The dynamic loading of user controls follows the same logic as in normal postback application. You can load custom user controls via AJAX requests by following the steps below:

1. You need to have an appropriate container for it, for example ASP:Panel.

2. Instantiate your user control in Page_Init or Page_Load and always recreate the last loaded User Control:

   • C#
   • VB.NET

   CopyC#

   UserControl MyControl = (UserControl)LoadControl(controlName);
   LoadMyUserControl(CurrentControl, Panel1);

   CopyVB.NET

   Dim MyControl As UserControl = Me.LoadControl(controlName)
   Me.LoadMyUserControl(Me.CurrentControl, Panel1)

3. Make sure you assign unique ID to the dynamically loaded User Control:

   • C#
   • VB.NET

   CopyC#

   string userControlID = controlName.Split('.')[0];
   MyControl.ID = userControlID.Replace("/", "").Replace("~", "");

   CopyVB.NET

   Dim userControlID As String = controlName.Split(".")(0)
   MyControl.ID = userControlID.Replace("/", "").Replace("~", "")

4. Place the instance inside the controls collection of the container:

   CopyCS/VB

   Parent.Controls.Add(MyControl)

5. Here is the code from a sample project:

CopyASPX

<telerik:RadScriptManager ID="RadScriptManager1" runat="server">
</telerik:RadScriptManager>
<asp:Button ID="Button1" runat="server" Text="Load WebUserControl1.ascx" OnClick="Button1_Click" />
<asp:Button ID="Button2" runat="server" Text="Load WebUserControl2.ascx" OnClick="Button2_Click" />
<asp:Panel ID="Panel1" runat="server">
</asp:Panel>
<telerik:RadAjaxManager ID="RadAjaxManager1" runat="server">
    <AjaxSettings>
        <telerik:AjaxSetting AjaxControlID="Button1">
            <UpdatedControls>
                <telerik:AjaxUpdatedControl ControlID="Panel1" />
            </UpdatedControls>
        </telerik:AjaxSetting>
        <telerik:AjaxSetting AjaxControlID="Button2">
            <UpdatedControls>
                <telerik:AjaxUpdatedControl ControlID="Panel1" />
            </UpdatedControls>
        </telerik:AjaxSetting>
    </AjaxSettings>
</telerik:RadAjaxManager>

And in the code-behind:

• C#
• VB.NET

CopyC#

protected void Page_Load(object sender, EventArgs e)
{
    if (this.CurrentControl != string.Empty)
    {
        LoadMyUserControl(CurrentControl, Panel1);
    }
}
protected void Button1_Click(object sender, EventArgs e)
{
    this.LoadMyUserControl("~/WebUserControl1.ascx", Panel1);
}
protected void Button2_Click(object sender, EventArgs e)
{
    this.LoadMyUserControl("~/WebUserControl2.ascx", Panel1);
}

private string CurrentControl
{
    get
    {
        return this.ViewState["CurrentControl"] == null ? string.Empty : (string)this.ViewState["CurrentControl"];
    }
    set
    {
        this.ViewState["CurrentControl"] = value;
    }
}

private void LoadMyUserControl(string controlName, Control parent)
{
    parent.Controls.Clear();
    UserControl MyControl = (UserControl)LoadControl(controlName);
    string userControlID = controlName.Split('.')[0];
    MyControl.ID = userControlID.Replace("/", "").Replace("~", "");
    parent.Controls.Add(MyControl);
    this.CurrentControl = controlName;
}

CopyVB.NET

Protected Sub Page_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Me.Load
    If (Not Me.CurrentControl Is String.Empty) Then
        Me.LoadMyUserControl(Me.CurrentControl, Panel1)
    End If
End Sub

Protected Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs)
    Me.LoadMyUserControl("~/WebUserControl1.ascx", Panel1)
End Sub

Protected Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs)
    Me.LoadMyUserControl("~/WebUserControl2.ascx", Panel1)
End Sub

Private Property CurrentControl() As String
    Get
        Return IIf(Me.ViewState("CurrentControl") Is Nothing, String.Empty, Me.ViewState("CurrentControl"))
    End Get
    Set(ByVal Value As String)
        Me.ViewState("CurrentControl") = Value
    End Set
End Property

Private Sub LoadMyUserControl(ByVal controlName As String, ByVal parent As Control)
    parent.Controls.Clear()
    Dim MyControl As UserControl = Me.LoadControl(controlName)
    Dim userControlID As String = controlName.Split(".")(0)
    MyControl.ID = userControlID.Replace("/", "").Replace("~", "")
    parent.Controls.Add(MyControl)
    Me.CurrentControl = controlName
End Sub

For full code working application you can check this example in RadAjax for ASP.NET AJAX Quick Start Framework:

http://demos.telerik.com/aspnet-ajax/Ajax/Examples/Common/LoadingUserControls/DefaultCS.aspx