How to convert Ext.data.TreeStore loading from file to loading from .net webservice?

I have a Ext.data.TreeStore which read data from text file :

this.store = Ext.create('Ext.data.TreeStore', {
        model: "GeoExt.data.LayerTreeModel",
        proxy: new Ext.data.HttpProxy({
            url: 'data/test.json',
            reader: {
                type: 'json',
                root: 'children',
                idProperty: 'Id'
            }
        }),
        folderSort: true
    });

This is the file content:

[
    {
        text: "Category1",
        leaf: false,
        expanded: true,
        checked: false,
        children: [
            {
                text: "A1",
                layer: "A2",
                name: "A3",
                leaf: true,
                checked: false,
                nodeType: "gx_layer"
            },
            {
                text: "B1",
                layer: "B2",
                name: "B3",
                leaf: true,
                checked: false,
                nodeType: "gx_layer"
            }
            ],
        nodeType: "gx_layer"
    },
    {
        text: "Category2",
        leaf: false,
        expanded: true,
        checked: false,
        children: [
            {
                text: "C1",
                layer: "C2",
                name: "C3",
                leaf: true,
                checked: false,
                nodeType: "gx_layer"
            }],
        nodeType: "gx_layer"
    }
]

I've noticed in firebug that the get doesn't return JSON struct , but plain text. but it works.

I want to get the same result but getting the tree nodes from webservice (which brings it back from postgres).

The first simplest thing I tried was (also tried text/json):

[WebMethod]        
        public void GetJSON()
        {
            HttpContext.Current.Response.Clear();
            HttpContext.Current.Response.ContentType = "text/plain";
            HttpContext.Current.Response.Charset = "utf-8";

            string res = File.ReadAllText(@"D:\test.json");
            HttpContext.Current.Response.Write(res);
        }     

And I'm getting error :

Request format is unrecognized for URL unexpectedly ending in '/getJSON'.

which make sense because the file isn't JSON (but this is how I managed to make it work).

I would like help with 2 things :

1. What is the correct JSON format I need to use to get the same result as I got in this example ?
2. How do I serve the same data from webservice ?

Thank you !