in this example code
public Company GetCompanyById(Decimal company_id)
{
IQueryable<Company> cmps = from c in db.Companies
where c.active == true &&
c.company_id == company_id
select c;
return cmps.First();
}
How should I handle if there is no data in cmps?
cmps will never be null, so how can I check for non existing data in a LINQ Query?
so I can avoid this
'cmps.ToList()' threw an exception of type ... {System.NullReferenceException}
when transforming it into, for example, a List
GetCompanyById(1).ToList();
Do I always need to wrap it up in a try catch block?
You can use Queryable.Any() (or Enumerable.Any()) to see if there is a member in cmps. This would let you do explicit checking, and handle it however you wish.
If your goal is to just return null if there are no matches, just use FirstOrDefault instead of First in your return statement:
return cmps.FirstOrDefault();
What about applying .Any or .Count() ?
Here's an example on MSDN
List<int> numbers = new List<int> { 1, 2 };
bool hasElements = numbers.Any();
Console.WriteLine("The list {0} empty.",
hasElements ? "is not" : "is");
Or just use the ?: operator
return myExample.Any() ? myExample.First() : null;
This will return the first one if there is one, or null if there isn't:
return (from c in db.Companies
where c.active == true &&
c.company_id == company_id
select c).FirstOrDefault();
Try return cmps.Count()==0?null:cmp.First()
That way if it is null it will simply return a null Company and if its not then it will return the first one in the list.
Check out http://en.wikipedia.org/wiki/Ternary_operation
var context = new AdventureWorksLT2008Entities();
var cust = context.Customers.Where(c => c.CustomerID == 1);
if (cust.Any())
{
Customer c = cust.First();
}
Related
For the following query:
var result = from sch in schemeDashboard
join exp in Expenditure on sch.schemeId equals exp.SchemeCode
into SchExpGroup
where sch.SectorDepartmentId == selectedDepartmentId &&
sch.YearCode == StateManager.CurrentYear
orderby sch.ADPId
select new
{
ModifiedAmounts = SchExpGroup.Select(a => a.ModifiedAmounts),
ProjectName = sch.schemeName,
ADPNo = sch.ADPId,
Allocation = sch.CurrentAllocation,
Expenditures = from expend in SchExpGroup
where expend.YearCode == StateManager.CurrentYear &&
expend.DepartmentId == selectedDepartmentId &&
InvStatus.Contains(expend.Status)
orderby expend.ADPId
group expend by expend.InvoiceId
};
I want to filter the above query on a condition so that result gives only those records where "ModifiedAmounts" are not null. I have tried as follow:
if (rbList2.SelectedIndex == 6)
{
result = result.Where(a => a.ModifiedAmounts != null));
}
but this gives error as:
Cannot compare elements of type
'System.Collections.Generic.IEnumerable`1'. Only primitive types,
enumeration types and entity types are supported.
Any suggestions as I am lost as how to rephrase the filtered query.
I think the problem is that ModifiedAmounts will never be null. Select will return an empty list. Unless SchExpGroup is null in which case you will get a null reference exception.
Try changing your code to
result = result.Where(a => a.ModifiedAmounts.Any());
if (rbList2.SelectedIndex == 6)
{
result = result.Where(a => a.!ModifiedAmounts.Any());
}
I have an array filled with long type values and for each value in the array I need to implement a query. I used foreach loop as you can see from the code below:
var result;
foreach(long id in PrdIdArr)
{
var mainQuery = (from o in db.OPERATIONs
join u in db.UNITs on o.OP_UNIT_ID equals u.UNIT_ID into smt
from s in smt
join x in db.XIDs on s.UNIT_ID equals x.UNIT_ID
where o.OP_OT_CODE == OtCode
where x.IDTYP_CD == "BSN"
where s.START_PRD_ID == id
where o.OP_UPD_DATE >= _StartDate
where o.OP_UPD_DATE <= _EndDate
select new
{
o.OP_ID,
o.OP_UPD_DATE,
x.EXTERNAL_ID,
o.OP_OS_CODE,
o.OP_START,
o.OP_ST_STATION,
s.START_PRD_ID
}).Take(_RowNumber);
//var result = mainQuery.ToList();
result.add(mainQuery.ToList());
}
data = this.Json(result);
data.JsonRequestBehavior = JsonRequestBehavior.AllowGet;
return data;
However, I have a problem in my code; I have to define a main list just before the foreach loop so that I could add results of each query to the that main list. my question is: How can I define this list as you can see at the beginning of my code? Thanks for the help...
How can I define this list as you can see at the beginning of my code?
Make
new {
o.OP_ID,
o.OP_UPD_DATE,
x.EXTERNAL_ID,
o.OP_OS_CODE,
o.OP_START,
o.OP_ST_STATION,
s.START_PRD_ID
}
into a concrete type (say QueryResult, although something a little more specific than that), and then just declare
var result = new List<QueryResult>();
Also, you should consider turning
foreach(long id in PrdIdArr)
and
where s.START_PRD_ID == id
into
where PrdIdArr.Contains(s.Start_PRD_ID)
var result = new List<object>();
foreach(long id in PrdIdArr)
{
....
result.Add(mainQuery.ToList());
}
You could do this:
var result = PrdIdArr.Select(id =>
from o in db.OPERATIONs
join u in db.UNITs on o.OP_UNIT_ID equals u.UNIT_ID into smt
from s in smt
join x in db.XIDs on s.UNIT_ID equals x.UNIT_ID
where o.OP_OT_CODE == OtCode
where x.IDTYP_CD == "BSN"
where s.START_PRD_ID == id
where o.OP_UPD_DATE >= _StartDate
where o.OP_UPD_DATE <= _EndDate
select new
{
o.OP_ID,
o.OP_UPD_DATE,
x.EXTERNAL_ID,
o.OP_OS_CODE,
o.OP_START,
o.OP_ST_STATION,
s.START_PRD_ID
}
.Take(_RowNumber)
.ToList()
).ToList();
I highly recommend performing some Extract Method refactorings, as the code is pretty complex and hard to understand/mange this way.
Just create the anonymous type outside with the same property names and the correct type
var result = Enumerable.Range(0, 0).Select(x => new
{
OP_ID = 1,
OP_UPD_DATE = DateTime.Now,
EXTERNAL_ID = 1,
OP_OS_CODE = 1,
OP_START = DateTIme.Now,
OP_ST_STATION = "",
START_PRD_ID = 1,
}).ToList();
And in your loop call AddRange
result.AddRange(mainQuery.ToList());
With Linq, can I use a conditional statement inside of a Where extension method?
var query = someList.Where(a => (someCondition)? a == "something" : true);
so, if 'someCondition' is false, 'Where' will be skipped.
Yes you can like:
var query = someList.Where(a => a == "something");
if (condition)
{
query = query.Where(b => b == "something else");
}
var result = query.ToList();
Because Where is producing an IQueryable, the execution is deferred until the ToList in my example so you can chain Wheres together as much as you want and then just execute it after you have passed all your conditions.
Make use of WhereIf extenstion method avaialbe in linq
Example
if (SearchControlMain.PostingID.HasValue)
query = query.Where(q => q.PostingID == SearchControlMain.PostingID);
instead of above go for the below
query = query.WhereIf(SearchControlMain.CategoryID.HasValue, q => q.CategoryID == SearchControlMain.CategoryID);
LINQ WhereIf Extension Method
LINQ to SQL Where Clause Optional Criteria
Not sure if this is appropriate but it is quite useful, you can use ifs quite handily with conditional where clauses:
var r = (from p in productinfo.tblproduct
where p.Accountid == accountid
select p);
if (uuf1 != null)
r = r.Where(p => p.UnitUserField1 == uuf1);
if (uuf2!= null)
r = r.Where(p => p.UnitUserField2 == uuf2);
So the where clause will be amended according to what is in UUF1 or UUF2 i.e. you might have only UUF1 with info, in which case it will take that and ignore the UUF2 where clause, you might have both in which it will take both or you might not have anything in UUF1 or 2 and your where clause will just take the accountid as the where clause.
In my case there were two "conditional" where depending on search keys, so I did:
var query = db.Package.Include("SomeThing")
.Where(item => searchString1 == null || searchString1 == "" || item.Contains(searchString1))
.Where(item => searchString2 == null || searchString2 == "" || item.Contains(searchString2));
...
from item in items
where condition1
&& (condition2 ? true : condition3)
select item
This is how can you can do it with the noob Linq syntax.
This applies the condition3 only if condition2 is false.
If condition2 is true, you are essentially doing && true which has no effect on the where clause.
So it is essentially doing this:
if(condition2)
{
from item in items
where condition1
select item
else
{
from item in items
where condition1
&& condition3
select item
}
I had a scenario like this where I had to check for null within the list itself. This is what I did.
items = from p in items
where p.property1 != null //Add other if conditions
select p;
// Use items the way you would use inside the if condition
But as Kelsey pointed out this would work too -
items = items.Where(a => a.property1 != null);
I'm not sure what the question is, but a possible answer could be:
Yes,
list.Where(item => { if (Foo(item)) return true; else return false; });
It would be a complicated way of saying something simple, though.
In my case, I wanted to keep the elements which met my criteria and log the ones that didn't without iterating multiple times.
var merchantsWithLocations = allMerchants.Where(m =>
{
if (m.Locations?.Any() != true)
{
_logger.Log("Merchant {merchantId} has no locations", m.Id);
return false;
}
return true;
};
Any time you want to do a side-effect per element (such as logging), breaking out the lambda into a statement body makes it easy to reason about.
tblUserRole Permission =
(oCurrenUserPermission.GetPermission(Convert.ToString(Session["Navigation"])));
if (Permission.IsInsert == 1)
{
}
public IQueryable GetPermission(string sPageName)
{
IQueryable query;
#region MyRegion
query = from r in this.Context.tblUserRoles
join p in this.Context.tblPageInfos on r.PageID equals p.PageID
where r.Record_Status == 2 && p.PageName == sPageName
select r;
return query;
#endregion
}
Above syntax show the bellow error:
Error 1 Cannot implicitly convert type 'System.Linq.IQueryable' to 'AkijBeverage.ServiceObject.tblUserRole'. An explicit conversion exists (are you missing a cast?) E:\Project-Akij\09-July-2010\AkijBeverage\AkijBeverage\SecurityUserControls\UCUserRole.ascx.cs 63 43 AkijBeverage
How to solve this ?
As far as I can tell, that query is just not compatible with tblUserRole.
It's going to return a IEnumereable<bool> instead of a tblUserRole.
The real problem is that you are selecting the IsInsert in two different spots: In the query, and again in the if()
The easiest way would be just return the full tblUserRole object:
return (from r in this.Context.tblUserRoles
join p in this.Context.tblPageInfos on r.PageID equals p.PageID
where r.Record_Status == 2 && p.PageName == sPageName
select r).SingleOrDefault();
Currently you're returning a query - not a single item. What do you want to return?
Here's one option, for example:
public tblUserRole GetPermission(string sPageName)
{
return (from r in this.Context.tblUserRoles
join p in this.Context.tblPageInfos on r.PageID equals p.PageID
where r.Record_Status == 2 && p.PageName == sPageName
select r).FirstOrDefault();
}
This will return the first matching role, or null if there aren't any.
If you do want to return a query, it would be better as a strongly typed IQueryable<T>:
public IQueryable<tblUserRole> GetPermissions(string sPageName)
{
return from r in this.Context.tblUserRoles
join p in this.Context.tblPageInfos on r.PageID equals p.PageID
where r.Record_Status == 2 && p.PageName == sPageName
select r;
}
Then you'd need to change your calling code to something like this:
tblUserRole Permission
oCurrenUserPermission.GetPermissions(Convert.ToString(Session["Navigation"]))
.FirstOrDefault();
The problem is you have an IQueryable and are attempting to assign to a "Type" without a cast.
Either change your method to:
public IQueryable<tblUserRole > GetPermission(string sPageName)
and then do (First/FirstOrDefault/Single/etc..)
oCurrenUserPermission.GetPermission(...).FirstOrDefault();
Or cast it and cross your fingers and select one.
(tblUserRole)oCurrenUserPermission.GetPermission(...).FirstOrDefault();
Side Note
I would suggest you work on naming your code in a more friendly maner. No need to prepend the "type" at the beginning. For example a possible better way to name everything:
string navigationValue = Convert.ToString(Session["Navigation"]);
UserRole permission =
currentUserPermission.GetPermission(navigationValue ).FirstOrDefault();
//If this isn't a boolean i would consider using one
if (permission.IsInsert == 1)
{
}
public IQueryable<UserRole> GetPermission(string sPageName)
{
#region MyRegion
var query = from r in this.Context.UserRoles
join p in this.Context.PageInformation on r.PageID equals p.PageID
where r.Record_Status == 2 && p.PageName == sPageName
select r;
return query;
#endregion
}
Assuming that we have the following table:
Person:
PersonID,
Name,
Age,
Gender
And we are providing a search function that allows users to search the table according to the name and/or the age.
The tricky part in writing the SQL ( or LINQ) query is that the users can choose to search for both field, or any one field, or no field. If he wants to search for all then he would just have to leave the textbox blank.
The logic to do this can be written as follows:
var p;
if(Name_TextBox=='')
{
p=from row in person
select row ;
}
else
{
p= from row in person
where row.Name=Name_TextBox
select row ;
}
// repeat the same for age
Now after a while the code gets very long and messy... How can I compress the above into a single query with no if-else?
Try code like this
string personName = txtPersonName.Text;
int personAge = Convert.ToInt32(txtAge.Text);
var opportunites = from p in this.DataContext.Persons
select new
{
p.PersonID,
p.Name,
p.Age,
p.Gender
};
if (personsID != 0)
opportunites = opportunites.Where(p => p.PersonID == personID);
if (personName != string.Empty)
opportunites = opportunites.Where(p => p.Name.StartsWith(personName));
if (personAge != 0)
opportunites = opportunites.Where(p => p.Age == personAge);
This will work fine. If personName is not given it will be not add to where, and if given then it will added.
One alternative which I have used in SQL which could be implemented in Linq too is
var p = from p in Person
where p.Name == Name_TextBox || Name_TextBox == String.Empty
select p;
(Note that your 'linq' is using SQL syntax, which won't compile. Also you can't declare a var as you are doing without directly assigning a value)
why not use the null coalescing operator? eg.
var products = from a in context.products
where a.ID == (productID ?? a.ID)
select a;
This works really well on my system