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'

1	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();
    }
}

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();
    }
}

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();
    }
}

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();
    }
}

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();

}

之前嘗試很多次都失敗，這次終於成功註冊Controller、Service與Repository了，來記錄一下設定檔。

AutofacConfig.cs

/// <summary>
/// DI設定檔
/// </summary>
public class AutofacConfig
{
	/// <summary>
	/// 註冊DI注入物件資料
	/// </summary>
	public static void Register()
	{
		// 容器建立者
		ContainerBuilder builder = new ContainerBuilder();

		// 註冊Controllers
		builder.RegisterControllers(Assembly.GetExecutingAssembly());

		// 註冊Service
		builder.RegisterAssemblyTypes(Assembly.GetExecutingAssembly())
			   .Where(t => t.Name.EndsWith("Service"))
			   .AsSelf();

		// 註冊Repository
		builder.RegisterAssemblyTypes(Assembly.GetExecutingAssembly())
			   .Where(t => t.Name.EndsWith("Repository"))
			   .AsSelf();

		// 建立容器
		IContainer container = builder.Build();

		// 解析容器內的型別
		AutofacDependencyResolver resolver = new AutofacDependencyResolver(container);

		// 建立相依解析器
		DependencyResolver.SetResolver(resolver);
	}
}

/// <summary>

/// DI設定檔

/// </summary>

public class AutofacConfig

{

/// <summary>

/// 註冊DI注入物件資料

/// </summary>

public static void Register()

{

// 容器建立者

ContainerBuilder builder = new ContainerBuilder();

// 註冊Controllers

builder.RegisterControllers(Assembly.GetExecutingAssembly());

// 註冊Service

builder.RegisterAssemblyTypes(Assembly.GetExecutingAssembly())

.Where(t => t.Name.EndsWith("Service"))

.AsSelf();

// 註冊Repository

builder.RegisterAssemblyTypes(Assembly.GetExecutingAssembly())

.Where(t => t.Name.EndsWith("Repository"))

.AsSelf();

// 建立容器

IContainer container = builder.Build();

// 解析容器內的型別

AutofacDependencyResolver resolver = new AutofacDependencyResolver(container);

// 建立相依解析器

DependencyResolver.SetResolver(resolver);

}

Global.asax

C#

protected void Application_Start() { //加入這行 AutofacConfig.Register(); }

1
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3
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5

protected void Application_Start()
{
//加入這行
AutofacConfig.Register();
}

DI

[C#][DI] 相互依賴造成循環注入死結

問題緣由

A1Service

A2Service

解決方法

A3Service

A4Service

[C#][ASP.NET MVC5][Autofac] DI Framework – AutofacConfig.cs