The first query returns a list of string, and I am passing them into another table to find corresponding items, but nothing happens. no error message or nothing
var classIds = _contextSpecRepo.Get(x => x.cId.Equals(cId)).Select(x => x.classNames).Distinct().ToList();
// issue happens in the following query
var classes= Repository.Get(x => x.Id.Equals(classIds)).ToList();
The call to Equals, which takes object, hides the problem: you are comparing a single Id to a list of Ids, rather than checking if the Id is present in a collection. This compiles, but yields no result.
Here is how you can fix it:
var classes= Repository.Get(x => classIds.Any(y => y == x.Id)).ToList();
or
var classes= Repository.Get(x => classIds.Contains(x.Id)).ToList();
If you must do it in 2 queries then you have to use contains
var classes= Repository.Get(x => classIds.Contains(x.Id)).ToList();
A better solution would be to use a join on the tables.
you can also skip .ToList()
var classes= Repository.Get(x => classIds.Contains(x.Id));
Related
I am using a predicate builder class and I need to invoke the contains method on an array of strings so in the code below instead of radio I would be passing in an array of strings:
wherePredicate = wherePredicate.Or(m => m.MediaType.Contains("Radio"));
the full code section:
if (param.iMediaGroupID > 0)
{
var wherePredicate = PredicateBuilder.False<MediaChannelModel>();
var ss = new NeptuneRepository<Lookup_MediaTypes>();
var mediagroups = ss.FindWhere(m => m.MediaGroupID == param.iMediaGroupID).Select(m => m.Name);
//problem area
wherePredicate = wherePredicate.Or(m => mediagroups.Contains(m.MediaType));
predicate = predicate.And(wherePredicate);
}
mediaGroups is: ["Radio","Tv","Magazine"]
If m.MediaType is any of these values then the predicate is true.
Is there a way to do this in C#?
I suspect you want something like:
wherePredicate = wherePredicate.Or(m => array.Contains(m.MediaType));
Or perhaps:
wherePredicate = wherePredicate.Or(m => array.Any(x => m.MediaType.Contains(x)));
If neither of those are what you're after, please clarify your requirements.
EDIT: The problem you're now facing is that you're not actually asking whether an array contains the value. You're asking whether a query contains a value. If you change it to an actual array, you may well find it works:
var mediagroups = ss.FindWhere(m => m.MediaGroupID == param.iMediaGroupID)
.Select(m => m.Name)
.ToArray();
However, if these are querying the same database, you'd be better off trying to do this in some kind of join.
Jon Skeet's answer worked perfectly for me. I had been struggling to make the .Contains search for a substring in a string array against the database, rather than try to find a substring in a single C# string object. Thank you!
Here's the modified code that worked for me:
var predicate = PredicateBuilder.False<ClientXMemberDetail>();
predicate = predicate.Or(x => strArrselectedCustomMemberNumbers.Any<string>(y => x.MemberID.Contains(y)));
CustomSearchMembersAlreadyMatched = ClientXContext.ClientXMemberDetails
.AsExpandable()
.Where(predicate)
.ToList()
.Select(r => r.MemberID.ToString()).ToList();
(ClientXContext above is an instance of the ObjectContext class, strArrselectedCustomMemberNumbers is a string array, ClientXMemberDetails is ObjectSet, where ClientXMemberDetail is the EntityObject)
Edit: Anonymized my client's name
I have a LINQ query against an XML, that gives me a list of nested lists, each sublist being a list of an elements("row") attributes.
var items = loadbodies.Descendants("row").Select(a => a.Attributes().Select(b => b.Value).ToList()).ToList();
This works as intended but, what I actually need to is query this against another list of values so as not to have sublists added where one of the elements attributes("messageID") is on the second list. I can do this for one value but need to check it against the entire second list.
The query to exclude a single sublist by a single hardcoded value from the second list is below.
var items = loadbodies.Descendants("row").Where(c => (string)c.Attribute("messageID") != "avaluefromthesecondlist").Select(a => a.Attributes().Select(b => b.Value).ToList()).ToList();
Any help would be much appreciated.
Just use Contains. Note that splitting lines helps readability considerably:
var ids = ...; // Some sequence of ids, e.g. a List<string> or HashSet<string>
var items = loadbodies
.Descendants("row")
.Where(row => ids.Contains((string) row.Attribute("messageId")))
.Select(a => a.Attributes()
.Select(b => b.Value)
.ToList())
.ToList();
Note that you could use a Join call too... but so long as you've got relatively few IDs, this should be fine.
If i have an object of type Photo and a Result set which is sorted in a particular order, is there a way for me to get the position of the current Photo object in the result set. and then get all objects that would follow it?
You can do something like this (not terribly efficient):
var result =
photos.Select((p, i) => new { Index = i, Photo = p })
.SkipWhile(x => x.Photo != photo).Skip(1);
This will give you all photos following photo combined with their index in the original collection.
If you're sorting against an Id:
// gets the previous photo, or null if none:
var previousPhoto = db.Photos
.Where(p => p.Id < currentPhotoId)
.OrderByDescending(p => p.Id)
.FirstOrDefault();
// gets the next photo, or null if none:
var nextPhoto = db.Photos
.Where(p => p.Id > currentPhotoId)
.OrderBy(p => p.Id)
.FirstOrDefault();
If you have custom ordering, you'd need to replace the OrderBy/OrderByDescending expression with your custom ordering. You'd also need to use the same ordering criteria in Where() to get only those photos before or after the current photo.
Not sure if I understand correctly but this might help:
var result = photos.Skip(100); // 100 would be the position of current object.
// if you don't know the index of the current object:
// This won't work on LINQ to SQL directly, do it on a list or something.
var result = photos.SkipWhile(x => x != currentObject).Skip(1);
In reality, if you are dealing with a database, there should be some identifier (a set of columns) you are sorting by. If you want to do the whole thing on the server side, you can grab the properties of the current object and filter the result set specifically for objects that would come after that in the sort order you want.
Index of the photo:
result.IndexOf(photo);
Items after it:
result.SkipWhile((q, i) => i <= result.IndexOf(photo));
Why does this yield an empty set?
Object[] types = {23, 234, "hello", "test", true, 23};
var newTypes = types.Select(x => x.GetType().Name)
.Where(x => x.GetType().Name.Equals("Int32"))
.OrderBy(x => x);
newTypes.Dump();
When you do your select you're getting an IEnumerable<String>. Then you're taking the types of each string in the list (which is all "String") and filtering them out where they aren't equal to "Int32" (which is the entire list). Ergo...the list is empty.
Equals works just fine, it's your query that isn't correct. If you want to select the integers in the list use:
var newTypes = types.Where( x => x.GetType().Name.Equals("Int32") )
.OrderBy( x => x );
Reverse the order of the operations:
var newTypes = types.Where(x => x is int)
.OrderBy(x => x)
.Select(x => x.GetType().Name);
(Notice this also uses a direct type check instead of the rather peculiar .GetType().Name.Equals(…)).
The thing with LINQ is you've got to stop thinking in SQL terms. In SQL we think like this:-
SELECT Stuff
FROM StufF
WHERE Stuff
ORDER BY Stuff
That is what your code looks like. However in LINQ we need to think like this :-
FROM Stuff
WHERE Stuff
SELECT Stuff
ORDER BY Stuff
var newTypes = types.Select(x => x.GetType().Name)
.Where(x => x.Equals("Int32"))
.OrderBy(x => x);
This doesn't work because the Select statement will convert every value in the collection to the name of the underlying type of that value. The resulting collection will contain only string values and hence they won't ever have the name Int32.
I can flatten the results of a child collection within a collection with SelectMany:
// a list of Foos, a Foo contains a List of Bars
var source = new List<Foo>() { ... };
var q = source.SelectMany(foo => foo.Bar)
.Select(bar => bar.barId)
.ToList();
this gives me the list of all Bar Ids in the Foo List. When I attempt to go three levels deep the incorrect result is returned.
var q = source.SelectMany(foo => foo.Bar)
.SelectMany(bar => bar.Widget)
.Select(widget => widget.WidgetId)
.ToList();
How should I be using SelectMany to get the list of all Widgets in all Bars in my list of Foos?
Edit
I miss-worded the above sentence, but the code reflects the goal. I am looking for a list of all Widget Ids, not widgets.
An "incorrect" result is not all of the widget ids are returned.
Your query is returning all the widget IDs, instead of all the widgets. If you just want widgets, just use:
var q = source.SelectMany(foo => foo.Bar)
.SelectMany(bar => bar.Widget)
.ToList();
If that's still giving "the incorrect result" please explain in what way it's the incorrect result. Sample code would be very helpful :)
EDIT: Okay, if you want the widget IDs, your original code should be fine:
var q = source.SelectMany(foo => foo.Bar)
.SelectMany(bar => bar.Widget)
.Select(widget => widget.WidgetId)
.ToList();
That could also be written as
var q = (from foo in source
from bar in foo.Bar
from widget in bar.Widget
select widgetId).ToList();
if you like query expression format.
This really should work - if it's not working, that suggests there's something wrong with your data.
We should have checked before - is this just LINQ to Objects, or a fancier provider (e.g. LINQ to SQL)?
var q = (
from f in foo
from b in f.Bars
from w in b.Widgets
select w.WidgetId
).ToList();
Also note that if you want the unique list, you can do .Distinct().ToList() instead.
var q = source.SelectMany(foo => foo.Bar)
.SelectMany(bar => bar.Widget,(bar,widget) => widget.WidgetId)
.ToList();
we can call this overload of SelectMany() with allow us to specify the projection using lambda experession