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
Related
I have a search page where you can enter 4 search criteria. User can enter all 4 or less and when I do the search I have to look for all 4 if they are entered and if not take all for the value not entered. Example: my search parameters are FirstName, LastName, City and Zip. Now if someone enters John and Zip 01803 I need to ignore other 2 and look for all Johns in that zip code. Can I do that in one query somehow? I know I can do that if I write multiple ones to check which values are null but that looks not so tedious. I want to know if I can use 'Any' or 'All' in LINQ to do that?
if (fname == null) fname = "";
if (lname == null) lname = "";
if (city == null) city = "";
var results = from c in context.Company.Where(d => d.CompanyCode == identifikator)
join p in context.Person.Where(d => (d.FirstName.Any(??) || d.FirstName.Contains(fname)) && d.LastName.Contains(lname) && d.City.Contains(city) && d.ZIP==zip) on c.CompanyId equals p.Identifikation
join ts in context.TSSession on p.PersonId equals ts.PersonId
select new
{
firstname = p.FirstName,
lastname = p.LastName,
mail = p.EMail,
order = tp.Order,
timestamp = ts.TimeStamp,
personid = p.PersonId
};
You can build your where expression like this:
var personsSubQuery = context.Persons;
if(fname != null)
personsSubQuery = personsSubQuery.Where(x=>x.FirstName.Contains(fname));
if(lname != null)
personsSubQuery = personsSubQuery.Where(x=>x.LastName.Contains(lname));
and so on.
then in your query just use personsSubQuery in place of your context.Persons.Where.
I don't see Any or All methods to be usable in this scenario.
I have a linq query like this :
var trfplanList = (from at in entities.tdp_ProviderAccomodationType
join ap in entities.tdp_ProviderAccomodationTariffPlan on at.PATID equals ap.FK_PATID
join ac in entities.tdp_ProviderAccomodationCategory on ap.FK_PACID equals ac.PACID
where at.FK_ProviderID == CityID && at.IsApproved == 0 && ap.IsApproved == 0 && ac.AccomodationCategory == "Double Occupy"
orderby at.AccomodationType,ap.FromDate,ap.SType
select new AccomodationTariff
{
AccomodationType = at.AccomodationType,
SType = ap.SType,
FromDate = Convert.ToDateTime(ap.FromDate),
ToDate = Convert.ToDateTime(ap.ToDate),
RoomTariff = Convert.ToDecimal(ap.Rate),
ExPAXRate = Convert.ToDecimal(at.PerPaxRate)
}).ToList();
I have two questions:
Can't I convert value while assigning in the select new {} block ? it is giving me an error in project.
I want use 'case' while selecting ExPAXRate from the database for example in SQL I used to write :
CASE ap.SType WHEN 'Off Season' THEN at.PerPaxRateOS ELSE at.PerPaxRate END AS ExPAXRate
Can I use something like this in linq query ?
Can't I convert value while assigning in the select new {} block
No, you can't (sadly). EF doesn't know how to translate it into SQL.
I want use 'case'
You can use the ternary operator (?):
ExPAXRate = at.OffSeason ? at.PerPaxRateOS : at.PerPaxRate
(assuming that at.OffSeason exists).
A solution for the conversion issue could be to project into an anonymous type first and then, in memory, to AccomodationTariff:
...
select new
{
AccomodationType = at.AccomodationType,
SType = ap.SType,
FromDate = ap.FromDate,
ToDate = ap.ToDate,
RoomTariff = ap.Rate,
ExPAXRate = at.PerPaxRate
}).AsEnumerable()
.Select(x => new AccomodationTariff
{
AccomodationType = x.AccomodationType,
SType = x.SType,
FromDate = Convert.ToDateTime(x.FromDate),
ToDate = Convert.ToDateTime(x.ToDate),
RoomTariff = Convert.ToDecimal(x.Rate),
ExPAXRate = Convert.ToDecimal(x.PerPaxRate)
}).ToList();
First Table is the View and Second is the result I want
This below query works fine
List<BTWStudents> students = (from V in db.vwStudentCoursesSD
where classIds.Contains(V.Class.Value)
select new BTWStudents
{
StudentId = V.StudentId
Amount= V.PaymentMethod == "Cashier Check" ? V.Amount: "0.00"
}).Distinct().ToList();
But I changed it to List to add string formatting(see below)
List<BTWStudents> students = (from V in db.vwStudentCoursesSD
where classIds.Contains(V.Class.Value)
select new {V}).ToList().Select(x => new BTWStudents
{
StudentId = V.StudentId
Amount= V.PaymentMethod == "Cashier Check" ? String.Format("{0:c}",V.Amount): "0.00"
}).Distinct().ToList();
With this Second Query I get this
Why is distinct not working in the second query?
When working with objects (in your case a wrapped anonymous type because you are using Select new {V} rather than just Select V), Distinct calls the object.Equals when doing the comparison. Internally, this checks the object's hash code. You'll find in this case, the hash code of the two objects is different even though the fields contain the same values. To fix this, you will need to override Equals on the object type or pass a custom IEqualityComparer implementation into the Distinct overload. You should be able to find a number of examples online searching for "Distinct IEqualityComparer".
Try this (moved your distinct to the first query and corrected your bugged if/then/else):
List<BTWStudents> students = (from V in db.vwStudentCoursesSD
where classIds.Contains(V.Class.Value)
select new {V}).Distinct().ToList().Select(x => new BTWStudents
{
classId = V.Class.HasValue ? V.Class.Value : 0,
studentName = V.StudentName,
paymentAmount = V.PaymentMethod == "Cashier Check" ? String.Format("{0:c}",x.V.AmountOwed): "0.00"
}).ToList();
You can get around using Distinct all together if you Group by StudentID
var studentsGroupedByPayment =
(from V in db.vwStudentCoursesSD
where classIds.Contains(V.Class.Value)
group V by V.StudentId into groupedV
select new
{
StudentID = groupedV.Key,
Amount = string.Format("{0:C}",
groupedV.First().PaymentMethod == "Cashier Check" ?
groupedV.First().Amount : 0.0)
}
).ToList();
How we can write a LINQ query for following select sql query:
string brandid="1,2,3"
string bodystyleid="1,2,3"
-------------------
-----------------
select * from car
where brandid in (brandid)
and bodystyleid in (brandid)
----------------------
-------------------
My specific requirement is that if brandid or bodystyleid is blank(if user does not select
any checkbox of a particular search option) query should return all record for that particular where condition.
Please guide me.
Thanks,
Paul
In order to fulfil your requirement about returning all items if none are specified, you need to check for the lists being empty.
var brands = brandid.Split(',').Select(x => Int32.Parse(x));
var styles = bodystyleid.Split(',').Select(x => Int32.Parse(x));
var result = from c in car
where (!brands.Any() || brands.Contains(c.brandid))
&& (!styles.Any() || styles.Contains(c.bodystyleid))
select c;
(similar to sgmoore's solution, but includes the check for no brand/style specified)
I've not actually checked how this gets converted back to SQL - it may be more efficient to use a flag to indicate whether there are any values:
var brands = ....; // As above
bool anyBrands = brands.Any()
var result = from c in car
where (!anyBrands || brands.Contains(c.brandid))
.....
Is bodystyleid meant to check brandid or bodystyleid? (I am assuming bodystyleid, however have wrote the query to match the query in the question (brandid))
As a start you could do:
var results = (from c in car
where c.brandid.Contains(brandid)
&& c.bodystyleid.Contains(brandid)
select c).ToList();
var brandids = brandid .Split(',').Select(n => int.Parse(n)).ToList();
var bodyStyleids = bodystyleid.Split(',').Select(n => int.Parse(n)).ToList();
var results =
(from c in car where
brandids.Contains(c.brandid) &&
bodyStyleids.Contains(c.bodystyleid)
select c
).ToList();
the Ids you have are as strings with comma delimiter, you need them to be collections like List of the same type as your Ids of the Car table, so if brandid column is int then brandids has to be List<long>, then you can do
var results = (
from c in cars
where brandids.Contains(c.brandid) && bodystyleid.Contains(c.bodystyleid)
select c).ToList();
I am using the following code to return an IList:
public IList<string> FindCodesByCountry(string country)
{
var query = from q in session.Linq<Store>()
where q.Country == country
orderby q.Code
select new {q.Code};
return (IList<string>) query.ToList();
}
However I keep getting this error:
Unable to cast object of type 'System.Collections.Generic.List1[<>f__AnonymousType01[System.String]]' to type 'System.Collections.Generic.IList`1[System.String]'.
What I am supposed to return here?
as long as q.code is a string this should work:
note that it is not creating an anonymous object, just the string is being selected.
public IList<string> FindCodesByCountry(string country)
{
var query = from q in session.Linq<Store>()
where q.Country == country
orderby q.Code
select q.Code;
return query.ToList();
}
Is there a reason you were selecting an anonymous type? If not try this...
var query = from q in session.Linq<Store>()
where q.Country == country
orderby q.Code
select q.Code;
How about
query.Select(s => s.ToString()).ToList();
Or
query.Cast<String>().ToList();
But I'm assuming that q.Code is a string? In which case you just want to change your LINQ expression:
var query = from q in session.Linq<Store>()
where q.Country == country
orderby q.Code
select q.Code;
In the query, instead of selecting an anonymous class containing a string, just select the string itself:
var query = from q in session.Linq<Store>()
where q.Country == country
orderby q.Code
select q.Code;
You can't cast a list of custom types to a list of strings like that. The easiest way would be to have your query object begin it's life as an iEnumerable list of strings, rather than a custom type. Change your select line to:
select new q.Code.toString();
and you'll be good. If q.Code is itself a string to begin with, then the .ToString() won't be necessary.
Try this:
return query.ToList<string>();