Question

I'm getting a strange NullReferenceException on a simple query with Dataobjects 4.4 (latest stable build)

Code:

  var query = Query.All<Document>()
                .Where(doc => doc.Container == TrashContainer)
                .Take(thresholdMany);

Exception:

System.NullReferenceException occurred
  Message=Object reference not set to an instance of an object.
  Source=Xtensive.Orm
  StackTrace:
       at Xtensive.Storage.Providers.Sql.BatchingCommandProcessor.ExecuteBatch(Int32 numberOfTasks, QueryRequest lastRequest)
       at Xtensive.Storage.Providers.Sql.BatchingCommandProcessor.ExecuteRequestsWithReader(QueryRequest request)
       at Xtensive.Storage.Providers.Sql.SessionHandler.<Xtensive.Storage.Providers.Sql.IQueryExecutor.ExecuteTupleReader>d__a.MoveNext()
       at Xtensive.Storage.Providers.Sql.SqlProvider.<OnEnumerate>d__0.MoveNext()
       at Xtensive.Storage.Rse.Providers.ExecutableProvider.<GetEnumerator>d__0.MoveNext()
       at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
       at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
       at Xtensive.Storage.Rse.RecordSet.<GetGreedyEnumerator>d__0.MoveNext()
       at System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext()
       at Xtensive.Core.EnumerableExtensions.<Batch>d__20`1.MoveNext()
       at Xtensive.Core.EnumerableExtensions.<ApplyBeforeAndAfter>d__28`1.MoveNext()
       at System.Linq.Enumerable.<SelectManyIterator>d__14`2.MoveNext()
       at System.Linq.Enumerable.First[TSource](IEnumerable`1 source)
       at lambda_method(Closure , Object[] , IEnumerable`1 , Session , Dictionary`2 , ParameterContext )
       at Xtensive.Core.DelegateBindExtensions.<>c__DisplayClass1f`6.<Bind>b__1e(T2 arg2, T3 arg3, T4 arg4, T5 arg5)
       at Xtensive.Orm.Linq.TranslatedQuery`1.Execute(Session session, ParameterContext parameterContext)
       at Xtensive.Orm.Linq.QueryProvider.Execute[TResult](Expression expression)
       at System.Linq.Queryable.Count[TSource](IQueryable`1 source)
       at Business.Tools.EmptyTrash.Process()

Workaround:

 Container trash = TrashContainer;
  var query = Query.All<Document>()
                .Where(doc => doc.Container == trash)
                .Take(thresholdMany);

Answer 1

Hello olorin,

Looks like this is a closure. As the query execution is lazy, the value of TrashContainer parameter is being calculated right before the execution, at that moment the value of the variable named TrashContainer is null.

To test the approach, try executing the query immediately, like this:

var query = Query.All<Document>()
            .Where(doc => doc.Container == TrashContainer)
            .Take(thresholdMany)
            .ToList();

This time you won't get any NullReferenceException.

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