Evaluation failure when using `Nullable` without a value in LINQ

[krzkowal]

Using Nullable<T>.HasValue in LINQ throws NHibernate.HibernateException: Evaluation failure on null.HasValue if no value is set.

Test case

[Test]
public void CanUseNullableWithoutAValue()
{
    bool? isActive = null;

    Assert.DoesNotThrow(
        () => _ = session.Query<Role>()
                     .Where(r => !isActive.HasValue || r.IsActive == isActive.Value)
                     .ToList());
}

Error details

NHibernate.HibernateException: Evaluation failure on null.HasValue ---> System.Reflection.TargetException: Non-static method requires a target.
   at System.Reflection.RuntimeMethodInfo.CheckConsistency(Object target)
   at System.Reflection.RuntimeMethodInfo.InvokeArgumentsCheck(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
   at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
   at System.Reflection.RuntimePropertyInfo.GetValue(Object obj, Object[] index)
   at NHibernate.Impl.ExpressionProcessor.FindValue(Expression expression) in C:\src\nhibernate-core\src\NHibernate\Impl\ExpressionProcessor.cs:line 269
   at NHibernate.Linq.Visitors.NhPartialEvaluatingExpressionVisitor.EvaluateSubtree(Expression subtree) in C:\src\nhibernate-core\src\NHibernate\Linq\Visitors\NhPartialEvaluatingExpressionVisitor.cs:line 151
   at NHibernate.Linq.Visitors.NhPartialEvaluatingExpressionVisitor.Visit(Expression expression) in C:\src\nhibernate-core\src\NHibernate\Linq\Visitors\NhPartialEvaluatingExpressionVisitor.cs:line 92
   --- End of inner exception stack trace ---
   at NHibernate.Linq.Visitors.NhPartialEvaluatingExpressionVisitor.Visit(Expression expression) in C:\src\nhibernate-core\src\NHibernate\Linq\Visitors\NhPartialEvaluatingExpressionVisitor.cs:line 99
   at System.Linq.Expressions.ExpressionVisitor.VisitUnary(UnaryExpression node)
   at System.Linq.Expressions.UnaryExpression.Accept(ExpressionVisitor visitor)
   at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)
   at NHibernate.Linq.Visitors.NhPartialEvaluatingExpressionVisitor.Visit(Expression expression) in C:\src\nhibernate-core\src\NHibernate\Linq\Visitors\NhPartialEvaluatingExpressionVisitor.cs:line 87
   at System.Linq.Expressions.ExpressionVisitor.VisitBinary(BinaryExpression node)
   at System.Linq.Expressions.BinaryExpression.Accept(ExpressionVisitor visitor)
   at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)
   at NHibernate.Linq.Visitors.NhPartialEvaluatingExpressionVisitor.Visit(Expression expression) in C:\src\nhibernate-core\src\NHibernate\Linq\Visitors\NhPartialEvaluatingExpressionVisitor.cs:line 87
   at System.Linq.Expressions.ExpressionVisitor.VisitLambda[T](Expression`1 node)
   at System.Linq.Expressions.Expression`1.Accept(ExpressionVisitor visitor)
   at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)
   at NHibernate.Linq.Visitors.NhPartialEvaluatingExpressionVisitor.Visit(Expression expression) in C:\src\nhibernate-core\src\NHibernate\Linq\Visitors\NhPartialEvaluatingExpressionVisitor.cs:line 87
   at System.Linq.Expressions.ExpressionVisitor.VisitUnary(UnaryExpression node)
   at System.Linq.Expressions.UnaryExpression.Accept(ExpressionVisitor visitor)
   at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)
   at NHibernate.Linq.Visitors.NhPartialEvaluatingExpressionVisitor.Visit(Expression expression) in C:\src\nhibernate-core\src\NHibernate\Linq\Visitors\NhPartialEvaluatingExpressionVisitor.cs:line 87
   at System.Linq.Expressions.ExpressionVisitor.VisitArguments(IArgumentProvider nodes)
   at System.Linq.Expressions.ExpressionVisitor.VisitMethodCall(MethodCallExpression node)
   at System.Linq.Expressions.MethodCallExpression.Accept(ExpressionVisitor visitor)
   at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)
   at NHibernate.Linq.Visitors.NhPartialEvaluatingExpressionVisitor.Visit(Expression expression) in C:\src\nhibernate-core\src\NHibernate\Linq\Visitors\NhPartialEvaluatingExpressionVisitor.cs:line 87
   at NHibernate.Linq.Visitors.NhPartialEvaluatingExpressionVisitor.EvaluateIndependentSubtrees(Expression expressionTree, PreTransformationParameters preTransformationParameters) in C:\src\nhibernate-core\src\NHibernate\Linq\Visitors\NhPartialEvaluatingExpressionVisitor.cs:line 60
   at NHibernate.Linq.NhRelinqQueryParser.PreTransform(Expression expression, PreTransformationParameters parameters) in C:\src\nhibernate-core\src\NHibernate\Linq\NhRelinqQueryParser.cs:line 70
   at NHibernate.Linq.NhLinqExpression..ctor(QueryMode queryMode, Expression expression, ISessionFactoryImplementor sessionFactory) in C:\src\nhibernate-core\src\NHibernate\Linq\NhLinqExpression.cs:line 50
   at NHibernate.Linq.NhLinqExpression..ctor(Expression expression, ISessionFactoryImplementor sessionFactory) in C:\src\nhibernate-core\src\NHibernate\Linq\NhLinqExpression.cs:line 43
   at NHibernate.Linq.DefaultQueryProvider.PrepareQuery(Expression expression, IQuery& query) in C:\src\nhibernate-core\src\NHibernate\Linq\DefaultQueryProvider.cs:line 204
   at NHibernate.Linq.DefaultQueryProvider.ExecuteList[TResult](Expression expression) in C:\src\nhibernate-core\src\NHibernate\Linq\DefaultQueryProvider.cs:line 107
   at NHibernate.Linq.NhQueryable`1.System.Collections.Generic.IEnumerable<T>.GetEnumerator() in C:\src\nhibernate-core\src\NHibernate\Linq\NhQueryable.cs:line 65
   at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
   at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
   at NHibernate.Test.Linq.WhereTests.<>c__DisplayClass59_0.<CanUseNullableWithoutAValue>b__0() in C:\src\nhibernate-core\src\NHibernate.Test\Linq\WhereTests.cs:line 760

[bahusoid]

You should always provide NHibernate version (looks like 5.4 dev is used)

As a workaround use null check instead : isActive == null

Just for reference (looks like similar issue): dotnet/coreclr@75d1d39

[oskarb]

Does NHibernate recognize the call to the Value property and internally understand that it can't actually call that if the nullable is null? (I don't know if that has been implemented).

Does this work:

.Where(r => isActive == null || r.IsActive == isActive)

[fredericDelaporte]

Another workaround would be to avoid using a condition related only to parameters values in the query. That generally generates queries with better performances. But it also generates more queries, which could be an issue in some circumstances.

var query = session.Query<Role>();
if (isActive.HasValue)
    query = query.Where(r => r.IsActive == isActive);
var results = query.ToList();

Oskar, the error message lets infer the trouble occurs when evaluating null.HasValue, so the issue is with the first condition. Maybe there would be an issue with the next one too, I have not checked. I am not sure the pre-evaluation of expressions which can be pre-evaluated does short-circuit on "lazy" boolean operands. (Or maybe we just ignore .Value calls as not needing to be preevaluated when the resulting value has to be translated to SQL, since SQL does not care about knowing the parameter type is nullable or not.)

I think NHibernate supports HasValue, translating it to null checks, when it is in on members, to be translated to SQL. But here on a parameter, the pre-evaluation fails. We should fix that.

[krzkowal]

.Where(r => isActive == null || r.IsActive == isActive)

Sadly also fails with HibernateException: Evaluation failure on null.Value.

I've created a pull request with both cases.

I'm using the if (isActive.HasValue)... workaround for now, thanks.

[bahusoid]

Sadly also fails with HibernateException: Evaluation failure on null.Value.

You are using r.IsActive == isActive.Value. It's not what's been suggested in quoted example .Where(r => isActive == null || r.IsActive == isActive)

[krzkowal]

@bahusoid Sorry, I've forgot to remove that 😅
Without the .Value part no errors are thrown.