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Null reference exception being thrown in EF LINQ query if-clause

I couldn't find the exact words to explain what's happening, so if this is a duplicated question, I apologize.
I tried to do a quite simple AND condition if-clause inside a LINQ Query, in order to check if an object is null and then verify if its property is equal or not the column I wanted to compare.
The code:
public IEnumerable<Plan> GetPlans(Plan plan)
{
return _context.Plans.Where(e =>
e.Situation == plan.Situation &&
e.Notes.Contains(plan.Notes) &&
(plan.Excercise != null && plan.Exercise.Year > 0 ? e.Exercise.Year == plan.Exercise.Year: true)).ToList();
}
I've already done this kind of check a dozen times before in .NET 4.5, without having any kind of issue.
But now, in the first .NET Core 2.0 project I'm working on, I had the following error:
An exception was thrown while attempting to evaluate a LINQ query parameter expression. To show additional information call EnableSensitiveDataLogging() when overriding DbContext.OnConfiguring.
The inner exception is clearer: NULL REFERENCE EXCEPTION.
After some tests, I found out that the error happens when plan.Exercise comes null, even if I try to avoid the exception by checking at first if it's null or not.
If I try to do the same check directly in Immediate Window, it returns "false", as it should be.
Am I missing something here? It could be an EF bug? Any particular reason why this works in .NET 4.5, for example, and not in .NET Core 2.0?
Thanks in advance.
UPDATE
Ivan's solution did the job:
Rewrite ? : constructs with equivalent ||
plan.Excercise == null || plan.Exercise.Year <= 0 || e.Excercise.Year == plan.Exercise.Year

It sounds like this might be a bug in EF Core (but I don't know this for sure).
One thing you might try is to fail fast if the base requirements of plan are not met, and more importantly, instead of using the ternary operator, use the traditional comparison operators along with parenthesis:
public IEnumerable<Plan> GetPlans(Plan plan)
{
if (plan == null) return new List<Plan>();
return _context.Plans
.Where(e =>
e.Situation == plan.Situation &&
e.Notes.Contains(plan.Notes) &&
(plan.Exercise == null ||
plan.Exercise.Year <= 0 ||
e.Excercise.Year == plan.Exercise.Year))
.ToList();
}

To avoid this issue, make sure that you are not evaluating on null object.
var exercice = await _repositoryExercice.FirstOrDefaultAsync(i => i.IsCurrent);
var depenses = _repositoryDepense.GetAll()
.Where( e => e.ExerciceId.Equals(exercice.Id))
.WhereIf(AbpSession.TenantId.HasValue, m => m.TenantId.Value.Equals(AbpSession.TenantId.Value))
.ToList();
The issue was causing by this line .Where( e => e.ExerciceId.Equals(exercice.Id)) because the variable exercice is null.
Best practice, I replaced that line by this :
...
.WhereIf(exercice != null, e => e.ExerciceId.Equals(exercice.Id))
...

how about simplifying your code into something like
public IEnumerable<Plan> GetPlans(int year)
{
return _context.Plans
.Where(e => e.Excercise.Year == year)
.ToList();
}

Ternary operator in lambda expression C#

I am trying to put a conditional expression with in a lambda query as below
GetPeers(bool? isConnected = true)
{
dbContext
.peers
.Where(m => isConnected.HasValue ? m.IsConnected == isConnected.Value : true)
.ToList();
}
Above statement gives me Nullable object must have a value. exception.
I dont get it! What is wrong in my query?
What I want if isConnected is null, then it should be returning all the records otherwise on the basis of m.IsConnected == isConnected.Value,

This is an issue with how Entity Framework is translating the ternary expression, it is attempting to create a SQL parameter using the result of isConnected.Value but isConnected is null so it throws.
My preferred option is to only apply the predicate if there is actually a predicate to apply:
IQueryable<Peer> peers = dbContext.peers;
if(isConnected.HasValue)
{
var isConnectedValue = isConnected.Value;
peers = peers.Where(m => m.IsConnected == isConnectedValue);
}
return peers.ToList();

You can change this
m.IsConnected == isConnected.Value
to this
m.IsConnected == (isConnected.HasValue && isConnected.Value)
The problem is that isConnected doesn't have always a value. It's a bool?. So it can has a value or not (null). If it has a value, it would be either true/false. The problem arises, when it has no value. This is happening due to conditional operator. According to the documentation:
The conditional operator (?:) returns one of two values depending on
the value of a Boolean expression. Following is the syntax for the
conditional operator.
condition ? first_expression : second_expression;
Either the type of first_expression and second_expression must be the
same, or an implicit conversion must exist from one type to the other.
For the full documentation, please have a look here.

Null coalesce not working in LINQ query

Take this:
int? item1 = null;
int? item2 = null;
someObjectList.Where(x => x.SomeItem1 == (item1 ?? x.SomeItem1)
&& x.SomeItem2 == (item2 ?? x.SomeItem2)
);
Where someObjectList is not empty and SomeItem1 and SomeItem2 is null in all the objects in the list.
Why is it returning nothing?
EDIT:
My Code:
public void GetPlacementsByMaterial(long clientMaterialID)
{
ClientMaterial clientMaterial = ((ApplicationEntityModel)NavigationItem.ObjectContext).ClientMaterial.FirstOrDefault(x => x.ClientMaterialID == clientMaterialID);
var list = GetPlacementList(supplier, mediaSpace);
PlacementsList = list.Where(x => x.MediaCategoryFormatID == (clientMaterial.MediaCategoryFormatID ?? x.MediaCategoryFormatID)
&& x.MediaCategorySizeID == (clientMaterial.MediaCategorySizeID ?? x.MediaCategorySizeID)
);
}
All ID's are Nullable<long>.
EDIT:
SQL Profiler:
SELECT *
FROM [dbo].[CampaignSchedulePlacements] AS [Extent5]
WHERE ([Extent5].[MediaCategoryFormatID] = [Extent5].[MediaCategoryFormatID]) AND ([Extent5].[MediaCategorySizeID] = [Extent5].[MediaCategorySizeID])
Note: cleaned up the `SQL.

In SQL, NULL is not equal to NULL.
You can interpret NULL as meaning: "there is value, but I don't know what it is". So if you're comparing two NULL values, you're really asking "is the first unknown value equal to the second unknown value?" Of course, there is no reason to assume they are, so SQL will say "no".
I am assuming that this is causing your problem. You can verify that by looking at the actual SQL produced. If it's using the SQL = operator, this is indeed the problem. You can verify that by running the SQL in a database tool, such as SQL Management Studio in case you're using SQL Server.
UPDATE:
The condition
([Extent5].[MediaCategoryFormatID] = [Extent5].[MediaCategoryFormatID])
will indeed return false when [Extent5].[MediaCategoryFormatID] is NULL.
That answers the question "Why is it returning nothing?"
However, another question come to mind: why would the entity framework generate that SQL from this linq query?
I'm afraid that linq to entities is not exactly known for the quality of its SQL generation, and this case seems to confirm that. You might consider Linq to SQL. Even if that seems to be a dead-end track in the long run, the current implementation if a lot better than linq to entities.
In either case, have you tried something like
someObjectList.Where(x =>
!item1.hasValue ||
x.SomeItem1.HasValue && x.SomeItem1.Value == item1.Value)
Make sure to verify that under the profiler as well though, linq to entities might mess it up too.

How can i use DateTime.AddXXXX functions in a Linq-to-Entities query?

I am trying to use AddMonths in a query
List<Entities.Subscriber> items = (from s in context.Subscribers
where s.Validated == false && s.ValidationEmailSent == true && s.SubscriptionDateTime < DateTime.Now.AddMonths(-1)
select s).ToList();
But I recieve an error :
LINQ to Entities does not recognize the method 'System.DateTime
AddMonths(Int32)' method, and this method cannot be translated into a
store expression.
Is there a way I can use this function inside my query?

The simplest fix to this is to work out the time limit once before using LINQ:
DateTime limit = DateTime.Now.AddMonths(-1);
List<Entities.Subscriber> items = (from s in context.Subscribers
where s.Validated == false && s.ValidationEmailSent == true &&
s.SubscriptionDateTime < limit)
select s).ToList();
Or more readably IMO:
var items = context.Subscribers
.Where(s => !s.Validated &&
s.ValidationEmailSent &&
s.SubscriptionDateTime < limit)
.ToList();
There's no benefit in using a query expression here, and explicit comparisons with true and false are ugly IMO (unless your properties are of type Nullable<bool> of course).

Jon Skeet has already provided a simple fix, but if you want the DateTime.Now.AddMonths bit to run on the database, try the EntityFunctions.AddMonths method.
This is a more general approach that is especially useful when you cannot replicate the expression cheaply or correctly on the client.

You can change your code to:
DateTime oneMonth = DateTime.Now.AddMonths(-1)
List<Entities.Subscriber> items = (from s in context.Subscribers
where s.Validated == false && s.ValidationEmailSent == true && s.SubscriptionDateTime < oneMonth
select s).ToList();
You have to do this because AddMonth is a .NET function that can't be translated into SQL by Linq to Entities. Perform the calculation in your code and then use the resulting datetime will work.

Linq where column == (null reference) not the same as column == null

I came across a rather strange problem with linq-to-sql. In the following example,
var survey = (from s in dbContext.crmc_Surveys
where (s.crmc_Retail_Trade_Id == tradeId) && (s.State_.Equals(state))
select s).First();
If tradeId is null, it doesn't behave as if I had specified null specifically like this instead,
var survey = (from s in dbContext.crmc_Surveys
where (s.crmc_Retail_Trade_Id == null) && (s.State_.Equals(state))
select s).First();
Which is my desired behavior. In fact it doesn't return anything unless both values are non-null. I can't figure out how to accomplish this short of several different linq queries. Any ideas?

Change where (s.crmc_Retail_Trade_Id == tradeId)
to
where (s.crmc_Retail_Trade_Id == tradeId ||
(tradeId == null && s.crmc_Retail_Trade_Id == null))
Edit - based on this post by Brant Lamborn, it looks like the following would do what you want:
where (object.Equals(s.crmc_Retail_Trade_Id, tradeId))
The Null Semantics (LINQ to SQL) MSDN page links to some interesting info:
LINQ to SQL does not impose C# null or
Visual Basic nothing comparison
semantics on SQL. Comparison operators
are syntactically translated to their
SQL equivalents. The semantics reflect
SQL semantics as defined by server or
connection settings. Two null values
are considered unequal under default
SQL Server settings (although you can
change the settings to change the
semantics). Regardless, LINQ to SQL
does not consider server settings in
query translation.
A comparison with the literal null
(nothing) is translated to the
appropriate SQL version (is null or is
not null).
The value of null (nothing) in
collation is defined by SQL Server;
LINQ to SQL does not change the
collation.

Another option to solve this, as I ran across this problem as well.
where (tradeId == null ? s.crmc_Retail_Trade_Id == null : s.crmc_Retail_Trade_Id == tradeId)

Not sure on this one, but I suspect when linq-to-sql translates that to an sql query string you get a slightly different expression specifying null directly such that at some point you end up comparing NULL to itself, and NULL=NULL is defined to be false.

I am not familiar with Linq, however in general:
NULL represents a missing, unknown, or undefined value. Strictly speaking, a variable cannot equal NULL; low-lvel languages which provide this construct usually do so as a convenience because there is no easy alternative -- at a higher level it's usually better to rely on ISNULL, defined, or whatever features your language supplies.
One undefined variable is not equal to another undefined variable (and the same applies to NULL == NULL). Joe Celko has a good example of writing a query to find all people whose hair colour matches the colour of the vehicle they drive. Should this query match a bald man who walks everywhere?

It's better to make sp for this purpose because linq will perform iteration it takes while for your assistance if you are using linq.
var c = lstQ_Buffer.Where(q => (((semesterId == 0 || semesterId == null ? q.fkSemesterId == null : q.fkSemesterId == semesterId) && (subjectSpecialtyId == 0 || subjectSpecialtyId == null ? q.fkSubSpecialtyId == null : q.fkSubSpecialtyId == subSpecialtyId) && (subTopicId == 0 || subTopicId == null ? q.fkSubTopicId == null : q.fkSubTopicId == subTopicId)) && (q.fkProgramId == programId && q.fkYearId == yearId && q.fkCourse_ModuleId == courseModuleId && q.fkSubject_SpecialtyId == subjectSpecialtyId && q.fkTopicId == topicId && q.fkDifficultyLevelId == diffucultyLevelId))).ToList();