Dynamically assign AsyncPostbackTrigger to ASP.Net RadioButtonList using C# and VB.Net

Hi counterkin,

You need to add the controls inside a UpdatePanel ContentTemplateContainer instead of PlaceHolder.

Then assign AsyncPostBackTrigger to the RadioButtonList. 

Finally, add the AsyncPostBackTrigger to the UpdatePanel and add the UpdatePanel to the PlaceHolder.

Refer below sample.

HTML

<asp:ScriptManager runat="server" />
<asp:PlaceHolder runat="server" ID="ContainerRBL" />

Code

C#

protected void Page_Load(object sender, System.EventArgs e)
{
    this.createRBLcontrols();
}

private void createRBLcontrols()
{
    Label lbl_variant = new Label();
    lbl_variant.ID = "lbl_variant_option_selected";
    lbl_variant.Text = "Red";

    Panel titleDiv = new Panel();
    titleDiv.ID = "titleDiv";
    titleDiv.CssClass = "wrap";
    titleDiv.Controls.Add(lbl_variant);

    RadioButtonList rbl = new RadioButtonList();
    rbl.ID = "rbl_product_option1";
    rbl.Items.Add("Red");
    rbl.Items[0].Attributes.Add("variant_name", "Red");
    rbl.Items.Add("Blue");
    rbl.Items[1].Attributes.Add("variant_name", "Blue");
    rbl.Items[0].Selected = true;
    rbl.RepeatDirection = RepeatDirection.Horizontal;
    rbl.AutoPostBack = true;
    rbl.SelectedIndexChanged += SelectedIndexChanged_rbl;

    AsyncPostBackTrigger asyncPost = new AsyncPostBackTrigger();
    asyncPost.ControlID = rbl.ID;
    asyncPost.EventName = "SelectedIndexChanged";

    UpdatePanel updatePanel = new UpdatePanel();
    updatePanel.ContentTemplateContainer.Controls.Add(titleDiv);
    updatePanel.ContentTemplateContainer.Controls.Add(rbl);
    updatePanel.Triggers.Add(asyncPost);

    this.ContainerRBL.Controls.Add(updatePanel);
}

private void SelectedIndexChanged_rbl(object sender, EventArgs e)
{
    RadioButtonList rbl = (this.ContainerRBL.FindControl("rbl_product_option1") as RadioButtonList);
    if ((rbl) != null)
    {
        (this.ContainerRBL.FindControl("lbl_variant_option_selected") as Label).Text = rbl.SelectedItem.Attributes["variant_name"];
    }
}

VB.Net

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
    createRBLcontrols()
End Sub

Private Sub createRBLcontrols()
    Dim lbl_variant As Label = New Label()
    lbl_variant.ID = "lbl_variant_option_selected"
    lbl_variant.Text = "Red"

    Dim titleDiv As Panel = New Panel()
    titleDiv.ID = "titleDiv"
    titleDiv.CssClass = "wrap"
    titleDiv.Controls.Add(lbl_variant)

    Dim rbl As RadioButtonList = New RadioButtonList()
    rbl.ID = "rbl_product_option1"
    rbl.Items.Add("Red")
    rbl.Items(0).Attributes.Add("variant_name", "Red")
    rbl.Items.Add("Blue")
    rbl.Items(1).Attributes.Add("variant_name", "Blue")
    rbl.Items(0).Selected = True
    rbl.RepeatDirection = RepeatDirection.Horizontal
    rbl.AutoPostBack = True
    AddHandler rbl.SelectedIndexChanged, AddressOf SelectedIndexChanged_rbl

    Dim asyncPost As AsyncPostBackTrigger = New AsyncPostBackTrigger()
    asyncPost.ControlID = rbl.ID
    asyncPost.EventName = "SelectedIndexChanged"

    Dim updatePanel As UpdatePanel = New UpdatePanel()
    updatePanel.ContentTemplateContainer.Controls.Add(titleDiv)
    updatePanel.ContentTemplateContainer.Controls.Add(rbl)
    updatePanel.Triggers.Add(asyncPost)

    Me.ContainerRBL.Controls.Add(updatePanel)
End Sub

Private Sub SelectedIndexChanged_rbl(ByVal sender As Object, ByVal e As EventArgs)
    Dim rbl As RadioButtonList = (TryCast(Me.ContainerRBL.FindControl("rbl_product_option1"), RadioButtonList))
    If (rbl) IsNot Nothing Then
        TryCast(Me.ContainerRBL.FindControl("lbl_variant_option_selected"), Label).Text = rbl.SelectedItem.Attributes("variant_name")
    End If
End Sub

Screenshot