可以'；无法获取组合框的SelectedItem或SelectedIndex

我开始爬这里的墙了。我已经用项目列表填充了一个组合框，我正在努力找出哪个项目被选中了。我还试图将组合框设置为自动选择列表中的第0项。但这也不起作用，我在GroupIndex和SelectedGroup上都遇到了绑定表达式路径错误。但是我不知道问题出在哪里

组合框Xaml:

<ComboBox     
Name="GroupComboBox"
SelectedIndex="{Binding Path=GroupIndex}"
SelectedItem="{Binding Path=SelectedGroup}"
DisplayMemberPath="Name"
SelectedValuePath ="Name"
ItemsSource="{Binding Path=Groups}"
Height="38"
HorizontalAlignment="Left"
Margin="159,115,0,0"
VerticalAlignment="Top"
Width="185"
FontSize="24" Text="Select a Group" IsEditable="False" IsReadOnly="False" />

这是填充组合框的代码。

    void webService_GroupListChanged(string response)
    {
        JavaScriptSerializer serializer = new JavaScriptSerializer();
        IList<JSONGroup> listOfGroups = new List<JSONGroup>();
        listOfGroups = serializer.Deserialize<List<JSONGroup>>(response);
        Application.Current.Dispatcher.BeginInvoke((Action)(() =>
        {
            Groups = new ObservableCollection<Group>();
            foreach(JSONGroup group in listOfGroups)
            {
                Groups.Add(new Group(group.name, group.id));
            }
        }));
    }
    private int groupIndex;
    private int GroupIndex
    {
        get { return this.groupIndex; }
        set
        {
            if (this.groupIndex != value)
            {
                this.groupIndex = value;
                this.OnPropertyChanged("GroupIndex");
            }
        }
    }

    private Group selectedGroup;
    private Group SelectedGroup
    {
        get { return this.selectedGroup; }
        set
        {
            if (this.selectedGroup != value)
            {
                this.selectedGroup = value;
                this.OnPropertyChanged("SelectedGroup");
            }
        }
    }

    private ObservableCollection<Group> groups;
    public ObservableCollection<Group> Groups
    {
        get { return this.groups; }
        set
        {
            if (this.groups != value)
            {
                this.groups = value;
                this.OnPropertyChanged("Groups");
            }
        }
    }