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}
 
"/>