What is C:\WINDOWS\assembly\NativeImages_v2.0.50727_32\index3a0.dat used for?

Question

I'm trying to diagnose why out .Net ActiveX control has a much slower load time on certain machines than others and I've discovered (using ProcMon) that on machines that load the control slowly the browser process seems to spend a lot of time doing something with this file.

In the slow process I see groups of events (4-8) in the above format that take around 200ms to complete:

6:02:17.333 CreateFile C:\WINDOWS\assembly\NativeImages_v2.0.50727_32\index3a0.dat NAME NOT FOUND
6:02:17.364 CreateFile C:\WINDOWS\assembly\NativeImages_v2.0.50727_32\index3a0.dat NAME NOT FOUND
6:02:17.396 CreateFile C:\WINDOWS\assembly\NativeImages_v2.0.50727_32\index3a0.dat NAME NOT FOUND
6:02:17.427 CreateFile C:\WINDOWS\assembly\NativeImages_v2.0.50727_32\index3a0.dat NAME NOT FOUND

However in a similar trace taken on a machine where the control loads quickly I see only a single line with a SUCCESS result.

What is this file?

Solution

I assume this has something to do with caching/quick lookup of GAC entries or something like that, on my system it is called index1c.dat (It is 0 bytes on this system)

I'm guessing the name comes from the LatestIndex value under HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Fusion\NativeImagesIndex\v2.0.50727_32

You could try windows update or try running "%windir%\Microsoft.NET\Framework\v2.0.50727\ngen" update

...but even if you could trigger a update of this cache, this might not be the cause of your problems.