I Have 10 Textboxes Among Them Based On 2 Textbox I Want To Generate Third Texbox Values Without Using Button

Use following code :

protected void TextBox1_TextChanged(object sender, EventArgs e)
  {

if(textbox1.text.trim()!='' && textbox2.text.trim()!='')
textbox3.text=Convert.ToInt32(textbox1.text)+Convert.ToInt32(textbox2.text);

  }
protected void TextBox2_TextChanged(object sender, EventArgs e)
  {

if(textbox1.text.trim()!='' && textbox2.text.trim()!='')
textbox3.text=Convert.ToInt32(textbox1.text)+Convert.ToInt32(textbox2.text);

  }

<title>













///////////////////////////////jquery function to add and display

$(document).ready(function () {
$("#Text1,#Text2").keyup(function () {
$('#Text3').val(parseInt($('#Text1').val()) + parseInt($('#Text2').val()));
});
});

I would suggest you to use the Javascript/JQuery function to do this calculation as this calculation is very simple and can be calculated at client end easily without creating any overhead to server.

<script type="text/javascript">

 $(document).ready(function () { 

/* Please remember to replace the following in whole code given below  
[TextBox1ID] with the ID of textbox 1
[TextBox2ID] with the ID of textbox 2
[TextBox3ID] with the ID of textbox 3*/

 // Event Handler to execute when focus goes out of textbox 1 or textbox 2
 $("#[TextBox1ID], #[TextBox2ID]").blur(function () {

   // Assigning the value of both textboxes to variable for further calculations..
   var txtVal1=$("#[TextBox1ID]").val();
   var txtVal2=$("#[TextBox2ID]").val();

   //Here we are validating the values of first two textboxes to make sure that both have integer value...
   if(txtVal1!='' && !isNaN(txtVal1) && txtVal2!='' && !isNaN(txtVal2)) 
   {
     // Here we convert the value of first two textboxes into INT datatype and then performing the SUM operation and assigning it's value to third textbox. 
     $('#[TextBox3ID]').val(parseInt(txtVal1) + parseInt(txtVal2));
   }
 });
});
  
 </script>

I hope this will help you.. :)