Passing an expression to a method in NHibernate results in Object of type 'ConstantExpression' cannot be converted to type 'LambdaExpression'

问题

This problem occurs in both NHibernate 2 and 3. I have a Class A that has a member set of class B. Querying the classes directly executes nicely. But when I pass one of the expressions involving class B into a method I get the following error:

System.ArgumentException: Object of type 'System.Linq.Expressions.ConstantExpression' cannot be converted to type 'System.Linq.Expressions.LambdaExpression'.

As far as I can see I am passing the exact same expression into the Any() method. But for some reason they are treated differently. I have done some debugging and it looks like in the first method, the expression is treated as an expression with NodeType 'Quote', while the same expression in the 2nd method seems to be treated as an expression with NodeType 'Constant'. The parent expression of the expression in the 2nd method has a NodeType 'MemberAccess'. So it looks like the expression tree is different in the different test methods. I just don't understand why and what to do to fix this.

Classes involvend:

public class A
{
   public virtual int Id { get; set; }
   public virtual ISet<B> DataFields { get; set; }
}

public class B
{
   public virtual int Id { get; set; }
}

Sample test code:

    [TestMethod]
    public void TestMethod1()
    {
        using (ISession session = sessionFactory.OpenSession())
        {
            var records = session.Query<A>()
                                 .Where<A>(a => a.DataFields
                                                 .Any(b => b.Id == 1));
            Console.Write("Number of records is {0}", records.Count());
        }
    }

    [TestMethod]
    public void TestMethod2()
    {
        GetAsWhereB(b => b.Id == 1);
    }

    private void GetAsWhereB(Func<B, bool> where) 
    {
        using (ISession session = sessionFactory.OpenSession())
        {
            var records = session.Query<A>()
                                 .Where(a => a.DataFields
                                              .Any(where));
            Console.Write("Number of records is {0}", records.Count());
        }
    }

回答1:

This is one problem:

private void GetAsWhereB(Func<B, bool> where)

That's taking a delegate - you want an expression tree otherwise NHibernate can't get involved. Try this:

private void GetAsWhereB(Expression<Func<B, bool>> where)

As an aside, your query is hard to read because of your use of whitespace. I would suggest that instead of:

var records = session.Query<A>().Where<A>(a => a.DataFields.
                                 Any(b => b.Id == 1));

you make it clear that the "Any" call is on DataFields:

var records = session.Query<A>().Where<A>(a => a.DataFields
                                                .Any(b => b.Id == 1));

I'd also suggest that you change the parameter name from "where" to something like "whereExpression" or "predicate". Some sort of noun, anyway :)

回答2:

Not quite sure if this is the proper solution or not. The problem feels like a bug and my solution like a workaround. Nonetheless the following works for me, which boils down to creating a 'copy' of the given expression by using its body and parameter to construct a new expression.

private void GetAsWhereB(Func<B, bool> where) 
{
    Expression<Func<T, bool>> w = Expression.Lambda<Func<T, bool>>(where.Body, where.Parameters);
    using (ISession session = sessionFactory.OpenSession())
    {
        var records = session.Query<A>()
                             .Where(a => a.DataFields
                                          .Any(w));
        Console.Write("Number of records is {0}", records.Count());
    }
}

来源：https://stackoverflow.com/questions/4738170/passing-an-expression-to-a-method-in-nhibernate-results-in-object-of-type-const

标签

linq

nhibernate

subquery

expression