Simple display with OnChange event on textbox

Question

How to use the OnChange event to display the result on the Label1 instantly after value have enter in Textbox1 and Textbox2??

    <div>
             <asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
             <asp:TextBox ID="TextBox2" runat="server"></asp:TextBox>
             <asp:Label ID="Label1" runat="server" Text="Label"></asp:Label>

         </div>
---------------------------------------------------------------------------------------
//Code behind

     int num1;
      if(!Int32.TryParse(TextBox1.Text, out num1))
      {
          Label1.Text = "Not a valid number";
          return;
      }
      int num2;

      if(!Int32.TryParse(TextBox2.Text, out num2))
      {
          Label1.Text = "Not a valid number";
          return;
      }
      sum = num1 + num2;
      Label1.Text = sum.ToString();

Answer 1

To use TextChangedEvent you need to add TextChangedEvent event handler in your code and set AutoPostBack=true in markup

<asp:TextBox ID="TextBox2" runat="server" OnTextChanged="TextBox2_TextChanged" AutoPostBack="true"></asp:TextBox>

Code behind

    protected void TextBox2_TextChanged(object sender,EventArgs e)
    {
       int num1;
       if(!Int32.TryParse(TextBox1.Text, out num1))
       {
          Label1.Text = "Not a valid number";
          return;
       }
       int num2;

      if(!Int32.TryParse(TextBox2.Text, out num2))
      {
         Label1.Text = "Not a valid number";
         return;
      }
      sum = num1 + num2;
      Label1.Text = sum.ToString();
    }

Answer 2

This will work for both textboxes :-

<div>
         <asp:TextBox ID="TextBox1" OnTextChanged="AnyTextBox_TextChanged" AutoPostBack="true" runat="server"></asp:TextBox>
         <asp:TextBox ID="TextBox2" OnTextChanged="AnyTextBox_TextChanged" AutoPostBack="true" runat="server"></asp:TextBox>
         <asp:Label ID="Label1" runat="server" Text="Label"></asp:Label>
</div>

Code behind:

protected void AnyTextBox_TextChanged(object sender, EventArgs e)
{
    int sum = 0;
    int num1;
    if (!Int32.TryParse(TextBox1.Text, out num1))
    {
        Label1.Text = "Not a valid number";
        return;
    }
    int num2;

    if (!Int32.TryParse(TextBox2.Text, out num2))
    {
        Label1.Text = "Not a valid number";
        return;
    }
    sum = num1 + num2;
    Label1.Text = sum.ToString();
}