Smart indent effect on completion (intelliSense) sessions - visual-studio

Question

I am writing a Visual Studio extension which provides intelliSense for a certain content type.
The problem that I am facing now is the effect of "Auto Indent" that Visual Studio provides on empty lines when user types a character.
Here a completion session started on an empty line (over virtual spaces):
Notice the tab symbols on the other lines and no tab on the line with caret on it.
Now when use starts typing, VS automatically and correctly adds necessary tab characters to the line:
Now the problem is those Added tabs apparently become part of the user input and as a result CurrentSession.SelectedCompletionSet.SelectBestMatch() or Filter() method cannot find the current item which starts with "C" here (thinking user has typed \t\tC instead).
If I start the session on anywhere else which does not require auto indent everything works fine.
Any idea?
Edit (more information): I used a code flow very similar to:
Ook here
vsLua here
vsClojure here
In Lua and Clojure you wouldn't face this problem because they never provide intelliSense on virtual spaces (meaning they always start after a certain set of characters) and if you start after a character virtual spaces are already turned into real spaces.
Ook on the other had has the same problem.

Answer 1

Revised Answer:
Ah, I see. I interpreted your question thinking that you were referring to completion triggering via typing, not from the explicit command. If you enable "show whitespace" for the C# editor, you can see what we do here: when you trigger the "show completion" command, we explicitly realize the whitespace so you're no longer floating around in virtual space. You should probably do this as well. (Alternatively, you could detect the scenario and fix it up on the first typing by adjusting your ApplicableTo span, but that's probably not worth the trouble.)
You can get the whitespace that should be inserted from IEditorOperations. So MEF import an IEditorOperationsFactoryService, and then do:
var editorOperations = editorOperationsFactoryService.GetEditorOperations(textView);
var whitespace = editorOperations.GetWhitespaceForVirtualSpace(textView.Caret.Position.VirtualBufferPosition);
if (whitespace.Length != 0)
{
textView.TextBuffer.Insert(textView.Caret.Position.BufferPosition, whitespace);
}
(Funny aside: as I answered this, I was curious to see how we handled this in the Roslyn C# and VB editors. The answer was "not", but filtering still worked by pure luck later in the code.)
Original Answer:
I suspect by your description of the problem that you are implementing your completion like this: you know a character is about to be typed (either via a keyboard filter or IOleCommandTarget) and you trigger an ICompletionSession, where the tracking span is an empty span on the current caret position.
The best approach to fixing this is to not trigger the session before the key is pressed and goes into the editor, but rather after it. This is what we do in the Roslyn implementation for C# and VB completion. Then, when you are in your AugmentCompletionSession call and creating your CompletionSet, compute the "applicable to" span which consists of the non-whitespace characters around your caret. The easiest way to compute this might just be to call GetWordExtent from the text structure navigator.
This allows for other scenarios to work right. Consider scenarios where the user types C, presses escape, and then continues to type your identifier. If you want to trigger completion again, you'd have to do the math to ensure that the "C" is counted as part of your span anyways.