Assembly runtime version v2.0 in Visual Studio 2010 .net 4 project

I downloaded a library(SquishIt) I want to use in a Console Application. I am working in Visual Studio 2010 so the Target framework is set to ".Net Framework 4" in the properties of the application. This causes the following error in my code:

The type or namespace name 'SquishIt' could not be found (are you missing a using directive or an assembly reference?)

When I switch the Targe开发者_运维百科t framework to 3.5 the problem goes away.

Could anybody explain why this happens?

IIRC, while .NET 3.0/3.5 added a lot of new functionality, it did not introduce any changes to the CLR, meaning that it was backwardly compatible with assemblies targeting the 2.0 framework. .NET 4 introduces some CLR changes, so no backward compatibility.