protected void imgbtnLogin_Click(object sender, ImageClickEventArgs e)
    {
        string excel = ConfigurationManager.AppSettings["ExcelPath"].ToString();
        DataTable dtExcel = ReadExcel(excel);
        string no = txtUserName.Text.Trim();
        string pass = txtPassword.Text.Trim();
        bool ok = false;

        foreach (DataRow row in dtExcel.Rows)
        {
            //2 no 4 şifre
            if (row[2].ToString().Trim() == no && row[4].ToString().Trim() == pass)
            {
                Response.Redirect("default.aspx");
                ok = true;
                Session.Add("UserName", "OK");
                Session.Timeout = 30;             }
        }//foreach
        if (!ok)
            Label1.Text = "Kullanıcı adı / şifre yanlış !";
    }

public static DataTable ReadExcel(string excelPath)
    {
        DataTable dtEXCEL = new DataTable();

        string strConn;
        strConn = "Provider=Microsoft.Jet.OLEDB.4.0;" +
                 "Data Source=" + excelPath + ";" +
                 "Extended Properties=Excel 8.0;";

        OleDbConnection oleConn = new OleDbConnection(strConn);
        oleConn.Open();
        object[] _obj = new object[] { null, null, null, "TABLE" };

        DataTable dtNAME = oleConn.GetOleDbSchemaTable(OleDbSchemaGuid.Tables, _obj);

        OleDbDataAdapter adp;

        if (dtNAME.Rows.Count > 0)
        {
            adp = new OleDbDataAdapter("SELECT * FROM [" + dtNAME.Rows[0]["TABLE_NAME"].ToString() + "]", oleConn);
            adp.Fill(dtEXCEL);
            adp.Dispose();
        }
        oleConn.Close();
        oleConn.Dispose();

        return dtEXCEL;
    }//ReadExcel