Where did HashSet<T> go in VS2012?

Question

I recently installed VS2012. A C++ project (with .Net 4.0) that compiles fine under VS2010 is not recognizing HashSet<T> on VS2012. I even tried being explicit with following declaration:

System::Collections::Generic::HashSet< String^ >^ _reasons;

But that only results in the error:

error C2039: 'HashSet' : is not a member of 'System::Collections::Generic

The documentation says it's in System.Collections.Generic. The C++ compiler doesn't think so.

Any ideas on where it went?

Answer 1

HashSet<> was a late addition to .NET, it became available in .NET 3.5. The namespace is older, mscorlib.dll contained classes in System::Collections::Generic since .NET 2.0, classes like Stack<> and Queue<>. HashSet<> got added in a new assembly for .NET 3.5, System.Core.dll, they didn't want to tinker with the 2.0 assemblies.

Accordingly, you must add a reference to System.Core to avoid the error message.

Always refer back to the MSDN documentation when you get an error like this, it shows you want assembly reference is required.