<%@ Register Assembly="AsyncControls" Namespace="DelvingWare.AsyncControls" TagPrefix="dw" %>

<dw:AsyncTextBox runat="server" ID="txtMain" 
Size="50" 
MaxLength="50" 
OnTextChanged="txtMain_TextChanged" />

<dw:AsyncLabel runat="server" ID="lblMain" 
RenderMode="paragraph">Type in the above AsyncTextBox.</dw:AsyncLabel>

using System;
using DelvingWare.AsyncControls;
...
protected void txtMain_TextChanged( object sender, AsyncEventArgs ae )
{
    // get the length of the text
    if ( txtMain.Text.Length >= 50 )
    {
        // update the AsyncLabel to show demo limit
        lblMain.Text = "You are limited to 50 chars or less in this demo.";
        return;
    } 

    // if the AsyncTextBox text is empty (null or all whitespace)
    if ( AsyncHelpers.IsEmpty(txtMain) )
        lblMain.Text = "Nothing...";
    else
        lblMain.Text = "You've typed: "+ txtMain.Text;
}

Imports System
Imports DelvingWare.AsyncControls
...

Protected Sub txtMain_TextChanged(ByVal sender As Object, ByVal ae As AsyncEventArgs)
    ' get the length of the text
    If (txtMain.Text.Length >= 50) Then
        ' update the AsyncLabel to show demo limit
        lblMain.Text = "You are limited to 50 chars or less in this demo."
        Return
    End If
    ' if the AsyncTextBox text is empty (null or all whitespace)
    If AsyncHelpers.IsEmpty(txtMain) Then
        lblMain.Text = "Nothing..."
    Else
        lblMain.Text = ("You've typed: " + txtMain.Text)
    End If
End Sub