向ObservableCollection List对象中添加对象

如何添加一个对象到ObservableCollection列表对象?我有一个叫做" Asset "的类，我已经创建了资产的ObservableCollection列表，我想要维护它，就像在ObservableCollection列表中添加和删除元素一样。现在我得到错误，当我试图添加单个元素到ObservableCollection。

这是我的代码。

    private static ObservableCollection<Assest> _collection = null;
    public ObservableCollection<Assest> AssestList
    {
        get
        {
            if (_collection == null)
            {
                _collection = new ObservableCollection<Assest>();
            }
            return _collection;
        }
        set { _collection = value; }
    }
    public static ObservableCollection<Assest> ToObservableCollection(List<Assest> assestList)
    {
        return new ObservableCollection<Assest>(assestList);
    }
    public MainWindow()
    {
        InitializeComponent();
        this.DataContext = this;
        LoadData();
        comboBox1.ItemsSource = AssestList;
    }
    private void LoadData()
    {
        Assest assest = new Assest() { AppID = "1", AssestName = "AppName", AppDescription = "Description" };
        Assest assest2 = new Assest { AppDescription = "Des2", AppID = "2", AssestName = "hi" };
        List<Assest> assList = new List<Assest> {assest, assest2};
        ObservableCollection<Assest> generatedAssestList = ToObservableCollection(assList);
        AssestList = generatedAssestList;
    }
    // Here I get an error.
    public static void AddAppToObservalCollection(Assest ass)
    {
        _collection.Add(ass);
    }

那么如何克服这种情况。谢谢所有人。