如何将选项卡控件内容的数据上下文绑定到可观察集合中的视图模型实例

TabControl 我的一个实例 StepViewModel 在一个 ObservableCollection 步骤。

我的ProcessViewModel:

pubic class ProcessViewModel : ViewModelBase
{
    public ObservableCollection<StepViewModel> Steps
    {
        get { return _steps; }
        set { _steps = value; OnPropertyChanged("Steps"); }
    }
    public StepViewModel SelectedStep
    {
        // like above...
    }
}

public class StepViewModel : ViewModelBase
{
    public string Name { get {...} set {...} }
    public object Media { get {...} set {...} }
    //...
}

我的TabControl(DataContext是ProcessViewModel):

<C1:C1TabControl
    ItemsSource="{Binding Steps}"
    SelectedItem="{Binding SelectedStep}"
    SelectionChanged="{tcSteps_OnSelectionChanged">
           <C1:C1TabControl.ContentTemplate>
                <DataTemplate>
                    <local:StepView
                        DataContext="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type vmns:ProcessViewModel}}, Path=SelectedStep}"
                        HorizontalAlignment="Stretch"
                        VerticalAlignment="Stretch">
                    </local:StepView>
                </DataTemplate>
            </C1:C1TabControl.ContentTemplate>
</C1:C1TabControl>

“System.Windows.Data错误:40:BindingExpression路径错误: 在“object”“StepViewModel”上找不到“SelectedStep”属性 (哈希代码=32952144)“。绑定表达式:路径=SelectedStep; DataItem=“StepViewModel”(哈希代码=32952144);目标元素是 'StepView'(名称='StepView');目标属性为“DataContext”(类型 '对象')“

有人知道我怎么解决这个问题吗? 谢谢