Error when converting Session to String in ASP.Net: Object reference not set to an instance of an object.

Your Session Tax is null and hence when you convert it to string ir returns error.

You have to write you code as below

C#

protected void Page_Load(object sender, EventArgs e)
{
    if (Session["Tax"] != null)
    {
        if (Session["Tax"].ToString() == "9")
        {
            lblTax.Text = Session["Tax"].ToString();
        }
    }
    else
    {
        lblTax.Text = "0";
    }
}