I have some little problem to understand how the assemblies work together.

I have a project P1 that reference a COM dll.
I generated a strong name for this project, and wrap the strong name for the
com dll.
Ok, it builds no pb.
I put my assembly P1 in the GAC.
In VS .NET i set the property "copy local" of the reference to false, not to
have all the dll in each bin folder.

Well, I have a new project P2 that references P1.
I also generated a strong name for P2.
Set "copy local" false.
Ok, i builds no pb.
Put the assembly P2 in the GAC

In the solution of P2, i added a project P3 that references P2 and P1.
Every time I want to rebuild P3, build fails and he says 'Cannot write
output file because it is used by another processus".

Can someone explain what I'm doing wrong?

I must say that if don't use gac and strong name, compiler generates me
warning between version of assembly, but it compile.
Mystery...