title: Why VS2012 project with referenced assembly can't target 4.0 automatically
body: <p>In a Visual Studio 2012 C# console application, I downgrade ".NET Framework Target" from 4.5 to 4.0. Win 7 Pro with both Frameworks installed.</p>
  
  <p>I then reference an assembly, which, through warnings complains the following:</p>
  
  <p><code>The primary reference "System.Threading.Tasks.Dataflow, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a, processorArchitecture=MSIL" could not be resolved because it has an indirect dependency on the framework assembly "System.Runtime, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" which could not be resolved in the currently targeted framework. ".NETFramework,Version=v4.0". To resolve this problem, either remove the reference "System.Threading.Tasks.Dataflow, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a, processorArchitecture=MSIL" or retarget your application to a framework version which contains "System.Runtime, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a".</code></p>
  
  <p>If I try to compile at this point, I error, becuase types and namespaces from referenced assembly aren't available, as if the assembly was not referenced at all.</p>
  
  <p>The "Add Reference" dialog doesn't have <strong>any</strong> System.Runtime choice, but if I manually brose to
  C:\Windows\Microsoft.NET\Framework\v4.0.30319\
  and reference the System.Runtime assembly found there, warnings go away and I am able to compile.</p>
  
  <p>Questions:</p>
  
  <ol>
  <li><p>Is such forcing of the System.Runtime version a potential issue down the road (deployment).</p></li>
  <li><p>If VS Project properties are seto to target Framework 4.0 (doesn't that relate to targeting the 4.0 SystemRuntime/CLR), why isn't the refferenced DLL picking that up and why manually adding the reference to my project fixes that issue?</p></li>
  </ol>