LINQ where clause with lambda expression having OR clauses and null values returning incomplete results

Question

the problem in short  we have a lambda expression used in the Where clause  which is not returning the  expected  result   quick summary  in the analysisObjectRepository object  there are certain objects which also contain the parent relationship in a property named Parent  we are querying this analysisObjectRepository to return some objects   detail  what the code below supposed to do is  returning the root  the first children  immediate children  and grandchildren of a specific object containing the ID value   in the code below  common sense says that all the results which makes any of the 3 seperate OR conditions true should be returned as in the results   List lt AnalysisObject gt  analysisObjects        analysisObjectRepository          FindAll            Where x   gt  x ID                  packageId                        x Parent ID           packageId                         x Parent Parent ID    packageId           ToList      but the above code only returns the children and grandchildren  while not returning the root objects  with a null Parent value  which make the  x ID    packageId   condition true    only objects which make the second  x Parent ID    packageId   and third   x Parent Parent ID    packageId   clauses are returned   If we only write the code to return the root object with the below code  it is returned  so we are totally sure that analysisObjectRepository contains all the objects  List lt AnalysisObject gt  analysisObjects        analysisObjectRepository          FindAll            Where x   gt  x ID    packageId            ToList      However  when we rewrite it as a delegate  we get the expected result  returning all the expected objects   List lt AnalysisObject gt  analysisObjects        analysisObjectRepository          FindAll            Where delegate AnalysisObject x                          return                 x ID    packageId                     x Parent    null  amp  amp  x Parent ID    packageId                         x Parent    null  amp  amp                      x Parent Parent    null  amp  amp                      x Parent Parent ID    packageId               ToList      question  Are we missing something in the lambda expression  it is a really simple 3 part OR condition and we think that any object that makes any of the three conditions true should be returned  we suspected that the root object having a null Parent value might cause a problem but couldn t figure it out exactly   any help would be great

User · Answer

Try writting the lambda with the same conditions as the delegate  like this     List lt AnalysisObject gt  analysisObjects        analysisObjectRepository FindAll   Where       x   gt           x ID    packageId          x Parent    null  amp  amp  x Parent ID    packageId          x Parent    null  amp  amp  x Parent Parent    null  amp  amp  x Parent Parent ID    packageId        ToList

User · Answer

Your second delegate is not a rewrite of the first in anonymous delegate  rather than lambda  format  Look at your conditions   First   x ID    packageId    x Parent ID    packageId    x Parent Parent ID    packageId   Second    x ID    packageId      x Parent    null  amp  amp  x Parent ID    packageId       x Parent    null  amp  amp  x Parent Parent    null  amp  amp  x Parent Parent ID    packageId    The call to the lambda would throw an exception for any x where the ID doesn t match and either the parent is null or doesn t match and the grandparent is null  Copy the null checks into the lambda and it should work correctly   Edit after Comment to Question  If your original object is not a List lt T gt   then we have no way of knowing what the return type of FindAll   is  and whether or not this implements the IQueryable interface  If it does  then that likely explains the discrepancy  Because lambdas can be converted at compile time into an Expression lt Func lt T gt  gt  but anonymous delegates cannot  then you may be using the implementation of IQueryable when using the lambda version but LINQ-to-Objects when using the anonymous delegate version   This would also explain why your lambda is not causing a NullReferenceException  If you were to pass that lambda expression to something that implements IEnumerable lt T gt  but not IQueryable lt T gt   runtime evaluation of the lambda  which is no different from other methods  anonymous or not  would throw a NullReferenceException the first time it encountered an object where ID was not equal to the target and the parent or grandparent was null   Added 3 16 2011 8 29AM EDT  Consider the following simple example   IQueryable lt MyObject gt  source           some object that implements IQueryable lt MyObject gt   var anonymousMethod    source Where delegate MyObject o    return o Name     Adam           var expressionLambda   source Where o   gt  o Name     Adam      These two methods produce entirely different results    The first query is the simple version  The anonymous method results in a delegate that s then passed to the IEnumerable lt MyObject gt  Where extension method  where the entire contents of source will be checked  manually in memory using ordinary compiled code  against your delegate  In other words  if you re familiar with iterator blocks in C   it s something like doing this   public IEnumerable lt MyObject gt  MyWhere IEnumerable lt MyObject gt  dataSource  Func lt MyObject  bool gt  predicate        foreach MyObject item in dataSource                if predicate item   yield return item            The salient point here is that you re actually performing your filtering in memory on the client side  For example  if your source were some SQL ORM  there would be no WHERE clause in the query  the entire result set would be brought back to the client and filtered there   The second query  which uses a lambda expression  is converted to an Expression lt Func lt MyObject  bool gt  gt  and uses the IQueryable lt MyObject gt  Where   extension method  This results in an object that is also typed as IQueryable lt MyObject gt   All of this works by then passing the expression to the underlying provider  This is why you aren t getting a NullReferenceException  It s entirely up to the query provider how to translate the expression  which  rather than being an actual compiled function that it can just call  is a representation of the logic of the expression using objects  into something it can use   An easy way to see the distinction  or  at least  that there is  a distinction  would be to put a call to AsEnumerable   before your call to Where in the lambda version  This will force your code to use LINQ-to-Objects  meaning it operates on IEnumerable lt T gt  like the anonymous delegate version  not IQueryable lt T gt  like the lambda version currently does   and you ll get the exceptions as expected   TL DR Version  The long and the short of it is that your lambda expression is being translated into some kind of query against your data source  whereas the anonymous method version is evaluating the entire data source in memory  Whatever is doing the translating of your lambda into a query is not representing the logic that you re expecting  which is why it isn t producing the results you re expecting

User · Answer

You are checking Parent properties for null in your delegate  The same should work with lambda expressions too   List lt AnalysisObject gt  analysisObjects   analysisObjectRepository          FindAll            Where x   gt                x ID    packageId                   x Parent    null  amp  amp                   x Parent ID    packageId                      x Parent Parent    null  amp  amp  x Parent Parent ID    packageId             ToList

[c#] LINQ where clause with lambda expression having OR clauses and null values returning incomplete results

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