I have the following LINQ-to-Entities query
from r in ctx.Rs
join p in ctx.Ps on r.RK equals p.RK
group r by r.QK into gr
select new { QK = (int)gr.Key, Num = gr.Count() }
that runs against this schema
Table P Table R Table Q
PK*
RK ----> RK*
Text QK ------> QK*
Text Text
and gives this message if there is any record in Q with no corresponding record in P: "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."
The problem is the gr.Count() in the last line, but I cannot find a solution. I have tried to test gr for null, but cannot find a way that works.
I have seen a number of solutions to a similar problem using Sum() instead of Count(), but I have not been able to adapt them to my problem.
I tried changing my query to look like the one in Group and Count in Linq issue, but I just got a different message.
I also looked at Group and Count in Entity Framework (and a number of others) but the problem is different.
group Key can't be null
var results = ctx.Rs.Where(r => r.QK != null)
.GroupBy(r => r.QK)
.Select(gr => new { Key = (int)gr.Key, Count = gr.Count() }
.ToList();
PS.
Mostly, You don't need 'JOIN' syntax in Entity Framework. see: Loading Related Entities
Writing descriptive-meaningful variable names would significantly improve Your codes and make it understandable. Readability does matter in real world production.
I'm having trouble reading your format. But can you try:
from r in ctx.Rs
join p in ctx.Ps.DefaultIfEmpty() on r.RK equals p.RK
group r by r.QK into gr
select new { QK = (int)gr.Key, Num = gr.Count(x => x.RK != null) }
With DefaultIfEmpty and x => x.RK != null being the changes.
I need to add a literal value to a query. My attempt
var aa = new List<long>();
aa.Add(0);
var a = Products.Select(p => p.sku).Distinct().Union(aa);
a.ToList().Dump(); // LinqPad's way of showing the values
In the above example, I get an error:
"Local sequence cannot be used in LINQ to SQL implementation
of query operators except the Contains() operator."
If I am using Entity Framework 4 for example, what could I add to the Union statement to always include the "seed" ID?
I am trying to produce SQL code like the following:
select distinct ID
from product
union
select 0 as ID
So later I can join the list to itself so I can find all values where the next highest value is not present (finding the lowest available ID in the set).
Edit: Original Linq Query to find lowest available ID
var skuQuery = Context.Products
.Where(p => p.sku > skuSeedStart &&
p.sku < skuSeedEnd)
.Select(p => p.sku).Distinct();
var lowestSkuAvailableList =
(from p1 in skuQuery
from p2 in skuQuery.Where(a => a == p1 + 1).DefaultIfEmpty()
where p2 == 0 // zero is default for long where it would be null
select p1).ToList();
var Answer = (lowestSkuAvailableList.Count == 0
? skuSeedStart :
lowestSkuAvailableList.Min()) + 1;
This code creates two SKU sets offset by one, then selects the SKU where the next highest doesn't exist. Afterward, it selects the minimum of that (lowest SKU where next highest is available).
For this to work, the seed must be in the set joined together.
Your problem is that your query is being turned entirely into a LINQ-to-SQL query, when what you need is a LINQ-to-SQL query with local manipulation on top of it.
The solution is to tell the compiler that you want to use LINQ-to-Objects after processing the query (in other words, change the extension method resolution to look at IEnumerable<T>, not IQueryable<T>). The easiest way to do this is to tack AsEnumerable() onto the end of your query, like so:
var aa = new List<long>();
aa.Add(0);
var a = Products.Select(p => p.sku).Distinct().AsEnumerable().Union(aa);
a.ToList().Dump(); // LinqPad's way of showing the values
Up front: not answering exactly the question you asked, but solving your problem in a different way.
How about this:
var a = Products.Select(p => p.sku).Distinct().ToList();
a.Add(0);
a.Dump(); // LinqPad's way of showing the values
You should create database table for storing constant values and pass query from this table to Union operator.
For example, let's imagine table "Defaults" with fields "Name" and "Value" with only one record ("SKU", 0).
Then you can rewrite your expression like this:
var zero = context.Defaults.Where(_=>_.Name == "SKU").Select(_=>_.Value);
var result = context.Products.Select(p => p.sku).Distinct().Union(zero).ToList();
I'm currently using a single query in two places to get a row from a database.
BlogPost post = (from p in dc.BlogPosts
where p.BlogPostID == ID
select p).Single();
The query is fine when retrieving the row to put data in to the text boxes, but it returns an error "Sequence contains no elements" when used to retrieve the row in order to edit it and put it back in to the database. I can't understand why it might find an appropriate row in one instance but not another.
(Using ASP.NET MVC and LINQ)
From "Fixing LINQ Error: Sequence contains no elements":
When you get the LINQ error "Sequence contains no elements", this is usually because you are using the First() or Single() command rather than FirstOrDefault() and SingleOrDefault().
This can also be caused by the following commands:
FirstAsync()
SingleAsync()
Last()
LastAsync()
Max()
Min()
Average()
Aggregate()
Please use
.FirstOrDefault()
because if in the first row of the result there is no info this instruction goes to the default info.
Well, what is ID here? In particular, is it a local variable? There are some scope / capture issues, which mean that it may be desirable to use a second variable copy, just for the query:
var id = ID;
BlogPost post = (from p in dc.BlogPosts
where p.BlogPostID == id
select p).Single();
Also; if this is LINQ-to-SQL, then in the current version you get a slightly better behaviour if you use the form:
var id = ID;
BlogPost post = dc.BlogPosts.Single(p => p.BlogPostID == id);
In addition to everything else that has been said, you can call DefaultIfEmpty() before you call Single(). This will ensure that your sequence contains something and thereby averts the InvalidOperationException "Sequence contains no elements". For example:
BlogPost post = (from p in dc.BlogPosts
where p.BlogPostID == ID
select p).DefaultIfEmpty().Single();
This will solve the problem,
var blogPosts = (from p in dc.BlogPosts
where p.BlogPostID == ID
select p);
if(blogPosts.Any())
{
var post = blogPosts.Single();
}
I had a similar situation on a function that calculates the average.
Example:
ws.Cells[lastRow, startingmonths].Value = lstMediaValues.Average();
Case Solved:
ws.Cells[lastRow, startingmonths].Value = lstMediaValues.Count == 0 ? 0 : lstMediaValues.Average();
Reason for error:
The query from p in dc.BlogPosts where p.BlogPostID == ID select p returns a sequence.
Single() tries to retrieve an element from the sequence returned in step1.
As per the exception - The sequence returned in step1 contains no elements.
Single() tries to retrieve an element from the sequence returned in step1 which contains no elements.
Since Single() is not able to fetch a single element from the sequence returned in step1, it throws an error.
Fix:
Make sure the query (from p in dc.BlogPosts where p.BlogPostID == ID select p)
returns a sequence with at least one element.
I've got a LINQ query going against an Entity Framework object. Here's a summary of the query:
//a list of my allies
List<int> allianceMembers = new List<int>() { 1,5,10 };
//query for fleets in my area, including any allies (and mark them as such)
var fleets = from af in FleetSource
select new Fleet
{
fleetID = af.fleetID,
fleetName = af.fleetName,
isAllied = (allianceMembers.Contains(af.userID) ? true : false)
};
Basically, what I'm doing is getting a set of fleets. The allianceMembers list contains INTs of all users who are allied with me. I want to set isAllied = true if the fleet's owner is part of that list, and false otherwise.
When I do this, I am seeing an exception: "LINQ to Entities does not recognize the method 'Boolean Contains(Int32)' method"
I can understand getting this error if I had used the contains in the where portion of the query, but why would I get it in the select? By this point I would assume the query would have executed and returned the results. This little ditty of code does nothing to constrain my data at all.
Any tips on how else I can accomplish what I need to with setting the isAllied flag?
Thanks
This poached from a previous answer...
Contains not supported.
IN and JOIN are not the same operator (Filtering by IN never changes the cardinality of the query).
Instead of doing it that way use the join method. It's somewhat difficult to understand without using the query operators, but once you get it, you've got it.
var foo =
model.entitySet.Join( //Start the join
values, //Join to the list of strings
e => e.Name, // on entity.Name
value => value, //equal to the string
(ModelItem ent, String str) => ent);//select the entity
Here it is using the query operators
var foo = from e in model.entitySet
join val in values on
e.Name equals val
select e;
Basically the entity framework attempts to translate your LINQ query into a SQL statement but doesn't know how to handle the Contains.
What you can do instead is retrieve your fleets from the database and set the isAllied property later:
var fleets = (from af in FleetSource
select new Fleet
{
fleetID = af.fleetID,
fleetName = af.fleetName,
userId = af.userId
}).AsEnumerable();
foreach (var fleet in fleets)
{
fleet.isAllied = (allianceMembers.Contains(fleet.userID) ? true : false);
}
Everyone above me is wrong!!! (No offense ...) It doesn't work because you are using the IList overload of "Contains" and not the IEnumerable overload of "Contains". Simply change to:
allianceMembers.Contains<int>(af.userID)
By adding the <int>, you are telling the compiler to use the IEnumerable overload instead of the IList overload.
var fleets = from af in FleetSource;
var x = from u in fleets.ToList()
select new Fleet
{
fleetID = u.fleetID,
fleetName = u.fleetName,
isAllied = (allianceMembers.Contains(u.userID) ? true : false)
}
calling ToList() on fleets the query is executed, later you can use Contains().
What is the best way to get the Max value from a LINQ query that may return no rows? If I just do
Dim x = (From y In context.MyTable _
Where y.MyField = value _
Select y.MyCounter).Max
I get an error when the query returns no rows. I could do
Dim x = (From y In context.MyTable _
Where y.MyField = value _
Select y.MyCounter _
Order By MyCounter Descending).FirstOrDefault
but that feels a little obtuse for such a simple request. Am I missing a better way to do it?
UPDATE: Here's the back story: I'm trying to retrieve the next eligibility counter from a child table (legacy system, don't get me started...). The first eligibility row for each patient is always 1, the second is 2, etc. (obviously this is not the primary key of the child table). So, I'm selecting the max existing counter value for a patient, and then adding 1 to it to create a new row. When there are no existing child values, I need the query to return 0 (so adding 1 will give me a counter value of 1). Note that I don't want to rely on the raw count of child rows, in case the legacy app introduces gaps in the counter values (possible). My bad for trying to make the question too generic.
Since DefaultIfEmpty isn't implemented in LINQ to SQL, I did a search on the error it returned and found a fascinating article that deals with null sets in aggregate functions. To summarize what I found, you can get around this limitation by casting to a nullable within your select. My VB is a little rusty, but I think it'd go something like this:
Dim x = (From y In context.MyTable _
Where y.MyField = value _
Select CType(y.MyCounter, Integer?)).Max
Or in C#:
var x = (from y in context.MyTable
where y.MyField == value
select (int?)y.MyCounter).Max();
I just had a similar problem, but I was using LINQ extension methods on a list rather than query syntax. The casting to a Nullable trick works there as well:
int max = list.Max(i => (int?)i.MyCounter) ?? 0;
Sounds like a case for DefaultIfEmpty (untested code follows):
Dim x = (From y In context.MyTable _
Where y.MyField = value _
Select y.MyCounter).DefaultIfEmpty.Max
Think about what you're asking!
The max of {1, 2, 3, -1, -2, -3} is obviously 3. The max of {2} is obviously 2. But what is the max of the empty set { }? Obviously that is a meaningless question. The max of the empty set is simply not defined. Attempting to get an answer is a mathematical error. The max of any set must itself be an element in that set. The empty set has no elements, so claiming that some particular number is the max of that set without being in that set is a mathematical contradiction.
Just as it is correct behavior for the computer to throw an exception when the programmer asks it to divide by zero, so it is correct behavior for the computer to throw an exception when the programmer asks it to take the max of the empty set. Division by zero, taking the max of the empty set, wiggering the spacklerorke, and riding the flying unicorn to Neverland are all meaningless, impossible, undefined.
Now, what is it that you actually want to do?
You could always add Double.MinValue to the sequence. This would ensure that there is at least one element and Max would return it only if it is actually the minimum. To determine which option is more efficient (Concat, FirstOrDefault or Take(1)), you should perform adequate benchmarking.
double x = context.MyTable
.Where(y => y.MyField == value)
.Select(y => y.MyCounter)
.Concat(new double[]{Double.MinValue})
.Max();
int max = list.Any() ? list.Max(i => i.MyCounter) : 0;
If the list has any elements (ie. not empty), it will take the max of the MyCounter field, else will return 0.
Since .Net 3.5 you can use DefaultIfEmpty() passing the default value as an argument. Something like one of the followings ways:
int max = (from e in context.Table where e.Year == year select e.RecordNumber).DefaultIfEmpty(0).Max();
DateTime maxDate = (from e in context.Table where e.Year == year select e.StartDate ?? DateTime.MinValue).DefaultIfEmpty(DateTime.MinValue).Max();
The first one is allowed when you query a NOT NULL column and the second one is the way a used it to query a NULLABLE column. If you use DefaultIfEmpty() without arguments the default value will be that defined to the type of your output, as you can see in the Default Values Table .
The resulting SELECT will not be so elegant but it's acceptable.
Hope it helps.
I think the issue is what do you want to happen when the query has no results. If this is an exceptional case then I would wrap the query in a try/catch block and handle the exception that the standard query generates. If it's ok to have the query return no results, then you need to figure out what you want the result to be in that case. It may be that #David's answer (or something similar will work). That is, if the MAX will always be positive, then it may be enough to insert a known "bad" value into the list that will only be selected if there are no results. Generally, I would expect a query that is retrieving a maximum to have some data to work on and I would go the try/catch route as otherwise you are always forced to check if the value you obtained is correct or not. I'd rather that the non-exceptional case was just able to use the obtained value.
Try
Dim x = (From y In context.MyTable _
Where y.MyField = value _
Select y.MyCounter).Max
... continue working with x ...
Catch ex As SqlException
... do error processing ...
End Try
Another possibility would be grouping, similar to how you might approach it in raw SQL:
from y in context.MyTable
group y.MyCounter by y.MyField into GrpByMyField
where GrpByMyField.Key == value
select GrpByMyField.Max()
The only thing is (testing again in LINQPad) switching to the VB LINQ flavor gives syntax errors on the grouping clause. I'm sure the conceptual equivalent is easy enough to find, I just don't know how to reflect it in VB.
The generated SQL would be something along the lines of:
SELECT [t1].[MaxValue]
FROM (
SELECT MAX([t0].[MyCounter) AS [MaxValue], [t0].[MyField]
FROM [MyTable] AS [t0]
GROUP BY [t0].[MyField]
) AS [t1]
WHERE [t1].[MyField] = #p0
The nested SELECT looks icky, like the query execution would retrieve all rows then select the matching one from the retrieved set... the question is whether or not SQL Server optimizes the query into something comparable to applying the where clause to the inner SELECT. I'm looking into that now...
I'm not well-versed in interpreting execution plans in SQL Server, but it looks like when the WHERE clause is on the outer SELECT, the number of actual rows resulting in that step is all rows in the table, versus only the matching rows when the WHERE clause is on the inner SELECT. That said, it looks like only 1% cost is shifted to the following step when all rows are considered, and either way only one row ever comes back from the SQL Server so maybe it's not that big of a difference in the grand scheme of things.
litt late, but I had the same concern...
Rephrasing your code from the original post, you want the max of the set S defined by
(From y In context.MyTable _
Where y.MyField = value _
Select y.MyCounter)
Taking in account your last comment
Suffice to say that I know I want 0
when there are no records to select
from, which definitely has an impact
on the eventual solution
I can rephrase your problem as: You want the max of {0 + S}.
And it looks like the proposed solution with concat is semantically the right one :-)
var max = new[]{0}
.Concat((From y In context.MyTable _
Where y.MyField = value _
Select y.MyCounter))
.Max();
Why Not something more direct like:
Dim x = context.MyTable.Max(Function(DataItem) DataItem.MyField = Value)
One interesting difference that seems worth noting is that while FirstOrDefault and Take(1) generate the same SQL (according to LINQPad, anyway), FirstOrDefault returns a value--the default--when there are no matching rows and Take(1) returns no results... at least in LINQPad.
Just to let everyone out there know that is using Linq to Entities the methods above will not work...
If you try to do something like
var max = new[]{0}
.Concat((From y In context.MyTable _
Where y.MyField = value _
Select y.MyCounter))
.Max();
It will throw an exception:
System.NotSupportedException: The LINQ expression node type 'NewArrayInit' is not supported in LINQ to Entities..
I would suggest just doing
(From y In context.MyTable _
Where y.MyField = value _
Select y.MyCounter))
.OrderByDescending(x=>x).FirstOrDefault());
And the FirstOrDefault will return 0 if your list is empty.
decimal Max = (decimal?)(context.MyTable.Select(e => e.MyCounter).Max()) ?? 0;
For Entity Framework and Linq to SQL we can achieve this by defining an extension method which modifies an Expression passed to IQueryable<T>.Max(...) method:
static class Extensions
{
public static TResult MaxOrDefault<T, TResult>(this IQueryable<T> source,
Expression<Func<T, TResult>> selector)
where TResult : struct
{
UnaryExpression castedBody = Expression.Convert(selector.Body, typeof(TResult?));
Expression<Func<T, TResult?>> lambda = Expression.Lambda<Func<T,TResult?>>(castedBody, selector.Parameters);
return source.Max(lambda) ?? default(TResult);
}
}
Usage:
int maxId = dbContextInstance.Employees.MaxOrDefault(employee => employee.Id);
// maxId is equal to 0 if there is no records in Employees table
The generated query is identical, it works just like a normal call to IQueryable<T>.Max(...) method, but if there is no records it returns a default value of type T instead of throwing an exeption
I've knocked up a MaxOrDefault extension method. There's not much to it but its presence in Intellisense is a useful reminder that Max on an empty sequence will cause an exception. Additionally, the method allows the default to be specified if required.
public static TResult MaxOrDefault<TSource, TResult>(this
IQueryable<TSource> source, Expression<Func<TSource, TResult?>> selector,
TResult defaultValue = default (TResult)) where TResult : struct
{
return source.Max(selector) ?? defaultValue;
}
I just had a similar problem, my unit tests passed using Max() but failed when run against a live database.
My solution was to separate the query from the logic being performed, not join them in one query.
I needed a solution to work in unit tests using Linq-objects (in Linq-objects Max() works with nulls) and Linq-sql when executing in a live environment.
(I mock the Select() in my tests)
var requiredDataQuery = _dataRepo.Select(x => new { x.NullableDate1, .NullableDate2 });
var requiredData.ToList();
var maxDate1 = dates.Max(x => x.NullableDate1);
var maxDate2 = dates.Max(x => x.NullableDate2);
Less efficient? Probably.
Do I care, as long as my app doesn't fall over next time? Nope.