可访问子项返回空值

我正在将使用AccessibleChildren方法的一些VBA代码转换为C#，但我遇到了一些问题。声明如下所示：

[DllImport("oleacc.dll")]
    private static extern uint AccessibleChildren(IAccessible paccContainer, int iChildStart, int cChildren, [Out] object[] rgvarChildren, out int pcObtained);

我正在运行的代码如下所示：

private static object[] GetChildren(IAccessible element)
{
    const int firstchild = 2;
    int numChildren;
    int numReturned;
    numChildren = element.accChildCount;
    object[] childrenArray = null;
    if (numChildren > 0)
    {
        childrenArray = new object[numChildren];
        AccessibleChildren(element, firstchild, numChildren, childrenArray, out numReturned);
    }
    return childrenArray;
}

问题是，当我在有效的 IAccessible（如 Word 中的功能区）上调用此方法时，childrenArray[0]等于 null 。有人有什么想法吗？我的一个想法（或者至少对我来说没有意义）是 VBA 代码传递数组的第一个元素：

Private Function GetChildren _
                 (Element As IAccessible) _
             As Variant()
    Const FirstChild        As Long = 0&
    Dim NumChildren         As Long
    Dim NumReturned         As Long
    Dim ChildrenArray()
    NumChildren = Element.accChildCount
    If NumChildren > 0 Then
    AccessibleChildren Element, FirstChild, NumChildren, _
                           ChildrenArray(0), NumReturned
    End If
    GetChildren = ChildrenArray
End Function

谢谢！