findControl in Naming container (Asp.net webform C#)

Question

I have a problem about findControl method in naming container. It's not the first trouble about that and I would like understand the theory. I found many solutions on website but nothing works

I have a DetailsView which contains controls. I put DefaultMode "Insert" and I add 2 radio buttons

<asp:DetailsView ID="DetailsView1" runat="server"
        ItemType="[...]"
        DefaultMode="Insert"
        [...]">
        <Fields>
            <asp:TemplateField>
                <InsertItemTemplate>
                    <asp:Panel ID="Panel1" runat="server" GroupingText="Create or Select">
                        <div class="Select">
                            <asp:RadioButton ID="RB_Select" runat="server" Text="Select" Checked="True" AutoPostBack="true" OnCheckedChanged ="RB_Select_CheckedChanged" />
                            <asp:DropDownList runat="server" ID="DDL_Select"
                                ItemType="[...]"
                                [...]
                                AutoPostBack="true">
                            </asp:DropDownList>
                        </div>
                        <div class="New">
                            <asp:RadioButton ID="RB_New" runat="server" Text="New" Checked="false" AutoPostBack="true" OnCheckedChanged="RB_New_CheckedChanged" />
                            <asp:TextBox ID="TXB_New" runat="server" Enabled="false" Text="<%# BindItem.Label %>"></asp:TextBox>
                        </div>
                    </asp:Panel>
                </InsertItemTemplate>
          </asp:TemplateField>
      </Fields>
</asp:DetailsView>

And for exemple in my behind Code, I Just want to test if radiobutton is check or not :

protected void RB_New_CheckedChanged(object sender, EventArgs e)
{
        var RadioButtonNew = (RadioButton)FindControl("RB_New");
        var RadioButtonSelect = (RadioButton)FindControl("RB_Select");

        RadioButtonSelect.Checked = !RadioButtonNew.Checked;
}

And I have a "System.NullReferenceException" because it doesn't find my controls.

Why it doesn't recognize my controls? And how to deal with this?

Thanks in advance

Answer 1

You are using FindControl on a Page level. But the Controls are inside a DetailsView, so you need to access that first.

TextBox tb = DetailsView1.FindControl("TXB_New") as TextBox;

//or

var RadioButtonSelect = (RadioButton)DetailsView1.FindControl("RB_Select");

Answer 2

Thank you again, I found the solution.

I didn't know but Panel element acted like a container. I just add a findControl :

var RadioButtonSelect = (RadioButton)DetailsView1.FindControl("Panel1").FindControl("RB_Select");