Strange problem with entity framework (null directly versus variable containing null)

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C#

entity-framework

Hi all,
I have a strange problem dealing with Entity Framework.
This problem never ever occured to me before, so I am kind of surprised.

It's all about the AgencyId, which is in fact null.
I verify this where I set agencyEqualsNull in the first line (it is true).

// verify that AgencyId indeed is null
bool agencyEqualsNull = AgencyId == null;

// here I do not get a result -; test is null
tblTest test = context.tblTest.FirstOrDefault(w => 
w.couponId == Id
&&
w.couponCode == Code
&&
w.customerId == CustomerId
&&
w.agencyId == AgencyId
);

if (test != null)
{
    // we never get here
}

// here I do get a result - test is not null
tblTest test2 = context.tblTest.FirstOrDefault(w => 
w.couponId == Id
&&
w.couponCode == Code
&&
w.customerId == CustomerId
&&
w.agencyId == null
);

if (test2 != null)
{
    //we get here
}

Now for the first try I receive null for test, for the second try I recieve a value for test2 (not null).
The only difference is that I do not pass the variable (which is in fact null) but null directly...

Do I overlook something? I ssumed the queries are both equal and should return a value...

// UPDATE
I fired up the SQL Server Profiler in order to enlight the situation
here is the query:

exec sp_executesql N'SELECT TOP (1) 
[Extent1].[id] AS [id], 
[Extent1].[code] AS [code], 
[Extent1].[customerId] AS [customerId], 
[Extent1].[employeeId] AS [employeeId],
FROM [dbo].[tblTest] AS [Extent1]
WHERE 
([Extent1].[Id] = @p__linq__0) AND 
([Extent1].[Code] = @p__linq__1) AND 
([Extent1].[customerId] = @p__linq__2) AND 
([Extent1].[employeeId] = @p__linq__3)',
N'@p__linq__0 bigint,
@p__linq__1 nvarchar(4000),
@p__linq__2 bigint,
@p__linq__3 bigint',
@p__linq__0=82,
@p__linq__1=N'ADVENT2012-CU88888',
@p__linq__2=88888,
@p__linq__3=NULL

In the last line you can see @p__linq__3=NULL
If I pass null directly it resolves to

([Extent1].[employeeId] IS NULL)

instead of

([Extent1].[employeeId] = @p__linq__3)

where

@p__linq__3=NULL

Now I am even more puzzled because I really assumed the generated query would be the same if I pass a variable containing null or null directly...

// UPDATE
I figured out that this will do the trick:
change

w.agencyId == AgencyId

to

(AgencyId == null) ? w.agencyId == null : w.agencyId == AgencyId

I somehow assumed that this would be done automatically...
This is actually not my code and now I am kind of afraid that I have to check everything that might be related to this problem in the complete codebase...

Does anybody has a suggestion regarding this one?


Any help is kindly appreciated,
best regards
Andy