I have the same issue - it seems to happen using a comparison on any generic type.

Here is a simple test that proves the error (assuming Repository<T> is some repository base that gives access to the NHibernate Session):

[TestFixture]
public class Class1
{
[Test]
public void ShouldBeAbleToGetItem()
{
var repo = new Repo<Entity1>();
var result = repo.Get();
Assert.That(result != null);
}
}

public class Repo<T> : Repository<T> where T : IMyGenericInterfaceType
{
public T Get()
{
var queryable = Session.Linq<T>();
return queryable.Where(item => item.Id == 1).First();
}
}

public class Entity1 : IMyGenericInterfaceType
{
public int Id { get; set; }
public DateTime Date { get; set; }
}

public class Entity2 : IMyGenericInterfaceType
{
public int Id { get; set; }
public DateTime Date { get; set; }
}

public interface IMyGenericInterfaceType
{
int Id { get; set; }
DateTime Date { get; set; }
}

I need a simple test.

I'm having exactly the same issue, I've abstracted my CRUD methods to a generic base class which adds a Convert() around elements in the lamba, which is then not treated correctly by the Visitors (possibly AssociationVisitor) Here's some of my code http://stackoverflow.com/questions/629480/strange-linq-to-nhibernate-issue-invalid-cast-from-system-int32

Any chance of a fix for this, or any clues on what to do to begin patching?

I removed the generics from the type, and discovered HasValue was not correct syntax, so i changed them to == null, and it works just fine without generics

public static Expression<Func<PlanVestingRuleset, bool>> WithinValidDateRange([NotNull]DateTime aDate)
{
return Linq.Expr<TData, bool>(v =>
(
v.ValidDateRange.BeginDate==null

v.ValidDateRange.BeginDate <= aDate
)
&&
(
v.ValidDateRange.BeginDate == null

v.ValidDateRange.EndDate >= aDate
)
);

}
}

but of course still doesn't work again if I add the generics back after fixing the HasValue issue

public static Expression<Func<TData, bool>> WithinValidDateRange<TData>([NotNull]DateTime aDate)
where TData : IValidDateRangedDataItem
{
return Linq.Expr<TData, bool>(v =>
(
v.ValidDateRange.BeginDate==null

v.ValidDateRange.BeginDate <= aDate
)
&&
(
v.ValidDateRange.BeginDate == null

v.ValidDateRange.EndDate >= aDate
)
);

}
}

We keep criteria types shared using static methods that return expressions. That's where the generics come in:

static public class Expr
{
public static Expression<Func<TData, bool>> WithinValidDateRange<TData>([NotNull]DateTime aDate)
where TData: IValidDateRangedDataItem
{
return Linq.Expr<TData, bool>(v =>
(
!v.ValidDateRange.BeginDate.HasValue

v.ValidDateRange.BeginDate <= aDate
)
&&
(
! v.ValidDateRange.EndDate.HasValue

v.ValidDateRange.EndDate >= aDate
)
);

}
}

then we use it like this tDB.PlanVestingRulesets.Where(Expr.WithinValidDateRange<PlanVestingRuleset>(asOfDate))