Validate DropDownList inside GRIDVIEW

In Coding Page

<asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="False" OnRowCancelingEdit="GridView1_RowCancelingEdit" OnRowEditing="GridView1_RowEditing" OnRowUpdating="GridView1_RowUpdating" OnRowDataBound="GridView1_RowDataBound" Font-Names="Verdana" Font-Size="Small" style="z-index: 100; left: 41px; position: absolute; top: 66px" DataKeyNames="productid">

<Columns>

<asp:BoundField DataField="productid" HeaderText="ProductID" />

<asp:BoundField DataField="productname" HeaderText="ProductName" />

<asp:BoundField DataField="price" HeaderText="Price" />

<asp:TemplateField HeaderText="Choose">

<ItemTemplate>

<asp:DropDownList ID="DropDownList1" runat="server" Width="100px">asp:DropDownList><br />

ItemTemplate>

<EditItemTemplate>

<asp:DropDownList ID="DropDownList1" runat="server" Width="100px">asp:DropDownList><br />

<asp:RequiredFieldValidator ID="RequiredFieldValidator2" runat="server" ControlToValidate="DropDownList1" Display="dynamic"

ErrorMessage="Please Select the Rating" InitialValue="Select Rating">asp:RequiredFieldValidator>

EditItemTemplate>

asp:TemplateField>

<asp:CommandField ShowEditButton="True" />

Columns>

asp:GridView>

In .CS File

using System.Data.SqlClient;

public partial class _Default : System.Web.UI.Page

{

SqlConnection con; SqlDataAdapter da; SqlCommand cmd; DataSet ds;

protected void Page_Load(object sender, EventArgs e)

{

con = new SqlConnection("server=FOCALPOI-392A7D\\SQLEXPRESS;user id=raji;password=chekuri;database=raji");

if (IsPostBack == false)

{

getdata();

}

}

protected void GridView1_RowDataBound(object sender, GridViewRowEventArgs e)

{

if (e.Row.RowType == DataControlRowType.DataRow)

{

DropDownList ddList = (DropDownList)e.Row.FindControl("DropDownList1");

//DropDown List First item is same as InitialValue propertie

ddList.Items.Add("Select Rating");

ddList.Items.Add("1");

ddList.Items.Add("2");

ddList.Items.Add("3");

ddList.Items.Add("4");

ddList.Items.Add("5");

}

}

public void getdata()

{

da = new SqlDataAdapter("select * from product", con);

ds = new DataSet();

da.Fill(ds, "product");

GridView1.DataSource = ds;

GridView1.DataBind();

}

protected void GridView1_RowEditing(object sender, GridViewEditEventArgs e)

{

GridView1.EditIndex = e.NewEditIndex;

getdata();

}

protected void GridView1_RowCancelingEdit(object sender, GridViewCancelEditEventArgs e)

{

GridView1.EditIndex = -1;

getdata();

}

//we are not doing any updation

protected void GridView1_RowUpdating(object sender, GridViewUpdateEventArgs e)

{

GridView1.EditIndex = -1;

getdata();

}

}