How to work with multiple datakeys in gridview?

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C#

ASP.NET

Gridview

I created a gridview with multiple (two) datakeys and tried to extract the datakey values in the code included below. When I load the gridview, the row["BookTaxId"] gives correct Id being used. But while I'm extracting its value from Gridview datakeys code the same column returns "1". Can anybody help me?
Code-->

Gridview:-

<asp:GridView runat="server" ID="TaxGV" DataKeyNames="Id, BookTaxId" AutoGenerateColumns="false" OnDataBound="TaxGV_DataBound" OnRowCommand="TaxGV_RowCommand" CssClass="table table-bordered table-hover table-responsive" ShowFooter="true">

LoadingGridview TaxGV:-

public void LoadTaxGridView_Table()
        {
            double Value = 0;
            int bkId = Convert.ToInt32(bookId);
            string str = ConfigurationManager.ConnectionStrings["ConStr"].ConnectionString;
            SqlConnection con = new SqlConnection(str);
            con.Open();
            SqlCommand cmd = new SqlCommand("select b.BookTaxId as BookTaxId, b.TaxId as Id, t.Name as Name, b.TaxValue as Value from BookingTax as b inner join Taxes as t on t.Id = b.TaxId where b.BookingId=@BookingId and b.Is_Delete=0", con);
            cmd.Parameters.AddWithValue("@BookingId", bkId);
            DataTable dt_BookTax = new DataTable();
            SqlDataReader reader = cmd.ExecuteReader();
            dt_BookTax.Load(reader);
            con.Close();
            foreach(DataRow row in dt_BookTax.Rows)
            {
                int Id = Convert.ToInt32(row["BookTaxId"]);//This gives correct Value of BookTaxId
                Value = Convert.ToDouble(row["Value"]);
                total_tax_Amount += (temp_Amount * Value / 100);

            }

            ViewState["BookTax_Table"] = dt_BookTax;
            TaxGV.DataSource = dt_BookTax;
            TaxGV.DataBind();
            TaxGV.Rows[TaxGV.Rows.Count - 1].Visible = false;

        }

Updaing Table through GridView TaxGV:-

public void Edit_Booking_Tax()
        {
            int bkId = Convert.ToInt32(bookId);
            string str = ConfigurationManager.ConnectionStrings["ConStr"].ConnectionString;
            SqlConnection con = new SqlConnection(str);
            con.Open();
            foreach(GridViewRow row in TaxGV.Rows)
            {
                int rowindex = row.RowIndex;
                int BookTaxId = Convert.ToInt32(TaxGV.DataKeys[rowindex].Values["BookTaxId"]);
                //This BookTaxId gives value=1; 
                
        }
}

What I have tried:

I've tried the above code but still can't collect true value of BookTaxId in

Edit_Booking_Tax()