How to show data in gridview asynchronously in ASP.NET?

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My code is properly work in desktop application show data asynchronously in data gridview but when I use same code in web form it throws exception GridView1 was null. at GridView1.DataSource = table;.While debugging data is completely working just want to put that data into webform gridview.What changes should i made to acheive my task.

What I have tried:

public partial class _Default : System.Web.UI.Page
{
    public _Default()
    {

        InitTable();
    }

    public class NameAndScore
    {
        public string Name { get; set; }
        public string Score { get; set; }
    }
   
    DataTable table;
    HtmlWeb web = new HtmlWeb();


    private async Task<List<NameAndScore>> GameRankingsFromPage(int pagenum)
    {
        string url = "https://www.ebay.com/sch/i.html?_nkw=xbox+one&_in_kw=1&_ex_kw=&_sacat=0&LH_Complete=1&_udlo=&_udhi=&_ftrt=901&_ftrv=1&_sabdlo=&_sabdhi=&_samilow=&_samihi=&_sadis=15&_stpos=&_sargn=-1%26saslc%3D1&_salic=1&_sop=12&_dmd=1&_ipg=50&_fosrp=1";
        //    string url = "https://www.gamerankings.com/browse.html";

        if (pagenum != 0)
            url = "https://www.ebay.com/sch/i.html?_sacat=0&LH_Complete=1&_udlo=&_udhi=&_ftrt=901&_ftrv=1&_sabdlo=&_sabdhi=&_samilow=&_samihi=&_sadis=15&_stpos=&_sop=12&_dmd=1&_fosrp=1&_nkw=xbox+one&_pgn=" + pagenum.ToString() + "&_skc=50&rt=nc";

        var doc = await Task.Factory.StartNew(() => web.Load(url));

        var namenodes = doc.DocumentNode.SelectNodes("//*[contains(@id,'item')]/h3/a");
        var scorenodes = doc.DocumentNode.SelectNodes("//*[contains(@id,'item')]/ul[1]/li[1]/span");
        var names = namenodes.Select(node => node.InnerText.Trim('\r', '\n', '\t'));
        var scores = scorenodes.Select(node => node.InnerText.Trim('\r', '\n', '\t'));


        if (namenodes == null || scorenodes == null)
            return new List<NameAndScore>();
       


        return names.Zip(scores, (name, score) => new NameAndScore() { Name = name.ToString(), Score = score.ToString() }).ToList();
    }
    protected async void Page_Load(object sender, EventArgs e)
    {
        int pageNume = 0;
        int id = 0;
        var rankings = await GameRankingsFromPage(0);
        while (rankings.Count > 0)
        {
            foreach (var ranking in rankings)
                
          table.Rows.Add(id++, ranking.Name, ranking.Score);
            pageNume++;
            rankings = await GameRankingsFromPage(pageNume);
        }
        
    }

    private void InitTable()
    {
        table = new DataTable("Xbox Prices");
        table.Columns.Add("ID", typeof(string));
        table.Columns.Add("Name", typeof(string));
        table.Columns.Add("Score", typeof(string));

        GridView1.DataSource =  table;
        GridView1.DataBind();

    }

Posted 24-Dec-18 1:07am

Arslan jappa

Updated 1-Jan-19 0:38am

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F-ES Sitecore 1-Jan-19 8:47am

If you think this is going to gradually fill the table in the browser then that's not going to happen. In asp.net the server waits until all your code has run and all your html has been generated before sending the html to the browser in one go.

2 solutions

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AnkushK1 · Answer 1 · 2019-01-01T00:39:00

I replicated your scenario on my system. I have to made few changes, like binding gridview after data has been mapped into table. Also, while block seems to be creating an infinite loop, so i have to comment out that (you might need to check that logic again). Here is the final code and it worked for me and i can see data on the web page

protected async void Page_Load(object sender, EventArgs e)
	{
		InitTable();
		int pageNume = 0;
		int id = 0;
		var rankings = await GameRankingsFromPage(0);
		//while (rankings.Count > 0)
		//{
			foreach (var ranking in rankings)

				table.Rows.Add(id++, ranking.Name, ranking.Score);
			pageNume++;
			rankings = await GameRankingsFromPage(pageNume);
		//}

		GridView1.DataSource = table;
		GridView1.DataBind();
	}

	private void InitTable()
	{
		table = new DataTable("Xbox Prices");
		table.Columns.Add("ID", typeof(string));
		table.Columns.Add("Name", typeof(string));
		table.Columns.Add("Score", typeof(string));

		//GridView1.DataSource = table;
		//GridView1.DataBind();

	}

AnkushK1 · Answer 2 · 2018-12-25T23:47:00

Solution 1

This is because, page constructor gets invoked before page_load event. You can not access any ASP.NET control in constructor because they are loaded after constructor call. In simple terms, if you want to use any control in ASP.NET, it must be in Page_Load or any other method of page life cycle rather than constructor. You might want to go through page life cycle events using below url

ASP.NET Page Life Cycle Overview | Microsoft Docs[^]

Posted 25-Dec-18 23:47pm

AnkushK1

Comments

Arslan jappa 28-Dec-18 1:14am

so where should I put my control to get output in this scenario.?

AnkushK1 28-Dec-18 12:51pm

You can either use Page_Init event which is the first to invoke when controls are initialized, however Page_Load should also work for you since it is raised after Page_Init. So you will be able to get your GridView in both of these events.

Arslan jappa 30-Dec-18 5:02am

I have put inittable in pageLoads still not working.

AnkushK1 30-Dec-18 11:42am

You mean still getting null exception? Can you share the code that you added?

Arslan jappa 1-Jan-19 1:26am

bro I just Put InitTable() function in pageload and remove from constructor.actually This code scrap data from website like get price and name of mobile etc and working perfectly.But I want to implement in web form.I think gridview load as soon as program start before getting data from website that the reason program throw null exception.I dont know what changes I can make to run in webform thats my goal.