Reference issues in .NET 4.0 project to .NET 2.0 dll/project

I just starting to use Visual Studio 2010 and .NET 4.0. I created a new WPF project a DLL built for .NET 2.0. When I build the project, I get a bunch of warnings like this one:

The referenced assembly
"MicroFour StrataFrame Business...processorArchitecture=MSIL"
could not be resolved because it has a dependency on
"System.Web, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a"
which is not in the currently targeted framework ".NETFramework,Version=v4.0,Profile=Client".
Please remove references to assemblies not in the targeted framework
or consider retargeting your project.

When I reference a project I created that targets 3.5, it can't find any of the namespaces (during compile), though they show up in the object browser and intellisen开发者_StackOverflow社区se correctly shows them.

Is there something I'm missing that needs to be done to reference an older .NET assembly?

There is very little point in targeting the client profile for .NET 4.0. The download is 41MB, the full version is 48MB, only 15% bigger. Unfortunately it is the default in VS2010, just change it with Project + Properties, Application tab, Target framework combo.

The client profile does make a lot of sense if you target 3.5, the full install is ~350 MB. The huge difference is explained by the prerequisites, .NET 4.0 requires at least XP SP3 or Vista SP1. But 3.5 installs on any version of Windows > 2000. The 3.5 installer thus contains lots of the required updates for unmanaged Windows components used by .NET. The web installer lessens that blow considerably btw.

Your 2.0 assembly has a reference to System.Web. The reference is being automatically forwarded to your target framework, 4.0. So it is attempting to use System.Web, version 4.0.0.0. The problem is with 4.0 (actually this started with 3.5), there is now the notion of a client profile. The client profile has a smaller set of reference assemblies, anything web/server related has been removed. This is to make the .NET framework a smaller/simpler download for end users.

With VS 2010 and .NET 4, the client profile is being pushed a lot harder, MS really wants us to use it. So your .NET 4 project is targeting the client profile by default. Your referenced assembly wants System.Web, so in that case you need to switch to the full profile.