ASP.Net: System.NullReferenceException: Object reference not set to an instance of an object

Hi irshad1231,

Write the CheckBox bind code inside !IspostBack condition.

Check this example. Now please take its reference and correct your code.

HTML

<asp:CheckBoxList ID="CheckBoxList1" runat="server" RepeatDirection="Horizontal">
</asp:CheckBoxList>
<asp:Button Text="Send" runat="server" OnClick="Button1_Click" />

C#

protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        CheckBoxList1.Items.Clear();
        var currentYear = DateTime.Today.Year;
        for (int i = 4; i >= 0; i--)
        {
            // Now just add an entry that's the current year minus the counter
            CheckBoxList1.Items.Add((currentYear - i).ToString());
        }
    }
}

protected void Button1_Click(object sender, EventArgs e)
{
    string yearly = "";
    for (int i = 0; i < CheckBoxList1.Items.Count; i++)
    {
        if (CheckBoxList1.Items[i].Selected)
        {
            yearly += CheckBoxList1.Items[i].Text + ",";
        }
    }
    Response.Redirect("~/InsertDatatoKPI.aspx?userID=" + yearly.Substring(0, yearly.Length - 1));
}

LandingPage

protected void Page_Load(object sender, EventArgs e)
{
    if (!string.IsNullOrEmpty(Request.QueryString["userID"]))
    {
        Response.Write(Request.QueryString["userID"]);
    }
}

Screenshot