Angular2：2服务依赖于彼此

import {Component} from 'angular2/core';
import {TempService} from '../services/tmp';
import {Temp2Service} from '../services/tmp2';

@Component({
    selector: 'my-app',templateUrl: 'app/app/app.component.html',providers: [TempService,Temp2Service]
})
export class AppComponent { (...) }

服务1：

import {Injectable} from 'angular2/core';
import {Temp2Service} from './tmp2';

@Injectable()
export class TempService {
  constructor (private _sessionService: Temp2Service) {}
}

服务2：

import {Injectable} from 'angular2/core';
import {TempService} from './tmp';

@Injectable()
export class Temp2Service {
  constructor (private _sessionService: TempService) {}
}

运行应用程序会导致以下错误：

EXCEPTION: Cannot resolve all parameters for
‘Temp2Service'(undefined). Make sure that all the parameters are
decorated with Inject or have valid type annotations and that
‘Temp2Service’ is decorated with Injectable

当在其中一个服务中注释构造函数时,应用程序运行正常.
所以我猜测这两个服务的“交叉参考”是造成这个问题的.
你有什么想法吗？
还是我的做法已经错了？

感谢您的建议！