How to retrieve value of chkbox from radio button selected index ?

<asp:DropDownList ID="ddSr" runat="server" AutoPostBack="true"
          EnableTheming="True" Height="16px"
          onselectedindexchanged="ddSr_SelectedIndexChanged" Width="190px">
      </asp:DropDownList>

<asp:RadioButtonList ID="rdoSr" runat="server" AutoPostBack="True"
                          onselectedindexchanged="rdoSr_SelectedIndexChanged">
          </asp:RadioButtonList>

protected void rdoSr_SelectedIndexChanged(object sender, EventArgs e)
    {

        SqlConnection conn = new SqlConnection(ConnString);
        SqlCommand cmd = new SqlCommand("Customer_FullJoin_State_Retrieve", conn);
        cmd.CommandType = CommandType.StoredProcedure;
        cmd.Parameters.Add("@SNo", rdoSr.SelectedItem.Value);


        conn.Open();
        SqlDataReader reader = cmd.ExecuteReader();
        reader.Read();
        {
//Error Showing Here
            chkSr.SelectedItem.Value = reader["State"].ToString();  


            reader.Close();
        }
        conn.Close();
    }

ALTER procedure [dbo].[Customer_FullJoin_State_Retrieve]
@SNo int
as
select Customer.City AS City1,State.City AS City2,Customer.Sr_No as Sr_No1, * from Customer full Join State on Customer.City = State.City
where Customer.Sr_No=@SNo
RETURN