Extjs4+asp.net mvc2 返回json有问题

public ActionResult testExtJsJson()

        {

            

            TreeModel ExtTree1 = new TreeModel();

            ExtTree1.id = "1";

            ExtTree1.text = "加工工艺管理";

            ExtTree1.leaf = true;

            ExtTree1.cls = "file";

            ExtTree1.href = "";

            ExtTree1.children = new List<TreeModel>();

            



            //string json = JsonConvert.SerializeObject(ExtTree1);



            //Response.ContentType = "text/html";

            //return View();

            return this.Json(ExtTree1, JsonRequestBehavior.AllowGet);

            //return Content("<pre>" + json+"</pre>", "text/html");

        }

Ext.require([

    'Ext.tree.*',

    'Ext.data.*',

    'Ext.tip.*'

]);



Ext.onReady(function() {

    Ext.QuickTips.init();

    

    var store = Ext.create('Ext.data.TreeStore', {

        proxy: {

            type: 'ajax',

            url: 'Home/testExtJsJson'

        },

        root: {

            text: 'Ext JS',

            id: 'src',

            expanded: true

        },

        folderSort: true,

        sorters: [{

            property: 'text',

            direction: 'ASC'

        }]

    });



    var tree = Ext.create('Ext.tree.Panel', {

        store: store,

        viewConfig: {

            plugins: {

                ptype: 'treeviewdragdrop'

            }

        },

        renderTo: 'tree-div',

        height: 300,

        width: 250,

        title: 'Files',

        useArrows: true,

        dockedItems: [{

            xtype: 'toolbar',

            items: [{

                text: 'Expand All',

                handler: function(){

                    tree.expandAll();

                }

            }, {

                text: 'Collapse All',

                handler: function(){

                    tree.collapseAll();

                }

            }]

        }]

    });

});

其实就是把官方的例子中的url改了一下！

页面总是显示：{"id":"1","text":"加工工艺管理","cls":"file","expanded":false,"leaf":true,"href":"","children":[]}

用firebug看body中是<pre style="word-wrap: break-word; white-space: pre-wrap; ">{"id":"1","text":"加工工艺管理","cls":"file","expanded":false,"leaf":true,"href":"","children":[]}</pre>

ps：用的是vs2010自带的asp.net mvc2 空项目