public ActionResult FillTypeAll() {
 
            IList<TypeInfo> types = TypeInfoService.SelectAll();
            string result = TypeInfoService.ListToJson<TypeInfo>(types);

            return this.Json(types);
}

  var typeStores = new Ext.data.Store({
                        proxy: new Ext.data.HttpProxy({
                        url: '<%= Url.Action("AddCommInfoByFile", "File") %>'
                        }),
                        reader: new Ext.data.JsonReader({
                           totalProperty:"totalProperty",
                           root: "result",
                           fields:[{name:'Id'},{name:'TypeName'}]})
                    });

                    typeStores.load();

                   // alert("-399999--");
                    //定义下拉框
                    var txtCombo = new Ext.form.ComboBox({
                       id: "Id",
                       editable:true,
                       store: typeStores,
                       emptyText:'请选择',
                       mode: 'local',
                       typeAhead:true,
                       triggerAction:'all',
                       valueField:'Id',
                       displayField: 'TypeName',
                       selectOnFocus:true,
                       width: 200,
                       resizable:true
                    });
问题是，界面上显示的下拉框没有值，数据是能查询出来的。
{"TypeInfo":[{"Id":4,"TypeName":"aaa","TypePid":0,"TypeDesc":"类型1"},{"Id":5,"TypeName":"bbb","TypePid":0,"TypeDesc":"类型2"},{"Id":6,"TypeName":"ccc","TypePid":0,"TypeDesc":"类型3"}]}