I have a pretty complex linq statement I need to access for different methods. Each of these methods may need to see the resulting data with different parameters. For one method it may be a project code, for another it may be language. The statement is pretty much the same it's just the where part which changes.
I have not been able to figure out how to use different where statements without duplicating the entire linq statement, and that just isn't dry enough for me.
For example (greatly simplified):
var r = from c in customer
where c.name == "some name"
// or it may be
var r = from c in customer
where c.customerId == 8
Is there a way to have both of these in the same statement so I can use one or the other based on what I am doing? I tried an if statement to use one of the where statements or the other, and that didn't go over very well.
You can do it like this:
var r = from c in customer
select c;
if (CustomerName != null)
r = r.Where(c => c.name == CustomerName);
if (CustomerID != null)
r = r.Where(c => c.customerId == CustomerID);
You could make these else if if only one should apply, in my example any criteria that wasn't null would be applied to the query to filter on.
what about something like this?
var useIdForFiltering = false;
var r = from c in customer
where (useIdForFiltering && c.customerId == 8) || (c.name == "some name")
You can pass in a Func delegate to your function (the Where clause takes a Func delegate with a boolean return type). Then use this delegate in the Where clause.
Related
Using Microsoft SQL Entity Framework I've got a query where sometimes I have a filter condition and sometimes I don't, so I tried to do what I've shown below. If the condition is not null then instead of doing the query as expected it queries everything from the Org_Hierarchy table, and then queries everything from the Workers table, and then dies as that takes too long:
void SomeMethod(Func<PRT, bool> whereClause) {
IQueryable<PRT> query;
if (whereClause != null) {
query = PRT.Where(whereClause).AsQueryable();
} else {
query = PRT.AsQueryable();
}
var data = from prt in query
// LEFT OUTER JOIN Worker a ON prt.assigned_to = a.WWID
join a_join in Worker on prt.assigned_to equals a_join.WWID into a_grp
from a in a_grp.DefaultIfEmpty()
// LEFT OUTER JOIN Worker c ON prt.closed_by = c.WWID
join c_join in Worker on prt.closed_by equals c_join.WWID into c_grp
from c in c_grp.DefaultIfEmpty()
// LEFT OUTER JOIN Worker r ON prt.requestor = r.WWID
join r_join in Worker on prt.requestor equals r_join.WWID into r_grp
from r in r_grp.DefaultIfEmpty()
// LEFT OUTER JOIN Org_Hierarchy o ON prt.org3 = o.OrganizationHierarchyUnitCd AND o.OrganizationHierarchyUnitTreeLevelNbr = 3 AND o.Active = true
join o in Org_Hierarchy on prt.org3 equals o.OrganizationHierarchyUnitCd
select new PrtInput {
If I change the query and put something direct in there, just for testing, like where prt.id == Guid.NewGuid() right above the last line shown then the query returns in one second. What's the trick to be able to dynamically add a where clause to the query?
The above code is from LinqPAD which is why the normal "context" stuff is all missing.
I'm not sure , but i think you should use something like this :
Expression<Func<PRT ,bool>> whereClause
Insted of:
Func<PRT ,bool> whereClause
When you using Func<> , first fetch data from db to memory then filter data in memory ,but if you use Epression<> filter send to sql and return result.
Also for the better performnce you can use AsNoTracking() like this:
if (whereClause != null) {
query = PRT.Where(whereClause).AsQueryable().AsNoTracking();
} else {
query = PRT.AsQueryable().AsNoTracking();
}
When you only want run query on yout database without any Insert ,update or delete on result , it better use AsNoTracking.
I hope this answers your question.
Three conditions in linq where clause using lambda expressions
List<Tbl_MVPConsultant> _objConsultants = _datalayer.Get_MVP_Consultants();
_objConsultants = _objConsultants.Where(p => p.Country.ToLower().Contains(SearchTextbox.ToLower()) ||
p.State.ToLower().Contains(SearchTextbox.ToLower()) ||
p.City.ToLower().Contains(SearchTextbox.ToLower())).ToList();
I'm trying to achieve a filter operation three times using the above query .. but i'm getting an error stating object reference not set to an instance of an object ..
Looking for a quick solution.Appreciate early efforts.
Thank you
You won't be able to call p.Country.ToLower if p.Country is null. Put p.Country != null && p.State != null && p.City != null && at the start of the lambda.
I am using LINQ to SQL to retrieve data, using boolean conditions (BIT columns in SQL). My LINQ query looks something like this:
var query = from r in db.Requests
select r;
query = query.Where(r => r.Completed == someBooleanVal);
query = query.Where(r => r.Cancelled == someOtherBool);
return query.ToList();
The 'Where()' gets applied in a different method, that's why I'm putting it in separately.
When the boolean values are given as false, the generated SQL looks something like this:
SELECT [t0].[col1], [t0].[col2], [t0].[col3], [t0].[etc]
FROM [dbo].[Requests] AS [t0]
WHERE (NOT(([t0].[Cancelled]) = 1) AND (NOT(([t0].[Completed]) = 1)
in stead of what I would use:
WHERE [t0].[Cancelled] = 0 AND [t0].[Completed] = 0
This runs very, very slowly. I strongly suspect that it is because of the negative conditions on the boolean values it generated (all the selected columns are covered by an index, and the two columns in the where clause have a separate index on them).
Why is it generating negative conditions? How can I fix it?
var query =
from r in db.Requests.Where(r => r.Completed == someBooleanVal && r.Cancelled == someOtherBool)
select r;
return query.ToList();
Hope it can help you and have a nice day.
I have 2 tables
1. Client
2. Operations
Operations can result in: Credits or Debits ('C' or 'D' in a char field) along with date and ammount fields.
I must calculate the balance for each client's account using linQ... The result should also show balance 0 for clients whom didn't make operations yet
I have the following function with linQ statement, but I know it could be done in a better and quicker, shorter way, right? Which will be?
public static double getBalance(ref ClasesDeDatosDataContext xDC, string SSN,
int xidClient)
{
var cDebits =
from ops in xDC.Operations
where (ops.idClient == xidClient) && (ops.OperationCode == 'D')
select ops.Ammount;
var cCredits =
from ops in xDC.Operations
where (ops.idClient == xidClient) && (ops.OperationCode == 'C')
select ops.Ammount;
return (double)(cCredits.Sum() - cDebits.Sum());
}
Thanks !!!
I don't know if the LINQ-to-SQL engine can handle the expression, but something like this should be possible:
return (
from ops in xDC.Operations
where ops.idClient == xidClient
select ops.operationCode == 'C' ? ops.Amount : -ops.Amount
).Sum(a => a);
Or perhaps:
return (
from ops in xDC.Operations
where ops.idClient == xidClient
select new { Sign = ops.operationCode == 'C' ? 1.0 : -1.0, Amount = ops.Amount }
).Sum(o => o.Sign * o.Amount);
If the collection is empty, the Sum method returns zero, so that takes care of clients without transactions.
Edit:
Corrected spelling in query: Ampont -> Amount
I have been playing with the Linq to Sql and I was wondering if it was possible to get a single result out? For example, I have the following:
using(DataClassContext context = new DataClassContext())
{
var customer = from c in context.table
where c.ID = textboxvalue
select c;
}
And with this I need to do a foreach around the var customer but i know that this will be a single value! Anyone know how I could do a textbox.text = c.name; or something along that line?
Yes, it's possible.
using(DataClassContext context = new DataClassContext())
{
var customer = (from c in context.table
where c.ID = textboxvalue
select c).SingleOrDefault();
}
This way you get 1 result or null if there isn't any result.
You can also use Single(), which throws an exception when there isn't a result.
First() will give you only the first result found, where Last() will give you only the last result, if any.
Here's an Overview of all Enumerable methods.
var customer = context.table.SingleOrDefault(c => c.ID == textboxvalue);