Add Rows in Temporary Table at runtime on button click (ASP.net C# Code)

Step 1 :  .aspx page

*********************************************************************************

Step 2: Code.cs

on page load


 if (!Page.IsPostBack)
        {
Session["tmpTable"] = DisPhotoTable();
}



 private DataTable DisPhotoTable()
    {
        dtTemp = new DataTable();
        dtTemp.Columns.Add("Name");
        dtTemp.Columns.Add("Age");
        dtTemp.Columns.Add("Sex");
        dtTemp.Columns.Add("ID Proof");
        dtTemp.Columns.Add("ID No");
        dtTemp.Columns.Add("Fare", typeof(decimal));
        return dtTemp;

    }



protected void btnadd_Click(object sender, EventArgs e)
    {
        DataRow dr;
        dtTemp = (DataTable)Session["tmpTable"];
        dr = dtTemp.NewRow();


        if (txtname.Text != string.Empty)
            dr[0] = txtname.Text.Trim();
        else
            dr[0] = "";
        if (txtage.Text != string.Empty)
            dr[1] = txtage.Text.Trim();
        else
            dr[1] = "";

        if (ddlsex.SelectedValue != string.Empty)

            dr[2] = ddlsex.SelectedValue;
        else
            dr[2] = "";
        if (ddlidtype.SelectedValue != string.Empty)
        {
            dr[3] = ddlidtype.SelectedValue;
        }
        else
        {
            dr[3] = "";
        }
        if (txtidno.Text != string.Empty)
        {
            dr[4] = txtidno.Text.Trim();
        }
        else
        {
            dr[4] = "0";
        }
        dr[5] = Convert.ToDecimal(lbltotalfare.Text); // fare eg : 2400
        dtTemp.Rows.Add(dr);

        BindTempGrid(dtTemp);
        decimal Total = Convert.ToDecimal(dtTemp.Compute("sum(Fare)", ""));
        total1.Text = Total.ToString();
 
    }

    private void BindTempGrid(DataTable dt)
    {
        gvGrid.DataSource = dt;
        gvGrid.DataBind();
        tddiv.Visible = true;
    }
    protected void gvGrid_RowDeleting(object sender, GridViewDeleteEventArgs e)
    {
        DataTable dt = (DataTable)Session["tmpTable"];
        DataRow dr = dt.Rows[e.RowIndex];
        dt.Rows.Remove(dr);
        gvGrid.EditIndex = -1;
        BindTempGrid(dt);
        string ToTAmt = dt.Compute("sum(Fare)", "").ToString();
        total1.Text = ToTAmt.ToString();
    }
    protected void gvGrid_RowCreated(object sender, GridViewRowEventArgs e)
    {
        if (e.Row.RowType == DataControlRowType.DataRow)
        {
            e.Row.Attributes.Add("onmouseover", "this.previous_color=this.style.backgroundColor;this.style.backgroundColor='#ffEECC'");
            e.Row.Attributes.Add("onmouseout", "this.style.backgroundColor=this.previous_color;");
        }
    }