider serviceProvider)
    {
        _serviceProvider = serviceProvider;

        if (guid == null)
        {
            guid = Guid.NewGuid().ToString();
        }
    }

    public void Run()
    {
        Console.WriteLine("A4Service: " + guid);
    }

    public void RunOther()
    {
        _serviceProvider.GetService<A3Service>().Run();
    }
}