asp.net-mvc – 为什么MVC提供的Default AccountController中有2

public class AccountController : Controller
{
    public IFormsAuthentication FormsAuth { get; private set; }
    public IMembershipService MembershipService { get; private set; }

    public AccountController()
        : this(null,null)
    {
    }
    public AccountController(IFormsAuthentication formsAuth,IMembershipService membershipService)
    {
        FormsAuth = formsAuth ?? new FormsAuthenticationService();
        MembershipService = membershipService ?? new AccountMembershipService();

        //---
    }

这很容易理解.

public AccountController(IFormsAuthentication formsAuth,IMembershipService membershipService)
    {
        FormsAuth = formsAuth ?? new FormsAuthenticationService();
        MembershipService = membershipService ?? new AccountMembershipService();
    }

这是什么？它的目的是什么？它是特定于帐户控制器还是其他控制器的要求？而且,我为什么要将它纳入我的项目？

public AccountController()
        : this(null,null)
    {
    }

他们似乎在其他两个地方使用这种类型的构造函数.

谢谢你的帮助