How to get the value of built, encoded ViewState? | I need to grab the `base64-encoded` representation of the `ViewState`. Obviously, this would not be available until fairly late in the request lifecycle, which is OK.


For example, if the output of the page includes:



```
<input type="hidden" name="__VIEWSTATE" 
  id="__VIEWSTATE" value="/wEPDwUJODU0Njc5MD...==" />

```

I need a way on the server-side to get the value `"/wEPDwUJODU0Njc5MD...=="`


To clarify, I need this value when the page is being rendered, not on PostBack. e.g. I need to know the ViewState value that is being *sent* to the client, not the ViewState I'm getting back from them.