Sharepoint WebMethod Session始终为null

Question

每个人都好。我有以下问题：

1. 我有一些带有一些ajax的webpart
2. 我有.ascx页面的webmethods。

问题是当我尝试使用以下任何JS函数时：

function getSessionData() {
    $.ajax({
        type: 'POST',
        url: '/_layouts/MyProject/WebService.aspx/getSessionData',
        data: "{}",
        contentType: "application/json; charset=utf-8",
        dataType: "json",
        success: function (msg) {
            // Do something interesting here.
            console.log(msg);
        }
        , error: function (xhr, ajaxOptions, thrownError) {
            console.log('error:');
            console.log(xhr.status);
            console.log(thrownError);
        }
    });
}

function setSessionData(str) {
    $.ajax({
        type: 'POST',
        url: '/_layouts/MyProject/WebService.aspx/setSessionData',
        data: "{'id': '" + str + "' }",
        contentType: "application/json; charset=utf-8",
        dataType: "json",
        success: function (msg) {
            // Do something interesting here.
            console.log(msg);
        }
        , error: function (xhr, ajaxOptions, thrownError) {
            console.log('error:');
            console.log(xhr.status);
            console.log(thrownError);
        }
    });
}

使用服务器端代码

    [WebMethod(EnableSession = true)]
    public static string getSessionData()
    {
        string wpsiid = "no data";
        try
        {
            wpsiid = HttpContext.Current.Session["wpsiid"].ToString();
        }
        catch (Exception)
        {
            wpsiid = "error occured";
            //throw;
        }
        return wpsiid;
    }

    [WebMethod(EnableSession = true)]
    public static string setSessionData(string id)
    {
        string ret = "ok";
        try
        {
            //HttpContext.Current.Session["wpsiid"] = id;
            HttpContext.Current.Session.Add("wpsiid",id);
        }
        catch (Exception)
        {
            ret = "error...";
            //throw;
        }

        return ret;
    }

我总是得到null引用异常。 那么男人的问题是如何解决它？