asp.net TreeView中checkBox的单选

如图

代码：

protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
                       TreeView1.Attributes.Add("onclick ", "OnClientTreeNodeChecked(event) "); 

        }
        
    }

//识别不同的浏览器 
    function getTargetElement(evt) {
        var elem
        if (evt.target) {
            elem = (evt.target.nodeType == 3) ? evt.target.parentNode : evt.target
        }
        else {
            elem = evt.srcElement
        }
        return elem
    }

    var lastC = null;
    function OnClientTreeNodeChecked(evt) {
        evt = (evt) ? evt : ((window.event) ? window.event : " ");
        if (evt == " ") {
            return;
        }
        var obj = getTargetElement(evt);
        var hasTreeNode = false;
        if (obj.tagName) {
            if (obj.tagName == "INPUT" && obj.type == "checkbox") {
                if (lastC) lastC.checked = false
                lastC = obj;
                obj.checked = true;

            }
        }
    }