1. Code behind.
protected void TextBox1_TextChanged(object sender, EventArgs e) { Label1.Text = Server.HtmlEncode(TextBox1.Text); }
2. Event trigger
xmlns:i="clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity"
<i:Interaction.Triggers>
<i:EventTrigger EventName="TextChanged">
<i:InvokeCommandAction Command="{Binding StartTextChanged}" />
</i:EventTrigger>
</i:Interaction.Triggers>
3. MVVM ( Prefered)
<TextBox Text="{Binding MyText,Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"/>