validate dropdown if checkbox checked inside gridview

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C#

ASP.NET

<asp:GridView ID="gvAttributes" runat="server"
              AutoGenerateColumns="false" EnableModelValidation="True"
              CssClass="GridViewStyle" onrowdatabound="gvAttributes_RowDataBound">
  <Columns>
    <asp:TemplateField HeaderText="Select">
      <ItemTemplate>
        <asp:CheckBox ID="cbxAttributescheck" runat="server" Text=''
        <%# Eval("AttributeName") %> AutoPostBack="true" OnCheckedChanged="cbxAttributescheck_CheckedChanged"/>
        <asp:HiddenField ID="hdnAttributeValue" runat="server" Value=''
        <%# Eval("AttributeID") %>' />
        <asp:Label ID="lblDelete" runat="server" Text=''
          <%# Eval("AttributeID") %>'   Visible="false"></asp:Label>
      </ItemTemplate>
    </asp:TemplateField>
    <asp:TemplateField HeaderText="Data Type" >
      <ItemTemplate>
        <asp:DropDownList ID="ddlAttributesDataType" runat="server">
        </asp:DropDownList>
        <asp:RequiredFieldValidator ID="rfvAttributeType" runat="server"
                                    ErrorMessage="*"  Enabled="false"
                                    ControlToValidate="ddlAttributesDataType" InitialValue="0">
        </asp:RequiredFieldValidator>
      </ItemTemplate>
    </asp:TemplateField>
    <asp:TemplateField HeaderText="Allow Nulls" >
      <ItemTemplate>
        <asp:CheckBox ID="cbxAttributesisnull" runat="server" />
      </ItemTemplate>
    </asp:TemplateField>
  </Columns>
</asp:GridView>

protected void cbxAttributescheck_CheckedChanged(object sender, EventArgs e)
{
    CheckBox cbx = (CheckBox)sender;
    string CurrentCbxId = ((CheckBox)sender).ClientID;
    foreach (GridViewRow Row in gvAttributes.Rows)
    {
        if (((CheckBox)Row.FindControl("cbxAttributescheck")).ClientID.Equals(CurrentCbxId) && cbx.Checked)
        {
            RequiredFieldValidator rfvAttributeType = (RequiredFieldValidator)Row.FindControl("rfvAttributeType");
            rfvAttributeType.Enabled = true;
        }
    }
}

I took grid view as above.
I want to enable the required field validator for drop down, when check box is checked.
I want to do this server side not through JavaScript.
I tried but unable to work out. I tried to enable the validator, when check box checked else it will be disabled.
I need assistance on this.