The NUnit reference goes away with Clean builds

In our build system, whenever we do a clean, it blows away the
reference to NUnit in the project. To get it to build again, we have
to manually add the reference in again. Part of the problem might be
that the team upgrades NUnit at an uneven pace, so we turn the
"Specific Version" flag off in the properties of the reference.

It may be that NUnit's decision to have a different directory every
time an upgrade is installed is part of the problem. Do you install
NUnit in it's default directory, or do you always install it in an
NUnit directory or something like that?

It seems strange to me that we would be having problems with this, and
I'm wondering if anyone has a recommended workaround for this issue.

Thanks!

-Kelly

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