It would be simpler to do this:
fixture.Register<DataSet>(() => new FakeDataSet());
but if you want to use a SpecimenBuilder, this should also work:
public class DataSetBuilder : ISpecimenBuilder
{
public object Create(object request, ISpecimenContext context)
{
var t = request as Type;
if (typeof(DataSet).Equals(t))
return new FakeDataSet();
return new NoSpecimen(request);
}
}
Personally, I'd use the first option.