Q:

Build step triggered by TeamCity always builds - even when there are no changes

The problem: I am setting up TeamCity as a build server for an ASP.NET MVC project. I am using Powershell with psake to run msbuild against our .csproj file and create a deployable package. From the build server, I can open up powershell, run the script and, because there are no source code changes, msbuild does not regenerate the project DLL files. BUT, when I call the exact same script from the TeamCity web interface, msbuild ALWAYS rebuilds and regenerates the DLL files even though there are no changes. Not what it should do AFAIK.
I have narrowed this problem down to a single step. To keep it simple, I have set up my TeamCity config so it is not using any source control, it runs a single "powershell" build step that calls my powershell script.
The powershell script runs a single command:
exec { &$msbuild $ProjectFile /t:Package "/p:PackageLocation=$PackageFile;OutDir=$TempPath;Configuration=$Config;SolutionDir=$BaseDir\Source\" /v:m }

When I call the script manually from a powershell command line, I see:
CoreCompile:
Skipping target "CoreCompile" because all output files are up-to-date with respect to the input files.

When I call the exact same script through TeamCity, I see:
[11:11:26]: CoreCompile:
[11:11:26]:   c:\WINDOWS\Microsoft.NET\Framework\v4.0.30319\Csc.exe /noconfig ...
<SNIP>
[11:11:32]: CopyFilesToOutputDirectory:
[11:11:32]:   Copying file from "obj\Demo\Website.Web.dll" to "d:\deploy\Build\package\Demo\temp\Website.Web.dll".
[11:11:32]:   Website.Web -> d:\deploy\Build\package\Demo\temp\Website.Web.dll
[11:11:32]:   Copying file from "obj\Demo\Website.Web.pdb" to "d:\deploy\Build\package\Demo\temp\Website.Web.pdb".
[11:11:32]: _CopyWebApplicationLegacy:
[11:11:32]:   Copying Web Application Project Files for Website.Web
[11:11:32]:   Copying file from "obj\Demo\Website.Web.dll" to "d:\deploy\Build\package\Demo\temp\_PublishedWebsites\Website.Web\bin\Website.Web.dll".
[11:11:32]:   Copying file from "obj\Demo\Website.Web.pdb" to "d:\deploy\Build\package\Demo\temp\_PublishedWebsites\Website.Web\bin\Website.Web.pdb".
[11:11:32]:   Copying file from "d:\deploy\Build\package\Demo\temp\Website.Data.dll" to "d:\deploy\Build\package\Demo\temp\_PublishedWebsites\Website.Web\bin\Website.Data.dll".
[11:11:32]:   Copying file from "d:\deploy\Build\package\Demo\temp\Website.Data.pdb" to "d:\deploy\Build\package\Demo\temp\_PublishedWebsites\Website.Web\bin\Website.Data.pdb".

Any ideas why running this script from TeamCity causes msbuild to detect changes and rebuild, but running the exact same script manually does not?
UPDATE:
Thinking this might be caused by some quirk with the TeamCity Powershell runner, I just tried making a batch file that passes the script into Powershell.exe and called it using the Command Line runner:
C:\WINDOWS\system32\WindowsPowerShell\v1.0\powershell.exe -NonInteractive -File D:\deploy\Build\run-build.ps1 && exit /b %ERRORLEVEL%

and I get the exact same behavior. If I call this batch file from the command line, the msbuild skips compilation. If I call it from TeamCity, the DLLs are recompiled.
UPDATE #2:
Eureka! I turned on diagnostic debugging in msbuild and found the cause of the forced recompile. It is caused by the GenerateTargetFrameworkMonikerAttribute target. Here is the key bits from the log output:
[15:23:28]: Target "GenerateTargetFrameworkMonikerAttribute" in file "c:\WINDOWS\Microsoft.NET\Framework\v4.0.30319\Microsoft.Common.targets" from project "d:\deploy\source\Website.Data\Website.Data.csproj" (target "BeforeCompile" depends on it):
[15:23:28]: Building target "GenerateTargetFrameworkMonikerAttribute" completely.
[15:23:28]: Output file "C:\TeamCity\buildAgent\temp\buildTmp\.NETFramework,Version=v4.0.AssemblyAttributes.cs" does not exist.
[15:23:28]: Using "WriteLinesToFile" task from assembly "Microsoft.Build.Tasks.v4.0, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a".
[15:23:28]: Task "WriteLinesToFile"
[15:23:28]: Done executing task "WriteLinesToFile".
[15:23:28]: Done building target "GenerateTargetFrameworkMonikerAttribute" in project "SMM.Data.csproj".

It looks like this target creates/updates an AssemblyAttributes file in the TEMP dir as specified in the TEMP environment variable. Apparently TeamCity overrides the TEMP environment variable and sets it to: C:\TeamCity\buildAgent\temp\buildTmp and this directory is cleaned before every build. 
I can see this if I call Get-ChildItem Env: from powershell:
TEMP                           C:\DOCUME~1\ADMINI~1\LOCALS~1\Temp
TMP                            C:\DOCUME~1\ADMINI~1\LOCALS~1\Temp

But if I call it from the powershell script as called from TeamCity:
TEMP                           C:\TeamCity\buildAgent\temp\buildTmp            
TMP                            C:\TeamCity\buildAgent\temp\buildTmp   

The key piece is that after this file is regnerated:
[15:23:28]: Building target "CoreCompile" completely.
[15:23:28]: Input file "C:\TeamCity\buildAgent\temp\buildTmp\.NETFramework,Version=v4.0.AssemblyAttributes.cs" is newer than output file "obj\Demo\SMM.Data.pdb".

And this is why the whole project is getting recompiled. 
When I run the script from Powershell, the temp directory is not changed or cleaned and the build runs as expected.
So, anyone know how I can either change the directory that this AssemblyAttributes file is created, or tell TeamCity to use a different TEMP dir? I have to believe that this is an issue that others have run into.
Thanks!

A:

So, as I mentioned in "Update #2" above, the problem seems to be caused by 2 things:
- TeamCity sets the TEMP and TMP environment vars to its own temp directory
- TeamCity "cleans" this temp directory prior to every build
- Part of the msbuild process runs a GenerateTargetFrameworkMonikerAttribute target that updates a specific file in the directory specified by the TEMP environment variable - causing the compiler to thing it needs to recompile the whole project
Once I figured this out, I found an applicable answer in this unrelated question:
In Visual Studio 2010 why is the .NETFramework,Version=v4.0.AssemblyAttributes.cpp file created, and can I disable this?
So I added:
<Target Name="GenerateTargetFrameworkMonikerAttribute" />

to both of the projects in my solution that compile to DLLs and it worked.

