How to implement zooming in and out using a “lasso” functionality in goview control?first is it possible?

You can use GoToolZooming for that. Although in GoDiagram Web the user won’t see the box during the drag.

can we draw the rectangle during the Drag? is there any overriding method?Thanks in advance.

Remember, overriding methods on the server doesn't give you additional drawing behavior on the client.
Well, it's certainly possible but a lot of work to try to implement that by modifying GoWeb.js. That's something we might do in the future.

Seen your post here which says lasso would be there in future.I also need something like the lasso finctionality, forming a lasso around objects.

i am using the 2.6.2 version.