Differences between Response.End() and Response.Flush()

Response.Flush

Forces all currently buffered output to be sent to the client. The Flush method can be called multiple times during request processing.

Response.End

Sends all currently buffered output to the client, stops execution of the page, and raises the EndRequest event.

You should try using this code if you are not doing any processing on the page after Response.Write and want to stop processing the page.

    context.HttpContext.Response.Clear();

    context.HttpContext.Response.Write(htmlString);              

    context.HttpContext.Response.Flush(); // send all buffered output to client 

    context.HttpContext.Response.End(); // response.end would work fine now.