Fuslogvw.exe and diagnosing .NET assembly binding issues

So let’s say that you are working with a large managed project and for some reason, one of your binary files just refuses to load. Perhaps you get some esoteric error message and perhaps not. If so then you may very well be in the world of managed assembly binding issues.

                For me, Microsoft was my first real programming job after college, so I was not used to having to debug assembly binding issues. But now that I work here I realize how much “debugging” time is really spent getting all of your assemblies to have the correct signing, versioning and to play nice together as opposed to actually stepping through code and fixing issues. As I’ve had to learn all of this in a work environment I thought that I would share some of the small tools and tricks that I’ve learned for diagnosing these issues. Please note that I’m still quite the rookie at these matters so if you are already a kung-fu managed developer then I’m probably not saying anything here that you don’t already know.

                One of the first tools that I turn to in these situations is fuslogvw.exe (great name, really rolls off the tongue easily ;-)). This tool keeps track of all managed assembly binding attempts that happen on your system. However, like its name, fuslogvw.exe is rather esoteric and its output can be a little hard to interpret. Hopefully this article will help to get you started with fuslogvw and understanding what it is telling you.

                When you first open fuslogvw you will see the screen below. In the main section is where you will see specific binding events, but only after you have turned on the proper settings.

                Click on the settings menu to bring up the settings for fuslogvw. As long as my application does not load too many dlls I like to turn on “Log all binds to disk” so that I can see all of the binds that are happening and not just the failures. Also, I sometimes get funny issue when not using “Enable custom log path” so I usually turn that on and point it at a log directory (this keeps it from using the IE cache for logging). For this example I’m going to purposefully delete one of my dependent dlls, select “Log bind failures to disk” and then try to run my application. When it crashes I will then move back to fuslogvw and click the refresh button. Now I will see an entry in the screen about the dll that failed to load.

                By double clicking on the entry I can get a text file of information about the file that failed to load. I’ve included the text file from my example below (some names omitted to protect the innocent). From the log we can see that we tried to look up the assembly in the GAC then we tried a few different extensions and locations to try to locate the file. Also, if the file was found but had incorrect signing we would see that here at this stage of the game. Hopefully this has helped to introduce you to fuslogvw and invaluable tool for diagnosing binding issues with managed .NET applications.

*** Assembly Binder Log Entry (7/12/2006 @ 3:27:59 PM) ***

The operation failed.

Bind result: hr = 0x80070002. The system cannot find the file specified.

Assembly manager loaded from: C:\WINDOWS\Microsoft.NET\Framework\v2.0.60425\mscorwks.dll

Running under executable C:\Program Files\Microsoft Visual Studio 8\Team Tools\Performance Tools\XXXXviewer.exe

--- A detailed error log follows.

=== Pre-bind state information ===


LOG: DisplayName = XXXXvisualization, Version=, Culture=neutral, PublicKeyToken=XXXXXXXXXXXX


LOG: Appbase = file:///C:/Program Files/Microsoft Visual Studio 8/Team Tools/Performance Tools/

LOG: Initial PrivatePath = NULL

LOG: Dynamic Base = NULL

LOG: Cache Base = NULL

LOG: AppName = XXXXviewer.exe

Calling assembly : XXXXviewer, Version=, Culture=neutral, PublicKeyToken=XXXXXXXXXXXX


LOG: This bind starts in default load context.

LOG: No application configuration file found.

LOG: Using machine configuration file from C:\WINDOWS\Microsoft.NET\Framework\v2.0.60425\config\machine.config.

LOG: Post-policy reference: XXXXvisualization, Version=, Culture=neutral, PublicKeyToken=XXXXXXXXXX

LOG: GAC Lookup was unsuccessful.

LOG: Attempting download of new URL file:///C:/Program Files/Microsoft Visual Studio 8/Team Tools/Performance Tools/XXXXvisualization.DLL.

LOG: Attempting download of new URL file:///C:/Program Files/Microsoft Visual Studio 8/Team Tools/Performance Tools/XXXXvisualization/XXXXvisualization.DLL.

LOG: Attempting download of new URL file:///C:/Program Files/Microsoft Visual Studio 8/Team Tools/Performance Tools/XXXXvisualization.EXE.

LOG: Attempting download of new URL file:///C:/Program Files/Microsoft Visual Studio 8/Team Tools/Performance Tools/XXXXvisualization/XXXXvisualization.EXE.

LOG: All probing URLs attempted and failed.