Response.Redirect issue with Asp.net async

This code will work in a brand new project, but, does not work with a new page in our existing code.

I assume your existing site has already been upgraded to .NET 4.5.

The first thing to check is that httpRuntime.targetFramework is set to 4.5. This is not set by default when you upgrade.

Edit from comments:

Another thing to check (just in case) is that Page.Async is set to true.

In this case, the solution was to call Response.Redirect("http://www.google.com", false), which explicitly passes false for the endResponse parameter. The default value of true is only for backwards-compatibility reasons as described here.