WPF MVVM ComboBox SelectedItem或SelectedValue不起作用

public class ViewModel : INotifyPropertyChanged
{
    private object _selectedItem;
    public object SelectedItem
    {
        get { return _selectedItem; }
        set
        {
            _selectedItem = value;
            OnPropertyChanged("SelectedItem");
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;

    protected virtual void OnPropertyChanged(string propertyName)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }
}

          SelectedItem="{Binding SelectedItem}" />

public MainWindow()
{
    InitializeComponent();
    DataContext = new ViewModel();
}