Question

我正在尝试动态创建一个基本的ASP.NET TreeView控件。但是当我调用RenderControl（）来获取其输出时，ASP.NET会抛出空引用异常。当我在.ascx / aspx中声明一个没有发生的TreeView时，ASP.NET在幕后做什么？

断裂：

    protected override void DoRender(HtmlTextWriter output)
    {
        TreeView treeview = new TreeView();
        treeview.SkipLinkText = String.Empty;  //omit this for exception on get_SpacerImageUrl
        treeview.ImageSet = TreeViewImageSet.BulletedList;

        TreeNode node = new TreeNode("Node 1");
        node.ImageUrl = string.Empty;
        node.ChildNodes.Add(new TreeNode("Subnode 1"));
        node.ChildNodes.Add(new TreeNode("Subnode 2"));
        treeview.Nodes.Add(node);

        node = new TreeNode("Node 2");
        node.ChildNodes.Add(new TreeNode("Subnode 1"));
        node.ChildNodes.Add(new TreeNode("Subnode 2"));
        node.ChildNodes.Add(new TreeNode("Subnode 3"));
        treeview.Nodes.Add(node);

        treeview.RenderControl(output);
    }

工作正常：

            <asp:TreeView ID="tvMenu" runat="server">
            <Nodes>
                <asp:TreeNode Text="Node 1">
                    <asp:TreeNode Text="Section 1"></asp:TreeNode>
                    <asp:TreeNode Text="Section 2"></asp:TreeNode>
                    <asp:TreeNode Text="Section 3"></asp:TreeNode>
                </asp:TreeNode>
                <asp:TreeNode Text="Node 2">
                    <asp:TreeNode Text="Section One"></asp:TreeNode>
                    <asp:TreeNode Text="Section Two"></asp:TreeNode>
                    <asp:TreeNode Text="Section Three"></asp:TreeNode>
                </asp:TreeNode>
            </Nodes>
        </asp:TreeView>

例外情况： [NullReferenceException：对象引用未设置为对象的实例。] System.Web.UI.WebControls.TreeView.GetImageUrl（Int32 index）+3089 System.Web.UI.WebControls.TreeNode.Render（HtmlTextWriter writer，Int32 position，Boolean [] isLast，Boolean enabled）+2329

Answer 1

您可以在CodeBehind中创建动态TreeView控件，如下所示：

 using (var db = new FORMS())
                {
                    //Get Chapters from selected form
                    var query = from b in db.CHAPTERS
                                select b;

                    //Create treeview hierarchy
                    foreach (var rootItem in query)
                    {
                        TreeNode myNode = new TreeNode(rootItem.titulo, rootItem.id.ToString());

                        var childQuery = from b in db.SECTIONS
                                         select b;
                        //Add childs
                        foreach (var childItem in childQuery)
                        {
                            TreeNode myChildNode = new TreeNode(childItem.titulo, childItem.id.ToString());
                            myNode.ChildNodes.Add(myChildNode);
                        }

                        ChapterTreeView.Nodes.Add(myNode);
                    }
                }

如何以编程方式呈现ASP.NET TreeView？

1 个答案: