Enable Disable ASP.Net RequiredFieldValidator in ASP.Net GridView based on DropDownList using C#

Refer below code.

HTML

<asp:GridView ID="gridview" runat="server" AutoGenerateColumns="false">
    <Columns>
        <asp:TemplateField HeaderText="t1" ItemStyle-CssClass="ddl_Class_new" ItemStyle-HorizontalAlign="Center">
            <ItemTemplate>
                <div>
                    <asp:DropDownList ID="ddl1" runat="server" AutoPostBack="true" OnSelectedIndexChanged="OnSelectedIndexChanged"
                        BackColor="Yellow" CssClass="ddl_Class_new">
                        <asp:ListItem Text="[ === === === ]" Value=""></asp:ListItem>
                        <asp:ListItem Text="OK" Value="OK"></asp:ListItem>
                        <asp:ListItem Text="KO" Value="KO"></asp:ListItem>
                    </asp:DropDownList>
                </div>
                <br />
            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="t2" ItemStyle-CssClass="ddl_Class_new" ItemStyle-HorizontalAlign="Center">
            <ItemTemplate>
                <div>
                    <asp:DropDownList ID="ddl2" runat="server" AutoPostBack="true" OnSelectedIndexChanged="OnSelectedIndexChanged"
                        BackColor="Yellow" CssClass="ddl_Class_new">
                        <asp:ListItem Text="[ === === === ]" Value=""></asp:ListItem>
                        <asp:ListItem Text="OK" Value="OK"></asp:ListItem>
                        <asp:ListItem Text="KO" Value="KO"></asp:ListItem>
                    </asp:DropDownList>
                </div>
                <br />
            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="t3" ItemStyle-CssClass="ddl_Class_new" ItemStyle-HorizontalAlign="Center">
            <ItemTemplate>
                <div>
                    <asp:DropDownList ID="ddl3" runat="server" AutoPostBack="true" OnSelectedIndexChanged="OnSelectedIndexChanged"
                        BackColor="Yellow" CssClass="ddl_Class_new">
                        <asp:ListItem Text="[ === === === ]" Value=""></asp:ListItem>
                        <asp:ListItem Text="OK" Value="OK"></asp:ListItem>
                        <asp:ListItem Text="KO" Value="KO"></asp:ListItem>
                    </asp:DropDownList>
                </div>
                <br />
            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="Note" ItemStyle-CssClass="ddl_Class_new" ItemStyle-HorizontalAlign="Center">
            <ItemTemplate>
                <div>
                    <br />
                    <asp:TextBox ID="t_txt" runat="server" CssClass="ddl_Class_new" TextMode="MultiLine"
                        Enabled="true"></asp:TextBox>
                    <asp:RequiredFieldValidator ID="rftxt" runat="server" ControlToValidate="t_txt" Enabled="false"
                        SetFocusOnError="true" ErrorMessage="Insert value" Display="None" ValidationGroup="Validation2"
                        CssClass="validation-summary-errors-one"></asp:RequiredFieldValidator>
                    <%--<asp:ValidationSummary ID="Validation2" ValidationGroup="Validation2" runat="server"
                        ShowMessageBox="true" CssClass="validation-summary-errors" ShowSummary="false" />--%>
                </div>
            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField>
            <ItemTemplate>
                <asp:ImageButton ID="btn" ImageUrl="/aspnet/img/btn.gif" ValidationGroup="Validation2"
                    runat="server" OnClick="btn_Click" CssClass="ddl_Class_new" Visible="true" />
            </ItemTemplate>
        </asp:TemplateField>
    </Columns>
</asp:GridView>

Code

protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        DataTable dt = new DataTable();
        dt.Columns.AddRange(new DataColumn[1] { new DataColumn("ID") });
        dt.Rows.Add("101");
        dt.Rows.Add("102");
        gridview.DataSource = dt;
        gridview.DataBind();
    }
}

protected void OnSelectedIndexChanged(object sender, EventArgs e)
{
    DropDownList ddl = (DropDownList)sender;
    GridViewRow row = (GridViewRow)ddl.NamingContainer;

    DropDownList t1 = (DropDownList)row.FindControl("ddl1");
    DropDownList t2 = (DropDownList)row.FindControl("ddl2");
    DropDownList t3 = (DropDownList)row.FindControl("ddl3");
    TextBox t_txt = (TextBox)row.FindControl("txt");
    RequiredFieldValidator t_rftxt = (RequiredFieldValidator)row.FindControl("rftxt");

    string sp_t1 = t1.SelectedValue;
    string sp_t2 = t2.SelectedValue;
    string sp_t3 = t3.SelectedValue;

    if (sp_t1.ToUpper() == "KO" || sp_t2.ToUpper() == "KO" || sp_t3.ToUpper() == "KO")
    {
        t_rftxt.Enabled = true;
    }
    else
    {
        t_rftxt.Enabled = false;
    }
}

protected void btn_Click(object sender, EventArgs e)
{

}

Screenshot