NAnt - A .NET Build Tool

The <solution> tasks fails if an assembly references
another assembly that does not exist but is not used at
all.

For instance, the Enterprise Library references
nunit.framework.dll. But the Debug, Release and
ReleaseFinal configurations exclude all test code, thus
the solution still builds under VS.NET 2003 even without
NUnit installed. VS.NET 2003 does not care that the
assembly reference is invalid, because no code uses
anything from the nunit.framework.dll assembly.

Only if the DebugUnitTests, DebugLongRunningTests,
ReleaseUnitTests or ReleaseLongRunningTests
configururations are built does VS.NET 2003 require
that the reference to nunit.framework.dll be valid and
therefore that NUnit is actually installed.