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How to filter a List<T> if it contains specific class data?

I need help with filtering list data in c#.
I got 3 class named Product.cs, Storage.cs and Inventory.cs.
public class Storage{
string StorageId;
string Name;
}
public class Inventory{
string InventoryId;
string StorageId;
string ProductId;
}
I got the filled List<Storage> mStorages, List<Product> mProduct and List<Inventory> mInventories.
I have trouble to print mStorages that contain with specific productId that only can be obtained from mInventories.
So, I tried this:
List<Storage> mFilteredStorage;
for(int i=0;i<mStorages.Count;i++){
if(mStorages[i] contain (productId from inventories)){
mFilteredStorage.add(mstorages[i]);
}
So I can get mFilteredStorage that contains specific product from inventories. (in inventories there are lot of product id).
What should I do to get that filteredStorage? I tried to use list.contains() but it only return true and at last there are duplicated storage at mFilteredStorage.
Really need your help guys. Thanks in advance.

I suggest you to read about lambda-expressions, that is what you are looking for.
mFilteredStorage.AddRange(mStorages.Where(storage => inventories.Any(inventory => inventory.productId == storage.productId)).ToList());
This returns you a list with your filtered conditions. So right after Where you iterate over each item in your list, I called this item storage. (you can name those what ever you want to) Then we iterate over your object inventories with another lambda expression. This, the second lambda expression, returns either true if any of inventories's productIds match the productId of the current iterating object of mStorages or false if they don't match.
So you once the productIds match you can imagine the code like the following:
mStorages.Where(storage => true);
And once the result of the second lambda expression is true, storage will be added to the IEnumerable you will get as a result of the Where method.
Since we get an IEnumerable as return, but we want to add those Storage objects to mFilteredStorage, I convert the IEnumerable to a list, by:
/*(the return object we get from the `Where` method)*/.ToList();

You can use LINQ to accomplish your goal. Since Storage has no ProductId, the query will match by StorageId.
var filteredStoragesQry =
from storage in mStorages
where inventories.Any(inventory => inventory.StorageId == storage.StorageId)
select storage;
mFilteredStorages = filteredStoragesQry.ToList();
This query is for LINQ to objects, but it will also work in Entity Framework, when you replace mStorages and inventories by the respective DbSet objects from the context.

mStorages.Join(mInventories, x => x.StorageId, y => y.StorageId, (x, y) => new { Storage = x, ProductId = y.ProductId})
.Where(z => z.ProductId == "specificProductId").Select(z => z.Storage).ToList()

I ended with this code.
mFilteredStorage = tempStorage.GroupBy(s => s.Id).Select(group => group.First()).ToList()
This code is what I want to show.

LINQ/lambda: How can I query a DB table based on information from another table? (many to many relationship)

I have a database scheme with 3 tables. One for requisitions, one for hospitals, and one joining the two (many-to-many relationship).
I'd like to list all requisitions in the database that are linked to a selected hospital.
This is what I have so far:
var valgtSykehus = Db.Sykehus.Where(n => n.Navn == sykehus).Single(); //this gives me a variable with my current hospital. I want to list all requistions that contains this.
var Rekvisisjoner = Db.Rekvisisjoner
.Where(r => r.Arkivert == true) //get only archived requsitions
.Include(p1 => p1.Sykehus) //include hospitals
.ToList() //this generates a list of -all- requisitions with the hospitals they are attached to.
.Where(x => x.Created > DateTime.Now.AddYears(-3)) /only go 3 years back
.Where(x => x.Sykehus.Contains(valgtSykehus)); //here is the problem. I want to discard all requisitions that does NOT contain the hospital in the valgtSykehus variable
Anyway, this gives me zero requistions, but if I skip the last line, I get all archived requistions.

x.Sykehus.Contains(valgtSykehus) executes in LINQ to Objects context (due to the intermediate ToList call) and most likely uses reference equality, which normally should work as soon as you use tracking queries.
Still, it's safer and also more efficient to do the whole thing with a single db query using Any condition with primitive key. Something like this:
var Rekvisisjoner = Db.Rekvisisjoner
.Include(r => r.Sykehus) //include hospitals
.Where(r => r.Arkivert == true) //get only archived requsitions the hospitals they are attached to.
.Where(r => r.Created > DateTime.Now.AddYears(-3)) /only go 3 years back
.Where(r => r.Sykehus.Any(s => s.Navn == sykehus));
If there is an issues with using DateTime.Now.AddYears(-3) inside the query, just put into variable outside of the query and use it inside.
var minDate = DateTime.Now.AddYears(-3);
var Rekvisisjoner =
// ...
.Where(r => r.Created > minDate)
//...

The issue may lie in the implementation of Contains. Contains has to check equality somehow. Anyway, if your valgtSykehus object is logically contained in x.Sykehus (i.e. has the same data), but not exactly the same object (i.e. the same reference), it's possible that Contains fails to find it, due to the default implementation of == in reference types (== is true, if the objects are exactly the same reference, false otherwise, even though all the data is the same).
You could try the following:
var Rekvisisjoner = Db.Rekvisisjoner
.Where(r => r.Arkivert == true)
.Include(p1 => p1.Sykehus)
.ToList()
.Where(x => x.Created > DateTime.Now.AddYears(-3))
.Where(x => x.Sykehus.Any(sh => sh.Id == valgtSykehus.Id));
If Id (or whatever your ID property is named) is a value field (most likely) this will return true whenever the ID of an Sykehus matches the ID of valgtSykehus.

Oh my.
I just realised that none of the archived requisitions contains any connections to the hospitals, as they apparently are removed one-by-one when the requisition is processed in the program.
I figured this out while trying to reverse the query, so thanks for that tip.

why cant I compare two id's in c#

I tried to compare two ids and get some result.it works for other strings.but not for this.
I tried like this.
var neededData = mainFaires.Where(c => c.trimacid == passId );
in here passId= OX20160330HAVHAV
and in the mainFaires list, in somewhere it includes this id.but it didn't give the result.I found in here
var x = mainFaires.ElementAt(27261);
this list include the same id.but didn't give result.I can't think why.

ElementAt is find the position.
You should use select to find the records
var x = mainFaires.Select(o => o.trimacid == 27261);

You should use .ToList() .First() or .FirstOrDefault() to actually commit the query and get a result. Your code only defined the query, but didn't actually submit it to the data collection.
If you expect only one item as a result, you're code should look like this:
var neededData = mainFaires.Where(c => c.trimacid == passId ).FirstOrDefault();
If there was no item found, neededData would be NULL or whatever the default value is. You may also check the Documentation here https://msdn.microsoft.com/en-us/library/system.linq.enumerable%28v=vs.100%29.aspx

asp.net MVC Where in List

I building my first application with c# and sp.net MVC 5, so far so good :)
Now I have a problem, we using 2 User Tables, first one contains the username, other the user data.
string user = User.Identity.Name;
var data = db.FE_Benutzer;
var collection = data.Where(o => o.Benutzername == user).Select(x => new
{
id = x.ID,
name = x.Name,
hauptbereiche = x.Hauptbereich.ToList()
});
var dataHauptbereich = db.Hauptbereich;
var collectionHauptbereich = dataHauptbereich.Where(o => collection.ElementAt(0).hauptbereiche.Contains(o)).Select(x => new
{
id = x.ID,
name = x.Name
});
return Json(collectionHauptbereich, JsonRequestBehavior.AllowGet);
I getting this error
LINQ to Entities does not recognize the method '<>f__AnonymousType63[System.Int32,System.String,System.Collections.Generic.List1[scorring.Models.Hauptbereich]] ElementAt[<>f__AnonymousType63](System.Linq.IQueryable1[<>f__AnonymousType63[System.Int32,System.String,System.Collections.Generic.List1[scorring.Models.Hauptbereich]]], Int32)' method, and this method cannot be translated into a store expression.
hauptbereiche = x.Hauptbereich.ToList()
contains a list of ids where the user have premission to.
When I fetching the data
dataHauptbereich.Where
I wont to include only the ids I have in the list
how is this possible?

Entity Framework doesn't know how to turn ElementAt into SQL. See this answer for more information: Getting the first result from a LINQ query - why does ElementAt<T>(0) fails when First<T>() succeeds?

Try
dataHauptbereich.Where(o => collection.ElementAt(0).hauptbereiche.Any(h => h.ID == o.ID))
Or
dataHauptbereich.Where(o => collection.Any(c => c.hauptbereiche.Any(h => h.ID == o.ID)))

I'm having a bit of a time deciphering exactly what you're trying to achieve with your code here, but it looks to me like your simply querying Hauptbereichs that belong to a particular user. Your first query selects an anonymous object composed of id, name and hauptbereiche, but of these you only ever use the hauptbereiche property. Then, in your second query, you merely selecting Hauptbereichs that match an item in this hauptbereiche property's collection. Actually, here, you're only comparing values from the first item in the original collection, which begs the question of why you're selecting anything other than the first item. That, and this second query is entirely redundant because if the items match that means you already had the items in the first place. You could get the same info directly from collection.ElementAt(0).hauptbereiche without issuing the second query.
So, here's a couple of simpler options:
If you're trying to get all the Hauptbereichs that belong to all the FE_Benutzers where Benutzername == user then just do:
var collectionHauptbereich = db.FE_Benutzer.Where(m => m.Benutzername == user)
.Include(m => m.Hauptbereich)
.SelectMany(m => m.Hauptbereich);
If you want just the first FE_Benutzer item's Hauptbereichs, then do:
var benutzer = db.FE_Benutzer.Where(m => m.Benutzername == user)
.Include(m => m.Hauptbereich)
.FirstOrDefault();
var collectionHauptbereich = benutzer != null
? benutzer.Hauptbereich.ToList()
: new List<Hauptbereich>();

LINQ sub list<> sometimes returns "NULL"

I have a LINQ query with a sublist in it which sometimes can return no returns, however can't manage to get the actual null instead of a error.
Any help in this would be appriciated.
var member_settings = from ml in _pMetaLanguages
join s in _settings
on ml.id equals s.setting_type_id
orderby ml.name descending
select new _class {
group_name = ml.name
, code = (ml.classes.Where(c => c.id == s.int_value)).FirstOrDefault().code
, name = (ml.classes.Where(c => c.id == s.int_value)).FirstOrDefault().name
, id = (ml.classes.Where(c => c.id == s.int_value)).FirstOrDefault().id
,
classUI = ml.metadataMUI
,
nameUI = ml.metadataMUI.Where(u => u.language_type_id.Equals(list_language_id))
.Select(i => new pMetaClasses
{
name = i.classes.Where(y => (y.bound_id.Equals(list_language_id))).FirstOrDefault().name
}).FirstOrDefault().name
, setting_type_id = s.setting_type_id
, int_value = s.int_value
};
EDITED
This part delivers the problems
nameUI = ml.metadataMUI.Where(u => u.language_type_id.Equals(list_language_id))
.Select(i => new pMetaClasses
{
name = i.classes.Where(y => (y.bound_id.Equals(list_language_id))).FirstOrDefault().name
}).FirstOrDefault().name
"metadataMUI" sometimes has records but not always, when no records it should be null (at least no error...

You have a collection of objects, it may or may not be empty. You want to pull out a property from the first item in that collection, if there is one. Currently whenever your query is face with this problem you are using this general approach:
sequence.FirstOrDefault().SomeMember
This code works fine if the sequence will always contain an item. It doesn't work if it may be empty. (Unless you can and want to get the member of the default value. For reference types, as you clearly are working with, this just results in null reference excpetions.) Fortunately there is a simple transformation to ensure that this works correctly. Simply use Select to transform the sequence into the sub-property that you are interested in and then get the first or default value of that sequence:
sequence.Select(item => item.SomeMember).FirstOrDefault();
This will work properly whether there are items or no items. The projection will never be called on default values using this approach.
Of course you do this in a lot of places, so you'll need to make this transformation throughout your query.