We’ve been trying to get the build server to pick up the Microsoft.Expression.Interactions.dll needed so we can use things like the ExtendedVisualStateManager and the DataStateBehavior. Adding the DataStateBehavior in Blend adds the reference to the project, so it all compiles fine on the local machine. Checking in the code into a CI server throws up some ugliness: d:\Builds\6\Source\MyFile.xaml (290): The tag 'DataStateBehavior' does not exist in XML namespace 'http://schemas.microsoft.com/expression/2010/interactions'. Errr, it should do…?? The reference is there, so… ahhhhh! A quick check of the properties and we’re using the dll from the c:\program files (x86)\ location, no wonder the build server can’t find it – let’s add the dll into our (ever expanding) ‘lib’ folder, reference that version, and check that bad boy in… No. Still No. Still get the same error. What the??? The reference is still pointing to the program files location?? Ok, so let’s modify the csproj file using the wonderful notepad, changing the reference from <Reference Include="Microsoft.Expression.Interactions, /* loadsa shizzle here */ /> to <Reference Include="Microsoft.Expression.Interactions"> <HintPath>..\Lib\Microsoft.Expression.Interactions.dll</HintPath> </Reference> Check that in, aaaand… Success!!! The reason for this (from what I can gather from here) is that we’re using the Productivity Power Tools, and they try to be clever about referencing dlls, and changed what we’d asked (i.e. for the local version) to use the original program files location.. Editing the file in notepad (sweet sweet notepad) gets around this issue… Irritating, took a while to figure this out… Meh :)