Common Gotcha: Slow VS 2005 Web Site Build Performance Because of "Dueling Assembly References"
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by Scott Guthrie
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Dueling Assembly Reference Problem

The problem I refer to as a “dueling reference” occurs when you setup multiple file-based references to assemblies from a VS 2005 Web Site Project that are each updated dynamically (using .refresh files), and which in turn have dependencies on different versions of a common shared assembly library.  

For example, assume your web-site has a reference to AssemblyA.dll and AssemblyB.dll – which in turn each reference AssemblyC.dll.  This will work without problems if the referenced AssemblyC.dll is the same version for both AssemblyA.dll and AssemblyB.dll.  But it will cause problems if the AssemblyC.dll being used is different between the two:

Figure 3

In cases where AssemblyC.dll is different, VS ends up copying the AssemblyC.dll file twice for each build – since it continually thinks that the assembly has been updated (once for each reference).  This ends up requiring all references to be recalculated by the compiler, and a full re-build to occur within the IDE.  This will cause build performance to slow down dramatically, and will cause builds to appear to pause as VS does this reference recalculation and fix-up on every single build.

How to Fix This Problem

There are a couple of ways to fix this problem:

1) The “most correct” way to fix this issue is to make sure your class library references are built against the same version of any dependent assemblies.   This is good to-do not just to fix the above build performance problem, but also because it decreases the likelihood of introducing hard to figure out bugs in your application (at runtime only one version of the shared assembly is going to be used –so if you don’t fix this at least one of your dependent assemblies will end up running against an assembly it wasn’t built/unit-tested with).

2) The “easiest quick fix” way to resolve this issue is to modify one or more of your assembly file-based references to not be “automatic refresh enabled”.  You can do this by deleting the .refresh files within your \bin directory that produce that shared conflict.  This will avoid having VS auto-update the assemblies, and so will prevent the dueling update conflict altogether.   You might find it useful to quickly disable this behavior by deleting the .refresh files as a stop-gap, and then re-enable the auto-refresh behavior once you fix the shared assembly conflicts.

Note that VS 2005 Web Application Projects don’t have this dueling reference issue (since they don’t use .refresh files for file-assembly references).  So you won’t have this build-performance problem with it (instead it will just pick one version of the shared assembly to use).   However, you still want to be careful about cases where you have two components built against two separate versions of a shared assembly – since this can still cause hard to understand behavioral bugs at runtime that end up bypassing your carefully written unit-tests (which were run against a different version of the shared assembly).

Hope this helps,


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User Comments

Title: Thanks   
Name: Matt
Date: 2007-01-14 2:19:51 AM
Exactly what I needed. Thank you.
Title: Thank you!   
Name: Uggis
Date: 2006-11-22 2:17:32 PM
This article saved me from hours of ripping my hair out! Thank you very much.

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