button怎么取的treeview的节点

button如何取的treeview的节点
button如何取的treeview的节点：

页面左边有个treeview用来显示单位
右边有button用来查询的，有一系列的下拉框用来拼接sql条件的

<asp:TreeView ID="TreeView1" runat="server" OnSelectedNodeChanged="TreeView1_SelectedNodeChanged"
                            OnLoad="TreeView1_Load">
                      
                            <SelectedNodeStyle ForeColor="Red" Font-Bold="True" />
                        </asp:TreeView>


我的代码为
protected void btnSearch_Click(object sender, EventArgs e)
        {
            ViewState["DropDownList1"] = DropDownList1.SelectedValue;
            ViewState["DropDownList2"] = DropDownList2.SelectedValue;
            string startdate = DropDownList1.SelectedValue + DropDownList2.SelectedValue;
 if (zzjg == "dydm")
            {
                if (TreeView1.SelectedNode.Depth == 2 && TreeView1.SelectedNode.ChildNodes.Count == 0)
                {
                    Dictionary<string, string> yqt = sql_Organization1(a);
                    foreach (var y in yqt)
                    {
                        if (yqt.Count > 0)
                        {
                            TreeView1.SelectedNode.ChildNodes.Add(new TreeNode(y.Value, y.Key));
                        }
                    }

                }
            }

 if (TreeView1.SelectedNode.ChildNodes.Count == 0)???????????
  我这个条件就取不到树形节点，老是为空错在这里的
｛
     ......  //井别分类提示 1 油井  2 气井 3  水井
                switch (DropDownList10.SelectedValue)

                {
                    case "1":
                        GridView2.Visible = true;
                        GV2Bind(sb.ToString());
                        break;
                    case "3":
                        GridView8.Visible = true;
                        waterBind(sWaterSql.ToString());
                        break;
                    default:
                        break;


                }
｝









                

TreeView 服务器
[解决办法]
因为你没有选中任何节点
你判断的依据是SelectedNode 而当你不选任何节点的时候 就会报NullReference错误了
[解决办法]