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我正在尝试运行本教程http://csharpsdk.org/docs/web/getting-started 并尝试在我的网站上进行 facebook 登录，但我无法运行教程中的最后一步，它给了我：系统。 NullReferenceException：对象引用未设置为对象的实例。for var accessToken = Session["AccessToken"].ToString(); 这是我所有的javascript

    window.fbAsyncInit = function () {
        FB.init({
            appId: '112249508938212', // App ID
            status: true, // check login status
            cookie: true, // enable cookies to allow the server to access the session
            xfbml: true  // parse XFBML
        });

        // Additional initialization code here
        FB.Event.subscribe('auth.authResponseChange', function (response) {
            if (response.status === 'connected') {
                // the user is logged in and has authenticated your
                // app, and response.authResponse supplies
                // the user's ID, a valid access token, a signed
                // request, and the time the access token 
                // and signed request each expire
                var uid = response.authResponse.userID;
                var accessToken = response.authResponse.accessToken;

                // TODO: Handle the access token
                // Do a post to the server to finish the logon
                // This is a form post since we don't want to use AJAX
                var form = document.createElement("form");
                form.setAttribute("method", 'post');
                form.setAttribute("action", '/FacebookLogin.ashx');

                var field = document.createElement("input");
                field.setAttribute("type", "hidden");
                field.setAttribute("name", 'accessToken');
                field.setAttribute("value", accessToken);
                form.appendChild(field);

                document.body.appendChild(form);
                form.submit();
            } else if (response.status === 'not_authorized') {
                // the user is logged in to Facebook, 
                // but has not authenticated your app
            } else {
                // the user isn't logged in to Facebook.
            }
        });
    };

    // Load the SDK Asynchronously
    (function (d) {
        var js, id = 'facebook-jssdk', ref = d.getElementsByTagName('script')[0];
        if (d.getElementById(id)) { return; }
        js = d.createElement('script'); js.id = id; js.async = true;
        js.src = "//connect.facebook.net/en_US/all.js";
        ref.parentNode.insertBefore(js, ref);
    }(document));

我也有 FacebookLogin.ashx 文件，其中包含网站提供的内容。

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