DropDown SelectedValue In GridView

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C#2.0

C#

ASP.NET

i have a drop down in gridview. i am binding it on gridviewrowdatabound event. The problem is in the same gridview i have checkbox. When checkbox is checked i am taking the selected value of dropdown for that row. The checkbox autopost event is true. But each time i am getting the first value where as i am binding this gridview on a button click i dont understand why each time selected value is same where i have change it on ui.
here is my code for checkboxcheckchange event

protected void chkDynamic_CheckedChanged(object sender, EventArgs e)
        {
            MessageCollection validationErrors = new MessageCollection();
            IabolMgmtSIOBO abolMgmt = new IabolMgmtSIOBO();
            UserBO user = (UserBO)Session["CurrentUser"];
            Message Msg = new Message();
            Msg.Text = "";
            foreach (GridViewRow gr in gvMain.Rows) 
            {
                
                CheckBox chkSel = (CheckBox)gr.FindControl("chkSel");
                DropDownList ddlService = (DropDownList)gr.FindControl("ddlService");
                if (chkSel != null && ddlService != null) 
                {
                    if (chkSel.Checked == true)
                    {
                        if (ddlService.SelectedItem.Value == "" || ddlService.SelectedItem.Value==null)
                        {
                            Msg.Text = "Select Service From DropDown First";
                            validationErrors.Add(Msg);
                            Master.ErrorMessages.Messages = validationErrors;
                            return;
                        }
                        else 
                        {
                            abolMgmt.AssignCustomRates(CurrentUser.LoginName, CurrentUser.Password, "237", int.Parse(ddlService.SelectedItem.Value), int.Parse(gvMain.DataKeys[gr.RowIndex].Value.ToString()));
                        }
                    }
                }
            
            }