I have faced a strange problem. When user comes to any page of my web app
I do check if user has permissions to access it, and provide trial period if its first time to come.
Here is my piece of code:
List<string> temp_workers_id = new List<string>();
...
if (temp_workers_id.Count > 6)
{
System.Data.SqlTypes.SqlDateTime sqlDate = new System.Data.SqlTypes.SqlDateTime(DateTime.Now.Date);
var rusers = dbctx.tblMappings.Where(tm => temp_workers_id.Any(c => c == tm.ModelID));
var permissions = dbctx.UserPermissions
.Where(p => rusers
.Any(ap => ap.UserID == p.UserID)
&& p.DateStart != null
&& p.DateEnd != null
&& p.DateStart <= sqlDate.Value
&& p.DateEnd >= sqlDate.Value);
if (permissions.Count() < 1)
{
permissions = dbctx.UserPermissions
.Where(p => rusers
.Any(ap => ap.UserID == p.UserID)
&& p.DateStart == null
&& p.DateEnd == null);
var used = dbctx.UserPermissions
.Where(p => rusers
.Any(ap => ap.UserID == p.UserID)
&& p.DateStart != null
&& p.DateEnd != null);
if (permissions.Count() > 0 && used.Count() < 1)
{
var p = permissions.First();
using (Models.TTTDbContext tdbctx = new Models.TTTDbContext())
{
var tp = tdbctx.UserPermissions.SingleOrDefault(tup => tup.UserID == p.UserID);
tp.DateStart = DateTime.Now.Date;
tp.DateEnd = DateTime.Now.Date.AddDays(60);
tdbctx.SaveChanges();
}
here the First() method throws exception:
Sequence contains no elements
how that even could be?
EDIT:
I dont think that user opens two browsers and navigate here at the same time, but could be the concurrency issue?
You claim you only found this in the server logs and didn't encounter it during debugging. That means that between these lines:
if (permissions.Count() > 0)
{
var p = permissions.First();
Some other process or thread changed your database, so that the query didn't match any documents anymore.
This is caused by permissions holding a lazily evaluated resource, meaning that the query is only executed when you iterate it (which Count() and First()) do.
So in the Count(), the query is executed:
SELECT COUNT(*) ... WHERE ...
Which returns, at that moment, one row. Then the data is modified externally, causing the next query (at First()):
SELECT n1, n2, ... WHERE ...
To return zero rows, causing First() to throw.
Now for how to solve that, is up to you, and depends entirely on how you want to model this scenario. It means the second query was actually correct: at that moment, there were no more rows that fulfilled the query criteria. You could materialize the query once:
permissions = query.Where(...).ToList()
But that would mean your logic operates on stale data. The same would happen if you'd use FirstOrDefault():
var permissionToApply = permissions.FirstOrDefault();
if (permissionToApply != null)
{
// rest of your logic
}
So it's basically a lose-lose scenario. There's always the chance that you're operating on stale data, which means that the next code:
tdbctx.UserPermissions.SingleOrDefault(tup => tup.UserID == p.UserID);
Would throw as well. So every time you query the database, you'll have to write the code in such a way that it can handle the records not being present anymore.
I was originally using a foreach loop and then for each element in the loop, I perform a LINQ query like so:
foreach (MyObject identifier in identifiers.Where(i => i.IsMarkedForDeletion == false))
{
if (this.MyEntities.Identifiers.Where(pi => identifier.Field1 == pi.Field1 && identifier.Field2 == pi.Field2 && identifier.Field3 == pi.Field3).Any())
{
return false;
}
}
return true;
Then I modified it like so:
if (identifiers.Any(i => !i.IsMarkedForDeletion && this.MyEntities.Identifiers.Where(pi => i.Field1 == pi.Field1 && i.Field2 == pi.Field2 && i.Field3 == pi.Field3).Any()))
{
return false;
}
return true;
My question is this still the wrong way to use LINQ? Basically, I want to eliminate the need for the foreach loop (which seems like I should be able to get rid of it) and also make the DB query faster by not performing separate DB queries for each element of a list. Instead, I want to perform one query for all elements. Thanks!
You can change your code in this way, and it will be converted to SQL statement as expected.
To prevent runtime errors during transformation, it will be better to save DBSet to the IQueryable variable; identifiers should be IQueryable too, so you should change your code into something like this (to be honest, Resharper converted your foreach in this short labda):
IQueryable<MyObject2> identifiers = MyEntities.Identifiers.Where(i => i.IsMarkedForDeletion == false);
IQueryable<MyObject2> ids = MyEntities.Identifiers.AsQueryable();
return identifiers.All(identifier => !ids.Any(pi => identifier.Field1 == pi.Field1 && identifier.Field2 == pi.Field2 && identifier.Field3 == pi.Field3));
If identifiers is in memory collection you can change code in this way (hope that fields are string):
IQueryable<MyObject2> ids = MyEntities.Identifiers.AsQueryable();
string[] values = identifiers.Where(i => i.IsMarkedForDeletion == false).Select(i => String.Concat(i.Field1, i.Field2, i.Field3)).ToArray();
return !ids.Any(i => values.Contains(i.Field1 + i.Field2 + i.Field3));
Unfortunately your modified version will be executed exactly the same way (i.e. multiple database queries) as in the original foreach approach because EF does not support database query with joins to in memory collection (except for primitive and enumeration type collections), so if you try the most logical way
bool result = this.MyEntities.Identifiers.Any(pi => identifiers.Any(i =>
!i.IsMarkedForDeletion &&
i.Field1 == pi.Field1 && i.Field2 == pi.Field2 && i.Field3 == pi.Field3));
you'll get
NotSupportedException: Unable to create a constant value of type 'YourType'. Only primitive types or enumeration types are supported in this context.
The only way to let EF execute a single database query is to manually build a LINQ query with Concat per each item from in memory collection, like this
IQueryable<Identifier> query = null;
foreach (var item in identifiers.Where(i => !i.IsMarkedForDeletion))
{
var i = item;
var subquery = this.MyEntities.Identifiers.Where(pi =>
pi.Field1 == i.Field1 && pi.Field2 == i.Field2 && pi.Field3 == i.Field3);
query = query != null ? query.Concat(subquery) : subquery;
}
bool result = query != null && query.Any();
See Logging and Intercepting Database Operations of how to monitor the EF actions.
I would use it as follows:
if (identifiers.Where(i => !i.IsMarkedForDeletion &&
this.MyEntities.Identifiers.Field1 == i.Field1 &&
this.MyEntities.Identifiers.Field2 == i.Field2 &&
this.MyEntities.Identifiers.Field3 == i.Field3).Any()))
{
return false;
}
return true;
I hope this helps. Even though it is more to type out, it is more understandable and readable then using multiple 'where' statements.
I have the below SQL Query
;with cte as(
select a.*
from [dbo].[AccountViewModel] a
where a.COLLECTORID = 724852
and a.MONTH = 12
and a.YEAR=2015)
select *
from cte c
where c.DispCode in ('Deceased','DND','WN','WI','NC','NORESPONSE','SKIP','SHIFTED','SFU')
OR (c.DispCode in('PTP','DIB','WCE','DP') and convert(varchar(11), c.PTPDate) >=convert(varchar(11), getdate()))
OR (MONTH(c.LastPaymentDate) = 12 and YEAR(c.LastPaymentDate)=2015)
I need to convert this into an equivalent Linq query (C#).
The Cte part is working fine with the below program (I have cross checked the records)
private List<AccountViewModel> GetAllAcountsForLoggedInAgents()
{
var allAcountsForLoggedInAgents = new List<AccountViewModel>();
allAcountsForLoggedInAgents = new ViewModelDatabase()
.Accounts
.Where(a =>
a.COLLECTORID == 724852 &&
a.MONTH == DateTime.Now.Month &&
a.YEAR == DateTime.Now.Year
)
.ToList();
return allAcountsForLoggedInAgents;
}
However the part outside CTE is not working correctly (means improper records)
GetAllAcountsForLoggedInAgents()
.Where
(
a =>
("Deceased,DND,WN,WI,NC,NORESPONSE,SKIP,SHIFTED,SFU".Split(',').Any(x => x.Contains(a.DispCode)))
|| ("PTP,DIB,WCE,DP".Split(',').Any(b => b.Contains(a.DispCode)) && a.PTPDate >= DateTime.Now)
|| (a.LastPaymentDate.Value.Month == 12 && a.LastPaymentDate.Value.Year == 2015)
)
I believe that may be I am using "ANY" in a wrong way.
This condition is not the same as the IN clause
("Deceased,DND,WN,WI,NC,NORESPONSE,SKIP,SHIFTED,SFU".Split(',').Any(x => x.Contains(a.DispCode)))
because it searches a.DispCode in one of the strings. You should use equality instead:
("Deceased,DND,WN,WI,NC,NORESPONSE,SKIP,SHIFTED,SFU".Split(',').Any(x => x == a.DispCode))
This is not ideal, because Split operation is not free, so you don't want to do it as part of your query. Making a static array of strings:
static readonly string[] DispCodeFilter = new string[] {
"Deceased", "DND", "WN", "WI", "NC", "NORESPONSE", "SKIP", "SHIFTED", "SFU"
};
...
(DispCodeFilter.Any(x => x == a.DispCode))
Your In condition is incorrect. It can be fixed by adding an extension method. I am using a generic method, but you could make it type specific if you only need/want it for strings. I am using params, so you can either provide the items one by one or via a split.
public static bool In<T>(this T item, params T[] items) {
return items.Any(i=> Equals(item, i));
}
GetAllAcountsForLoggedInAgents().Where( a => a.DispCode.In
("Deceased","DND","WN","WI","NC","NORESPONSE","SKIP","SHIFTED","SFU")
|| (a.DispCode.In("PTP,DIB,WCE,DP".Split(',')) &&
a.PTPDate >= DateTime.Now)
|| (a.LastPaymentDate.Value.Month == 12 && a.LastPaymentDate.Value.Year == 2015)
)
One difference between this and the sql version, and a reason you may not want it to be generic, is that it is case sensitive: "wi" doesn't equal "WI".
Here are 2 simple rules for converting SQL to Linq
SQL Linq
============ ==========
IN (...) Contains
EXISTS (...) Any
where Contains is the corresponding Enumerable/Queryable method (not to be mixed with string.Contains).
According to this, your Linq criteria should be something like this
var DispCodes1 = new [] { "Deceased", "DND", "WN", "WI", "NC", "NORESPONSE", "SKIP", "SHIFTED", "SFU" };
var DispCodes2 = new [] { "PTP", "DIB", "WCE", "DP" };
GetAllAcountsForLoggedInAgents()
.Where
(
a =>
DispCodes1.Contains(a.DispCode)
|| (DispCodes2.Contains(a.DispCode)) && a.PTPDate >= DateTime.Now)
|| (a.LastPaymentDate.Value.Month == 12 && a.LastPaymentDate.Value.Year == 2015)
)
dasblinkenlight answer contains a good point, so you can make DispCodes1 and DispCodes2 static, but that's not essential.
Another thing to mention is that the way you did the "CTE part" is not equivalent to the SQL query, where cte is just a named subquery and the whole query executes in the database, while in your implementation the cte part is executed in the database, then gets materialized in the memory and the additional query is executed in the memory using Linq To Objects. To make it fully equivalent and let the whole query execute in the database, change the GetAllAcountsForLoggedInAgents result type to IQueryable<AccountViewModel> and remove ToList call.
I am trying to extend my linq query with additional search criteria to filter the data by sending also a List<Listitem> to the function for processing. The List can contain 1 or more items and the objective is to retreive all items which match any criteria.
Since i am sending several search criteria to the function the goal is to make a more accurate filter result the more information i am sending to the filter. If one or several criterias are empty then the filter will get less accurate results.
Exception is raised every time i execute following code, and I cant figure out how to solve the using statement to include the List<ListItem>. Appreciate all the help in advance!
Exception: Unable to create a constant value of type 'System.Web.UI.WebControls.ListItem'. Only primitive types or enumeration types are supported in this context.
using (var db = new DL.ENTS())
{
List<DL.PRODUCTS> products =
(from a in db.PRODUCTS
where (description == null || description == "" ||
a.DESCRIPTION.Contains(description)) &&
(active == null || active == "" || a.ACTIVE.Equals(active, StringComparison.CurrentCultureIgnoreCase)) &&
(mID == null || mID == "" || a.MEDIA_ID == mID) &&
(mID == null || objTypes.Any(s => s.Value == a.OBJECTS)) //Exception here!
select a).ToList<DL.PRODUCTS>();
return products;
}
Pass collection of primitive values to expression:
using (var db = new DL.ENTS())
{
var values = objTypes.Select(s => s.Value).ToArray();
List<DL.PRODUCTS> products =
(from a in db.PRODUCTS
where (description == null || description == "" || a.DESCRIPTION.Contains(description)) &&
(active == null || active == "" || a.ACTIVE.Equals(active, StringComparison.CurrentCultureIgnoreCase)) &&
(mID == null || mID == "" || a.MEDIA_ID == mID) &&
(mID == null || values.Contains(a.OBJECTS))
select a).ToList<DL.PRODUCTS>();
return products;
}
That will generate SQL IN clause.
Note - you can use lambda syntax to compose query by adding filters based on some conditions:
var products = db.PRODUCTS;
if (!String.IsNullOrEmpty(description))
products = products.Where(p => p.DESCRIPTION.Contains(description));
if (!String.IsNullOrEmpty(active))
products = products.Where(p => p.ACTIVE.Equals(active, StringComparison.CurrentCultureIgnoreCase)));
if (!String.IsNullOrEmpty(mID))
products = products.Where(p => p.MEDIA_ID == mID);
if (mID != null)
products = products.Where(p => values.Contains(p.OBJECTS));
return products.ToList();
Linq isn't able to convert the predicate on ListItem to something useful to Sql.
I would suggest that you pre-project the values of the ListItems into a simple List<string> before using this with Contains (which is converted to IN)
var listValues = objTypes.Select(_ => _.Value).ToList();
List<DL.PRODUCTS> products = ...
listValues.Contains(a.OBJECTS))
I am using Linq to filter some things and some times I need to use reflection to get the value. Here is the example:
//...
PropertyType[] properties = myType.GetProperties();
var filtered = properties.Where(p=>p.PropertyType==typeof(MyMetaData)
&& ((MyType)p.GetValue(obj)).Name=="name"
&& ((MyType)p.GetValue(obj)).Length==10
).ToList();
//...
In my example I am using GetValue() method more than one time. Is there way if I can use variable to store it? I think that will help with performance.
It looks like that to include some variable in a LINQ we have to use the expression query, not method query, like this:
var filtered = (from x in properties
let a = (x.PropertyType is MyType) ? (MyType) x.GetValue(obj) : null
where a != null && a.Name == "name" && a.Length == 10).ToList();
I think this also works for method query with some Select:
var filtered = properties.Select(p=> new {p, a = (p.PropertyType is MyType) ? (MyType) p.GetValue(obj) : null})
.Where(x=>x.a != null && x.a.Name == "name" && x.a.Length == 10)
.Select(x=>x.p).ToList();
Something like following should work with method expression(lambda syntax)
var filtered = properties.Where(p =>
{
if(!p.PropertyType is MyMetaData)
{
return false;
}
var item = (MyType) p.GetValue(obj);
return item.Name == "name"
&& item.Length == 10
}
).ToList();