How do I pass null as a value to a string (varchar) column from linq to sql? I'm trying to query the table where some rows have NULL for a column (say, Col_A) and others have "NotNull" (string) as value.
string s = null;
var query = (from r in context.Table1
where r.Col_A == s
select r).ToList();
This returns 0 rows. But if I set s to "NotNull", I get the rows back as expected.
Any pointers will be helpful.
There is an annoying bug, where the following can work differently in some cases:
where x.Foo == null
vs
where x.Foo == s
where s happens to be null at execution. Basically, in some use-cases, it can incorrectly end up using:
WHERE [x].[Foo] = #p3
with #p3 a null value, but clearly that is not sensible in ANSI compliant SQL.
I would suggest if you are doing a test where the parameter value might be null, special-case it and code the query with a literal null in the C#. I did have some code that would use an expression-visitor to apply this after-the-fact, but it is easier just to special-case null. So:
var query = // some core query
if(s == null) { query = query.Where(x => x.Foo == null); }
else { query = query.Where(x => x.Foo == s); }
have you tried DBNull.Value? That's .net's DB equivalent to null.
Related
I have the following code. I'm getting error:
"The cast to value type 'Int32' failed because the materialized value is null. Either the result type's generic parameter or the query must use a nullable type."
when CreditHistory table has no records.
var creditsSum = (from u in context.User
join ch in context.CreditHistory on u.ID equals ch.UserID
where u.ID == userID
select ch.Amount).Sum();
How can I modify the query to accept null values?
A linq-to-sql query isn't executed as code, but rather translated into SQL. Sometimes this is a "leaky abstraction" that yields unexpected behaviour.
One such case is null handling, where there can be unexpected nulls in different places. ...DefaultIfEmpty(0).Sum(0) can help in this (quite simple) case, where there might be no elements and sql's SUM returns null whereas c# expect 0.
A more general approach is to use ?? which will be translated to COALESCE whenever there is a risk that the generated SQL returns an unexpected null:
var creditsSum = (from u in context.User
join ch in context.CreditHistory on u.ID equals ch.UserID
where u.ID == userID
select (int?)ch.Amount).Sum() ?? 0;
This first casts to int? to tell the C# compiler that this expression can indeed return null, even though Sum() returns an int. Then we use the normal ?? operator to handle the null case.
Based on this answer, I wrote a blog post with details for both LINQ to SQL and LINQ to Entities.
To allow a nullable Amount field, just use the null coalescing operator to convert nulls to 0.
var creditsSum = (from u in context.User
join ch in context.CreditHistory on u.ID equals ch.UserID
where u.ID == userID
select ch.Amount ?? 0).Sum();
Had this error message when I was trying to select from a view.
The problem was the view recently had gained some new null rows (in SubscriberId column), and it had not been updated in EDMX (EF database first).
The column had to be Nullable type for it to work.
var dealer = Context.Dealers.Where(x => x.dealerCode == dealerCode).FirstOrDefault();
Before view refresh:
public int SubscriberId { get; set; }
After view refresh:
public Nullable<int> SubscriberId { get; set; }
Deleting and adding the view back in EDMX worked.
Hope it helps someone.
I have used this code and it responds correctly, only the output value is nullable.
var packesCount = await botContext.Sales.Where(s => s.CustomerId == cust.CustomerId && s.Validated)
.SumAsync(s => (int?)s.PackesCount);
if(packesCount != null)
{
// your code
}
else
{
// your code
}
You are using aggregate function which not getting the items to perform action , you must verify linq query is giving some result as below:
var maxOrderLevel =sdv.Any()? sdv.Max(s => s.nOrderLevel):0
I see that this question is already answered. But if you want it to be split into two statements, following may be considered.
var credits = from u in context.User
join ch in context.CreditHistory
on u.ID equals ch.UserID
where u.ID == userID
select ch;
var creditSum= credits.Sum(x => (int?)x.Amount) ?? 0;
Got this error in Entity Framework 6 with this code at runtime:
var fileEventsSum = db.ImportInformations.Sum(x => x.FileEvents)
Update from LeandroSoares:
Use this for single execution:
var fileEventsSum = db.ImportInformations.Sum(x => (int?)x.FileEvents) ?? 0
Original:
Changed to this and then it worked:
var fileEventsSum = db.ImportInformations.Any() ? db.ImportInformations.Sum(x => x.FileEvents) : 0;
I was also facing the same problem and solved through making column as nullable using "?" operator.
Sequnce = db.mstquestionbanks.Where(x => x.IsDeleted == false && x.OrignalFormID == OriginalFormIDint).Select(x=><b>(int?)x.Sequence</b>).Max().ToString();
Sometimes null is returned.
I am using Visual Studio 2012 with MySQL 5.7 Community Edition. I am getting object reference not set to an instance of object on the below query.
var oList = (
from dm in dbContext.document_master
join um in dbContext.user_master on dm.Alocated_CAA equals um.UserId
where (dm.DocumentHandle != null)
select new TaskLogReport
{
documentDate = dm.Document_Date,
documentHandle = dm.DocumentHandle,
fileNumber = dm.FileNumber,
statusRemarks = dbContext.statushistories
.Where(x => x.DocumentHandle == 12345678).FirstOrDefault().Remarks
}).ToList();
In the above query If I get change 12345678 to dm.DocumentHandle, then getting object reference not set to an instance object.
Try with MS-SQL server , working fine.
You get the error because FirstOrDefault() returns null, because there are no objects who's DocumentHandle equals dm.DocumentHandle. Changing dm.DocumentHandle to 12345678 works because there is one matching element.
This line is the problem:
.Where(x => x.DocumentHandle == 12345678).FirstOrDefault().Remarks
C#'s Enumerable.FirstOrDefault() method returns either the first matching element of an enumerable, or, if there aren't any, the default value of the type. In the case of nullable types, FirstOrDefault() returns null if no elements are found.
The reason you get no error when you use 12345678 is because there is at least one matching value with that DocumentHandle. However, when you change that to dm.DocumentHandle, no matching elements are found, causing FirstOrDefault to return null. Then you essentially do null.Remarks, which causes the error.
You should change your code to this:
.FirstOrDefault(x => x.DocumentHandle == dm.DocumentHandle)?.Remarks
There are two differences here:
Got rid of the where and moved the predicate to the FirstOrDefault call. This is just a cleaner way of doing what you were doing before.
Added the ? at the end of the FirstOrDefault call. That is the null propagation operator.
What it does is it turns this:
int foo;
if (bar != null)
{
foo = bar.foo;
}
Into this:
int foo = bar?.foo;
Now, if there are matching elements, statusRemarks will be equal to the first one's Remarks. Otherwise, statusRemarks will be null.
EDIT: The null propagation operator was implemented in C# 6.0, which is .Net 4.6 onwards, so it won't work in .Net 4.0.
You'll have to modify your query and implement a check, like so:
var oList = (
from dm in dbContext.document_master
join um in dbContext.user_master on dm.Alocated_CAA equals um.UserId
where (dm.DocumentHandle != null && dbContext.statushistories.Count(x => x.DocumentHandle == dm.DocumentHandle) > 0)
select new TaskLogReport
{
documentDate = dm.Document_Date,
documentHandle = dm.DocumentHandle,
fileNumber = dm.FileNumber,
statusRemarks = dbContext.statushistories.First(x.DocumentHandle == dm.DocumentHandle).Remarks
}).ToList();
I am struggling to figure out how to get a LINQ statement to produce a specific WHERE clause in SQL in a single statement.
I am after it producing something like this:
SELECT ColA, ColB, ColC, ColN...
FROM Orders
WHERE Client = #ClientId
AND (#CompanyId IS NULL OR #CompanyId = CompanyId)
My (failing) LINQ statement looks like this:
var includeAllCompanies = company == null;
var data = context.Orders.Where(o => o.Client.Id == clientId
&& (includeAllCompanies
|| (c.Company != null && c.Company.Id == company.Id)).ToList();
However, it always throws an exception when the variable company is NULL (it works fine when it has been initialised). The exception being:
Non-static method requires a target.
My current fix is to split my LINQ statement into two. One using an Expression<Func<>> (to be transformed to a SQL statement with partial filtering). Then another that uses Func<> to perform the remaining filters on the returned list.
Expression<Func<>> to let SQL do some of the work (excluding nullable objects)
var data = context.Orders.Where(o => o.Client.Id == clientId).ToList();
Func<> to then filter out the nullable objects
data = data.Where(c => (territory == null
|| (c.Territory != null && c.Territory.Id == territory.Id))).ToList();
This works, however, I want SQL to be performing this query.
The problem is that, company is server-side variable. Regardles includeAllCompanies value, EF has to translate whole LINQ query to SQL - and in this case SQL doesn't know what is company.Id - so EF has to always get company.Id value in order to put into SQL query. Even if company is null (so that is why you get exception). I hope you see my point, if not - I'll try to give some sample.
In order get rid of exception you can do the following:
var companyId = company == null ? null : (int?)company.Id;
var data = context.Orders.Where(o => o.Client.Id == clientId
&& (companyId == null
|| (c.Company != null && c.Company.Id == companyId)).ToList();
I stumbled across a null ref exception today in one of my Linq-To-Entitites queries and wondered how that could be possible. It seems an conditional OR used as an OR-gate has no effect in Linq-To-Entities. A simplified example of my query is this:
from a in db.Articles
where a.Author == "John Doe"
&& (tag == null || a.Tags.Any(t => t.TagName == tag.TagName))
select a;
Now when tag is NULL, the right side of the where query still gets executed and a NULL reference exception occurs on tag.TagName. Maybe this is because Linq-To-Entities always translates the complete statement into SQL?
Anyhow ... how to get around this issue?
Thanks very much :)
var query = db.Articles.Where(x => x.Author == "John Doe");
query = tag == null
? query
: query.Where(x => x.TagName == tag.TagName);
or:
var query = from a in db.Articles
where a.Author == "John Doe"
select a;
query = tag == null
? query
: from a in query
where a.Tags.Any(t => t.TagName == tag.TagName)
select a;
Personally, I find the first one cleaner
Think about sql. Linq converts your code as a whole into sql query passing 'outside' objects as parameters and evaluating them. That's why it fails on evaluation of your null object.
It's a good practice to construct a linq query bit by bit based on conditions to reduce number of unnecessary code in resulted query, so it's better to split your query:
var query = db.Articles.Where(x => x.Author == "John Doe");
if( tag != null)
query = query.Where(x => x.TagName == tag.TagName);
Because your query will be evaluated and executed on selection you are welcome to add more conditions without worrying about multiple requests.
I have a table called "test", which only has 1 column, "NullableInt" (nullable int type)
The records are: 1, 2, null
int? nullableInt = null;
var t = db.tests.Where(x => x.NullableInt == null).ToList(); // returns 1 record
var t2 = db.tests.Where(x => x.NullableInt == nullableInt).ToList(); // returns 0 records
For some reason, t2 returns 0 records, even tho it's using "nullableInt" variable, which has a value of null, just like t, which is comparing against "null"
Any help would be greatly appreciated!
Yep - it's a bug in LINQ-to-SQL / Entity Framework. IS NULL queries will only be generated if you hardcode null into the query, instead of a variable that happens to currently be null.
The second query will generate
SELECT .......
WHERE NullableInt == #someParam
WHERE #someParam is null.
Where the first will generate the appropriate IS NULL in the WHERE clause.
If you're using LINQ-to-SQL, you can log your queries to Console.Out to see for yourself, and if you're using EF, then ToTraceString() should show you the same info (or SQL Server profiler)
tl;dr
If you use DbContext in EF6 this is fixed.
If you're using EF5 (or ObjectContext in EF6) you need to set ObjectContext.ContextOptions.UseCSharpNullComparisonBehavior to true. To do that on DbContext use this:
((IObjectContextAdapter)db).ObjectContext.ContextOptions.UseCSharpNullComparisonBehavior = true;
.
More details
The root cause of this issue is a difference in how the database compares null values and how C# compares null values. Because you write your query in C# you want to use the semantics of C#.
In EF5 we introduced ObjectContext.ContextOptions.UseCSharpNullComparisonBehavior, which allowed you to opt in to using C# semantics instead of database semantics. The default is false (so that existing queries don't magically start returning different results when you upgrade to EF5). But you can set it to true and both your queries will return rows.
If you are using DbContext in EF5 you need to drop down to the ObjectContext to set it:
((IObjectContextAdapter)db).ObjectContext.ContextOptions.UseCSharpNullComparisonBehavior = true;
If you are using EF6, then it's already set to true on DbContext so you are good to go. We decided this causes so much confusion it was worth taking the potential impact on existing queries.
Queries could be built in this way:
var q = db.tests;
if(nullableInt.HasValue)
{
q = q.Where(x => x.NullableInt == nullableInt.Value);
}
else
{
q = q.Where(x => x.NullableInt == null);
}
var t2 = q.ToList();
There is another solution that will always work, albeit with a small caveat:
int? nullableInt = null;
var t2 = db.tests.Where(x => object.Equals(x.NullableInt, nullableInt)).ToList();
When the value is null you will get the proper IS NULL query, however when its not null you will get something like:
SELECT ...
WHERE ([t0].[NullableInt] IS NOT NULL) AND ([t0].[NullableInt] = #p0)
Obviously it has a condition extra (the source of which is kind of puzzling). That being said, SQL Server's query optimizer should detect that, since #p0 is a non-null value, the first condition is a superset and will cut the where clause.
Would doing:
var t2 = db.tests.Where(x => x.NullableInt == nullableInt ?? null).ToList();
Work?
It seems like utter madness though.