script Editor Control In ASP.Net

Such control is simply the element <textarea ... />.
 
To make is a script editor, you need a separate script which gets the value of the property value of this element (it is a string representing the text typed in the text area) and simply call JavaScript function eval, passing this value as an argument. Get a the result of the function and present is some other element.
 
Here is a complete example: http://sakryukov.org/freeware/calculator/[^].
 
You can add a lot of other detail and features…
 
Enjoy,

—SA