asp.net mvc - How can a .NET 4.5.2 app still compile on a build server with only 4.5.1 installed? -

this has me baffled. testing upgrading our build infrastructure .net 4.5.2, on .net 4.5.1.

my first step create asp.net mvc project on developer machine targeting .net 4.5.2. using vs 2013 , have 4.5.2 developer pack installed. have written superfluous code utilizing methods supported in .net 4.5.2:

public actionresult index() {     if (!response.headerswritten)         response.addonsendingheaders((c) => { c.response.addheader("foo", "bar"); });      return view(); } 

response.headerswritten , response.addonsendingheaders introduced in .net 4.5.2 version of system.web.dll. expected, compiles fine locally.

prior upgrading build server 4.5.2 thought i'd run build of above project on build server source control see fail. build completes successfully, reporting following warning:

the reference assemblies framework ".netframework,version=v4.5.2" not found. resolve this, install sdk or targeting pack framework version or retarget application version of framework have sdk or targeting pack installed. note assemblies resolved global assembly cache (gac) , used in place of reference assemblies. therefore assembly may not correctly targeted framework intend.

i pull down compiled binaries drop local machine , app runs fine , adds foo header.

how 4.5.2 code compile! there no local references system.web checked source being built , there nothing being pulled in through nuget package or such.

i've tracked down version of system.web.dll in gac being referenced during build. despite documentation stating response.headerswritten , response.addonsendingheaders introduced in .net 4.5.2, following security update .net 4.5/4.5.1 installed version of system.web.dll gac include methods:

https://support.microsoft.com/en-us/kb/2894854

this explains why compilation still succeeds. when forcing build compile targeting 4.5.1, using /p:targetframeworkversion=v4.5.1 msbuild switch, system.web.dll reference older version pulled following location, not include methods , therefore fails compile expected:

c:\program files (x86)\reference assemblies\microsoft\framework\.netframework\v4.5.1\system.web.dll