How to Bind to a List Member

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C#

WPF

Hi!
I use this Service:

public Service()
{
        BList = new ObservableCollection<object>()
            {new CommonData() { MenuName="name1",Id="s01"}
            ,new BOption() {MenuName="nam2"}
            ,new BOption(){MenuName="nam3"}};
}
        public ObservableCollection<object> BList { get; private set; }

Then this ViewModel:

public class CommonScreenViewModel
    {
        public ViewModel(IService Service)
        {
            this.Service=Service;
            this.ImportCommand = new DelegateCommand<object>(this.OnImport, this.CanImport);
        }

        public IService Service;
        public ICommand ImportCommand { get; private set; }
        private void OnImport(object arg) {
            MessageBox.Show("Importing...");
        }
        private bool CanImport(object arg) { return true; }
    }

I set the dataContext in the View.xaml.cs like this:

public View(ViewModel viewModel)
        {
            InitializeComponent();
            this.Loaded += (s, e) =>
            { this.DataContext = viewModel; };
        }

In the XAML file i want to do some binding like this:

<Button Content="Command" IsEnabled="{Binding CanImport}" >
        <i:Interaction.Triggers>
                <i:EventTrigger EventName="Click">
                <i:InvokeCommandAction Command="{Binding ImportCommand}"/>
                </i:EventTrigger>
        </i:Interaction.Triggers>
</Button>
<TextBox Text="{Binding Path=Service.BList[0].Id, Mode=TwoWay}"/>

My problem is that the Button Binding works fine but the textbox does not.

If I change the view.xaml.cs to

public View(ViewModel viewModel)
        {
            InitializeComponent();
            this.Loaded += (s, e) =>
            { this.DataContext = viewModel.Service.BList[0]; };
        }

and bind my textbox with just

<TextBox Text="{Binding Path=Id, Mode=TwoWay}"/>

then the TextBox binds fine but the button doen't work

Any suggestions? (thnx in advance)