Question

We are planning to move a legacy application which uses enterprise library 4.1 which uses .Net 3.5 in a .Net 4.0 web application.

We are wondering will this cause any performance problems? Will the .net 3.5 code run in a different
application pool?

Answer 1

I have finally found answer to this. The performance problem I previously mentioned my team discovered is due to something else. Not loading .NET 3.5 on .NET 4.0 App Domain.

Reading this article:
http://msdn.microsoft.com/en-us/magazine/ee819091.aspx

In-Proc SxS does not solve the
compatibility problems faced by
library developers. Any libraries
directly loaded by an
application–either via a direct
reference or an Assembly.Load–will
continue to load directly into the
runtime and AppDomain of the
application loading it. This means
that if an application is recompiled
to run against the .NET Framework 4
runtime and still has dependent
assemblies built against .NET 2.0,
those dependents will load on the .NET
4 runtime as well. Therefore, we still
recommend testing your libraries
against all version[s] of the
framework you wish to support. This is
one of the reasons we have continued
to maintain our high level of backward
compatibility.

So, there’s no problem loading .NET 3.5 assemblies directly into .NET 4.0 apps without recompiling them into .NET 4.0.