After updating moq from 4.10.1 to 4.11, mocking nhibernate session throws a System.NullReferenceException.

[ronenfe]

 public class UnitTest1
    {
        [Fact]
        public void TestMethod1()
        {
            // Arrange
            Guid itemId = Guid.NewGuid();
            var session = new Mock<NHibernate.ISession>();
            var repo = new Repository();
            session.Setup(x => x.QueryOver<Item>().Where(b => b.Id == itemId).List()).Returns(new List<Item>());

            // Act
            var res = repo.GetById(itemId, session.Object);

            // Verify
            //...
        }
    }
  class Repository
    {
        public IList<Item> GetById(Guid id, ISession session)
        {
            var items = session.QueryOver<Item>()
                .Where(b => b.Id == id)
                .List();

            return items;
        }
    }
    public class Item
    {
        public Guid Id { get; set; }
    }

4.11 and above:
getting: System.NullReferenceException: 'Object reference not set to an instance of an object.'

4.10.1 and beyond:
works.

Attached is a sample solution. See exception when running the test. Downgrade to 4.10.1 and see error is gone.
UnitTestProject2.zip

[stakx]

... and what would "doesn't work" mean exactly? Sorry, your report doesn't make it clear enough what exactly the problem is.

Also, please test with the latest version of Moq, which at this time is 4.13.1.

[ronenfe]

I am revising the report but couldn't delete it, give me a while for analyzing.

[ronenfe]

Hi, I edited the description and attached a sample solution.

[ronenfe]

Same error for latest version of Moq.

[stakx]

Thanks for the repro, @ronenfe. This does look like a regression. Seems like quoted expressions might no longer be treated correctly in all cases. I'll take a closer look soon.

[stakx]

@ronenfe, not a regression, after all! Starting with Moq 4.11.0, matching of multi-part expressions such as your x => x.QueryOver<Item>().Where(b => b.Id == itemId).List() has become more accurate, in earlier versions Moq didn't really match arguments of each individual method part correctly / not at all—now it does, and finds that b => b.Id == itemId from your test and b => b.Id == id inside your repository class are not equal LINQ expression trees. That in turn is due to captured variables (itemId and id), which haven't been evaluated prior to expression tree comparison. That is, the way Moq currently sees it, these expressions are different because they catch different variables (regardless of what values they contain at the time of comparison).

I'll need to check whether it makes sense to evaluate captured variables prior to comparing the expression trees; if yes, your use case can likely be fixed; otherwise, I'll close this issue.

[ronenfe]

Thanks stakx, is it possible to overcome this with the current version by changing the usage? or I need to wait for next release?

[stakx]

is it possible to overcome this with the current version by changing the usage?

Yes, although the resulting code will be less semantically correct than what you have now. You simply need to make sure that you pass the same (as in object.ReferenceEquals) Expression<Func<Item, bool>> to both .Where(...) calls. That is, create that expression once, store it to a variable that's accessible to both code sites, then use it in both places.

[ronenfe]

I added the expression to the Item class as a property but I also had to extract the itemId to a property as I can't create an expression without having an id variable when creating it.
If you have better suggestion please let me know, these are the changed class and test:

using System;
using System.Collections.Generic;
using Moq;
using Xunit;


namespace UnitTestProject2
{
    public class UnitTest1
    {
        [Fact]
        public void TestMethod1()
        {
            // Arrange
            Guid itemId = Guid.NewGuid();
            var session = new Mock<NHibernate.ISession>();
            var repo = new Repository();
            session.Setup(x => x.QueryOver<Item>().Where(repo.Expression).List()).Returns(new List<Item>());
            // Act
            var res = repo.GetById(itemId, session.Object);
            // Verify
            //...
        }
    }
}

using System;
using System.Collections.Generic;
using System.Linq;
using System.Linq.Expressions;
using System.Text;
using System.Threading.Tasks;
using NHibernate;

namespace UnitTestProject2
{
    public class Repository
    {
        public Guid ID { get; set; } = new Guid();
        public Expression<Func<Item, bool>> Expression;


        public Repository()
        {
            Expression = b => b.Id == ID;
        }
        public IList<Item> GetById(Guid id, ISession session)
        {
            ID = id;
            var items = session.QueryOver<Item>()
                .Where(Expression)
                .List();

            return items;
        }
    }
}

[stakx]

There you go @ronenfe—starting with the next version of Moq (4.14.0), your scenario should once again work like it used to.