[DefaultValue(true)]
public virtual bool Enabled { get; set; }

<DefaultValue(True)>
Public Overridable Property Enabled As Boolean

<style>
    .sendCommentBlock {
        margin: 20px 0;
    }

    .comment {
        border-bottom: 1px dashed #888888;
    }
</style>
<script>
    function SendCommentCallback(s, e) {
        CallbackPanel.PerformCallback();
    };

    function OnBeginCallback(s, e) {
        LoadingPanel.Show();
    };

    function OnEndCallback(s, e) {
        LoadingPanel.Hide();
    };
</script>
<dx:ASPxButton ID="Button" runat="server" Text="Post Comment" AutoPostBack="false">
    <ClientSideEvents Click="SendCommentCallback" />
</dx:ASPxButton> 




<dx:ASPxTextBox ID="TextBox" runat="server" Width="170px"></dx:ASPxTextBox>
...
<dx:ASPxCallbackPanel ID="CallbackPanel" ClientInstanceName="CallbackPanel" 
    runat="server"
    CssClass="pnl"
    Width="300px"
    OnCallback="CallbackPanel_Callback"
    OnInit="CallbackPanel_Init">
    <SettingsLoadingPanel Enabled="false" />
    <ClientSideEvents BeginCallback="OnBeginCallback" EndCallback="OnEndCallback" />
    <PanelCollection>
        <dx:PanelContent>
            <dx:ASPxLabel ID="CountLabel" runat="server" Text="Comments Count : "></dx:ASPxLabel>
            

            

            <dx:ASPxLabel ID="NoCommentsLabel" runat="server" Text="No Comments" ForeColor="Gray"></dx:ASPxLabel>
        </dx:PanelContent>
    </PanelCollection>
</dx:ASPxCallbackPanel>
...
<dx:ASPxLoadingPanel ID="LoadingPanel" ClientInstanceName="LoadingPanel"
    runat="server"
    Modal="true"
    HorizontalAlign="Center"
    VerticalAlign="Middle">
    <Image Url="Images/load.gif" Height="50px" Width="50px"></Image>
</dx:ASPxLoadingPanel>

using DevExpress.Web;

public partial class _Default : System.Web.UI.Page {
    protected void CallbackPanel_Callback(object sender, DevExpress.Web.CallbackEventArgsBase e) {
        ASPxCallbackPanel panel = (ASPxCallbackPanel)sender;
        NoCommentsLabel.Visible = false;
        Thread.Sleep(3000);

        ASPxLabel comment = new ASPxLabel() {
            Text = string.Format("[{0}]\n{1}\n\n",
                DateTime.Now.ToLocalTime(),
                !string.IsNullOrEmpty(TextBox.Text) ? TextBox.Text : "Empty comment"
            ),
            CssClass = "comment"
        };

        List<ASPxLabel> comments = (List<ASPxLabel>)Session["comments"] ?? new List<ASPxLabel>();

        comments.Add(comment);
        panel.Controls.Add(comment);
        Session["comments"] = comments;

        CountLabel.Text = "Comments Count : " + comments.Count;
    }

    protected void CallbackPanel_Init(object sender, EventArgs e) {
        if(!IsPostBack && !IsCallback)
            Session.Clear();
        RecreateComments(sender);
    }

    private void RecreateComments(object sender) {
        List<ASPxLabel> comments;
        if((comments = (List<ASPxLabel>)Session["comments"]) != null) {
            CountLabel.Text = "Comments Count : " + comments.Count;
            ASPxCallbackPanel panel = (ASPxCallbackPanel)sender;
            NoCommentsLabel.Visible = false;
            foreach(ASPxLabel comment in comments) {
                panel.Controls.Add(comment);
            }
        }
    }
}

Imports DevExpress.Web

Partial Public Class _Default
    Inherits System.Web.UI.Page

    Protected Sub ASPxCallbackPanel1_Callback(ByVal sender As Object, ByVal e As DevExpress.Web.CallbackEventArgsBase)
        Dim panel As ASPxCallbackPanel = DirectCast(sender, ASPxCallbackPanel)

        ' Perform long task
        Thread.Sleep(5000)

        ' Add/Modify/Hide controls inside callback panel
        TimeLabel.Text = Date.Now.ToLongDateString()
        TimeLabel.ForeColor = Color.Green
        TextBoxToHide.Visible = False
        ButtonToDisable.Enabled = False

        panel.Controls.Add(New ASPxHyperLink() With {.NavigateUrl = "https://www.devexpress.com/sc", .Text = "Navigate to DevExpress support center"})
    End Sub
End Class