interface INetworkJoin : INotifyPropertyChanged
{
    string Name {get;set;}
    void Join();
}

class DomainJoin : INetworkJoin
{
    string Name {get;set;}
    string Username {get;set;}
    SecureString {get;set;}
    void Join()
    {
        // Domain join code
    }
}

class WorkgroupJoin : INetworkJoin
{
    string Name {get;set;}
    void Join()
    {
        // Workgroup join code
    }
}

class NetworkJoinViewModel : INotifyPropertyChanged
{
    private DomainJoin _domainJoin;
    private WorkgroupJoin _workgroupJoin;

    public NetworkJoinViewModel(INetworkJoin defaultNetworkJoin)
    {
        switch (defaultNetworkJoin)
        {
            case DomainJoin dj:
                _domainJoin = dj;
                _workgroupJoin = new WorkgroupJoin();
                break;
            case WorkgroupJoin wj:
                _domainJoin = new DomainJoin;
                _workgroupJoin = wj;
                break;
        }

        ActiveJoin = defaultNetworkJoin;
    }

    public INetworkJoin ActiveJoin { get; private set; }

    public string Name
    {
        get => ActiveJoin.Name;
        set => ActiveJoin.Name = value;
    }

    public string Username
    {
        get => ActiveJoin is DomainJoin dj ? dj.Username : null;
        set
        {
            if (ActiveJoin is DomainJoin dj)
            {
                dj.Username = value;
            }
        }
    }

    public string Password
    {
        get => ActiveJoin is DomainJoin dj ? dj.Password: null;
        set
        {
            if (ActiveJoin is DomainJoin dj)
            {
                dj.Password= value;
            }
        }
    }

    public void Join()
    {
        ActiveJoin.Join();
    }
}