Problem newrow in GridView

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C#

ASP.NET

I have problems when creating new rows.I have a GridView 3 columns:
DropDownList (column name: ProductName), textbox (name column: Number), TextBox (column name: value) and Button Field to add new row with CommandName="addrow"
when the page loads,I bind Data into DropDownList.
The user will select the product name in DropDownList and enter the value into the textbox in the column named price,quantity.
After clicking LinkButtonField,a new row is added to the GridView.
But the value of previous row not stored.
Because for loop re-create controls of previous row(Dropdownlist,TextBox.
How to value of previous row not lost?
My code:

//Product class
clsProduct p=new clsProduct();
DataTable dt; 
void LoadData()
{
        dt = //get data from table Product(idPro,ProName,.....)
        GridView2.DataSource = tbl;//table containt a row
        GridView2.DataBind();
        //Problem is here
        for (int i= 0; i < GridView2.Rows.Count; i++)
        {
            //Find DropDownList ProductName
            DropDownList cbo = (DropDownList)GridView2.Rows[cborow].Cells[0].FindControl("DrdProName");
            cbo.DataSource = dt;
            cbo.DataValueField = "idPro";
            cbo.DataTextField = "TProName";
            cbo.DataBind();
}

Make table for GridView:

 void MakeTableGridView()
{
        if (tbl.Rows.Count == 0 || tbl==null)
        {
            tbl.Columns.Add("idPro", typeof(string));
            tbl.Columns.Add("quantity", typeof(int));
            tbl.Columns.Add("price", typeof(int));
            RowNew();
        }
}

New row:

void RowNew()
    {

        dr = tbl.NewRow();
        dr[0] = "";
        dr[1] = "1";
        dr[2] = "0";
        tbl.AcceptChanges();
        tbl.Rows.Add(dr);
    }

Event when click AddRow LinkButtonField:

protected void GridView2_RowCommand(object sender, GridViewCommandEventArgs e)
{
        string cmd = e.CommandName;
        if (cmd =="addrow")
        {
            //Add New row in datatable
            RowNew();
            LoadData();
        }
}