null reference exception in Count of linq to nhibernate query

Question

I have a linq query by NHibernate.

var q = SessionInstance.Query<Identity>()
       .Count(x => MyPerson != null && x.Person.Id == MyPerson.Id);

But Check condition in Count Is not Valid. Because if value of MyPerson be null ,expected result of count is 0 but this query has a NullReferenceException By this message :

Object reference not set to an instance of an object.

Why?

Stack trace is :

   at lambda_method(Closure )
   at Remotion.Data.Linq.Parsing.ExpressionTreeVisitors.PartialEvaluatingExpressionTreeVisitor.EvaluateSubtree(Expression subtree)
   at Remotion.Data.Linq.Parsing.ExpressionTreeVisitors.PartialEvaluatingExpressionTreeVisitor.VisitExpression(Expression expression)
   at Remotion.Data.Linq.Parsing.ExpressionTreeVisitor.VisitBinaryExpression(BinaryExpression expression)
   at Remotion.Data.Linq.Parsing.ExpressionTreeVisitor.VisitExpression(Expression expression)
   at Remotion.Data.Linq.Parsing.ExpressionTreeVisitors.PartialEvaluatingExpressionTreeVisitor.VisitExpression(Expression expression)
   at Remotion.Data.Linq.Parsing.ExpressionTreeVisitor.VisitBinaryExpression(BinaryExpression expression)
   at Remotion.Data.Linq.Parsing.ExpressionTreeVisitor.VisitExpression(Expression expression)
   at Remotion.Data.Linq.Parsing.ExpressionTreeVisitors.PartialEvaluatingExpressionTreeVisitor.VisitExpression(Expression expression)
   at Remotion.Data.Linq.Parsing.ExpressionTreeVisitor.VisitLambdaExpression(LambdaExpression expression)
   at Remotion.Data.Linq.Parsing.ExpressionTreeVisitor.VisitExpression(Expression expression)
   at Remotion.Data.Linq.Parsing.ExpressionTreeVisitors.PartialEvaluatingExpressionTreeVisitor.VisitExpression(Expression expression)
   at Remotion.Data.Linq.Parsing.ExpressionTreeVisitor.VisitUnaryExpression(UnaryExpression expression)
   at Remotion.Data.Linq.Parsing.ExpressionTreeVisitor.VisitExpression(Expression expression)
   at Remotion.Data.Linq.Parsing.ExpressionTreeVisitors.PartialEvaluatingExpressionTreeVisitor.VisitExpression(Expression expression)
   at Remotion.Data.Linq.Parsing.ExpressionTreeVisitor.VisitAndConvert[T](T expression, String methodName)
   at Remotion.Data.Linq.Parsing.ExpressionTreeVisitor.<>c__DisplayClass6`1.<VisitAndConvert>b__5(T expression)
   at Remotion.Data.Linq.Parsing.ExpressionTreeVisitor.VisitList[T](ReadOnlyCollection`1 list, Func`2 visitMethod)
   at Remotion.Data.Linq.Parsing.ExpressionTreeVisitor.VisitAndConvert[T](ReadOnlyCollection`1 expressions, String callerName)
   at Remotion.Data.Linq.Parsing.ExpressionTreeVisitor.VisitMethodCallExpression(MethodCallExpression expression)
   at Remotion.Data.Linq.Parsing.ExpressionTreeVisitor.VisitExpression(Expression expression)
   at Remotion.Data.Linq.Parsing.ExpressionTreeVisitors.PartialEvaluatingExpressionTreeVisitor.VisitExpression(Expression expression)
   at Remotion.Data.Linq.Parsing.ExpressionTreeVisitors.PartialEvaluatingExpressionTreeVisitor.EvaluateIndependentSubtrees(Expression expressionTree)
   at NHibernate.Linq.NhLinqExpression..ctor(Expression expression)
   at NHibernate.Linq.NhQueryProvider.PrepareQuery(Expression expression, IQuery& query, NhLinqExpression& nhQuery)
   at NHibernate.Linq.NhQueryProvider.Execute(Expression expression)
   at NHibernate.Linq.NhQueryProvider.Execute[TResult](Expression expression)
   at System.Linq.Queryable.Count[TSource](IQueryable`1 source, Expression`1 predicate)
   at RCISP.NHibernate.Repository.IdentityRepositoryNh.IsExistOriginalIdentityForThisAlias(Identity item)
   at RCISP.Domain.Services.IdentityService.CreateIdentity(IRepositoryLocator locator, Person person)
   at RCISP.Domain.Services.IdentityService.LoadByPersonIdentificationCode(IRepositoryLocator locator, String identificationCode, Nullable`1 autoCreate)
   at RCISP.Domain.Services.EvidenceRequestService.Update[T](IRepositoryLocator locator, EvidenceRequestDto dto)
   at RCISP.Domain.Services.EvidenceRequestService.UpdateEntityCommand[T,TR](IRepositoryLocator locator, EvidenceRequestDto dto)
   at RCISP.Domain.Services.WeaponLicenseRequestService.<>c__DisplayClass4.<UpdateEntity>b__3(IRepositoryLocator locator)
   at RCISP.NHibernate.TransManager.TransManagerNh.ExecuteCommand[TResult](Func`2 command)

It could be x.Person that is null - try checking for that as well — Simon Ejsing, Jun 30 '12 at 05:26
@SimonEjsing: I checked it. My problem Exactly is in MyPerson. — Ehsan, Jun 30 '12 at 05:31

Damith · Answer 1 · 2012-06-30T05:37:26.393

1

if(MyPerson != null)
{
  var q = SessionInstance.Query<Identity>()
       .Count(x => x.Person != null && x.Person.Id == MyPerson.Id);
}else
{
  // if you know when MyPerson is null count is zero, why you need to Query?
  // here count is zero..

}

EDIT

  var q = SessionInstance.Query<Identity>()
       .Count(x => MyPerson != null && x.Person != null && x.Person.Id == MyPerson.Id);

edited Jun 30 '12 at 05:37

answered Jun 30 '12 at 05:23

Damith

62,401
13
102
153

I know it, but my question is in the case how check condition in linq provider – Ehsan Jun 30 '12 at 05:27
@Ehsan have you check my second Query? are you still get the exception ? – Damith Jun 30 '12 at 05:41
I checked it. My problem exactly is in MyPerson and is not in `x.Person` – Ehsan Jun 30 '12 at 05:45

score 0 · Answer 2 · answered Jun 30 '12 at 16:36

Whenever you have a NullReferenceException in any line of code and it's not clear where it can originate, start to split your long line of code into smaller ones, and do null checks.

For instance:

var q = SessionInstance.Query<Identity>();

if (q != null && MyPerson != null) {
   var count = q.Count(x => 
       x != null &&
       x.Person != null &&
       x.Person.Id == MyPerson.Id
   );
}
else {
   throw new InvalidOperationException("Something went wrong");
}

Your answer is duplicate because my question is in the case how check condition in linq provider. — Ehsan, Jul 03 '12 at 03:35

null reference exception in Count of linq to nhibernate query

2 Answers2

EDIT