Question

I am using ASP.NET Callbacks (that implements the ICallbackEventHandler) and in the handler, I try to set a value of the text box:

txtName.text = "Test";

but this value is not set. Is this a limitation with callback? It appears I cannot do much in a callback handler other than sending back a string to the client side (ofcourse I can access the Session etc)

Answer 1

Even though you are calling back to the server, the whole page is not processed and sent back to the client. Thus, changes you make to controls server-side will not be reflected client-side.

Controls like the GridView uses callbacks in order to do some processing on the server while not incurring a full postback. Think of it as a form of AJAX. The GridView, upon receiving the string response you mention, is responsible for interpreting the string and updating its own state client-side using javascript.

Clarification: as described here:

the page…runs an abbreviated
version of its normal life cycle to
process the callback

so no, ASP.Net will not do a full page lifecycle, and no, no html will be returned to the client. Unless you return some html yourself in the GetCallbackResult method.

Idea: instead of doing callbackeventhandlers, take a look at JQuery with ASP.Net AJAX and more on the state of things here. This would enable your scenario with much of the plumbing already in place done by Microsoft.