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Not able to compare two Lists using LINQ

I need to Compare two List types based on its column values Id and Name. List1 has names and ids and List2 also contains Id ,Name and other columns. Now I want to check a condition that one input Name receive in a method. That Name is not exist in List2 but It may exist in List1.The condition should validate that List1 Id does not exist in List2 Id and the external Name might exist in List1. I have used below query for achieve this.
var IsNameExists= db.List1.Where(a => List2.Any(b=>b.Id!=a.Id) && a.Name== InputName).ToList();
When I am using this query, I am getting exception like
Unable to create a constant value of type 'ProjectName.DBEntities.List2'. Only primitive types or enumeration types are supported in this context.
I am not sure what i have done wrong on my query. Please guide me how can I achieve this to check the Name existence on List1.
Corrected query and Answer is:
var IsNameExists= db.List1.ToList().Where(a => !(List2.Select(x => x.Id).Contains(a.Id) && a.Name== InputName).ToList();

You could do something like this i guess, basically its just selecting out the ids and sending them to SQL as primitives to make a nice simple sql query
The fundamental problem here is trying to mix whats in memory with LinqToSql which in essence tries to convert you statement to a query and runs it on the database. The database wont know about your List that you have memory, so the solve is either give it something it understands or do it in memory. The following example extracts the ids out, and gives the data base something it does know, a list of int
var ids = List2.Select(x => x.Id).ToList();
var IsNameExists= db.List1.Where(a => !ids.Contains(a.Id) && a.Name== InputName)
.ToList();

LINQ Intersection of two different types

I have two different list types. I need to remove the elements from list1 that is not there in list2 and list2 element satisfies certain criteria.
Here is what I tried, seems to work but each element is listed twice.
var filteredTracks =
from mtrack in mTracks
join ftrack in tracksFileStatus on mtrack.id equals ftrack.Id
where mtrack.id == ftrack.Id && ftrack.status == "ONDISK" && ftrack.content_type_id == 234
select mtrack;
Ideally I don't want to create a new copy of the filteredTracks, is it possible modify mTracks in place?

If you're getting duplicates, it's because your id fields are not unique in one or both of the two sequences. Also, you don't need to say where mtrack.id == ftrack.Id since that condition already has to be met for the join to succeed.
I would probably use loops here, but if you are dead set on LINQ, you may need to group tracksFileStatus by its Id field. It's hard to tell by what you posted.
As far as "modifying mTracks in place", this is probably not possible or worthwhile (I'm assuming that mTracks is some type derived from IEnumerable<T>). If you're worried about the efficiency of this approach, then you may want to consider using another kind of data structure, like a dictionary with Id values as the keys.

Since the Q was about lists primarily...
this is probably better linq wise...
var test = (from m in mTracks
from f in fTracks
where m.Id == f.Id && ...
select m);
However you should optimize, e.g.
Are your lists sorted? If they are, see e.g. Best algorithm for synchronizing two IList in C# 2.0
If it's coming from Db (it's not clear here), then you need to build your linq query based on the SQL / relations and indexes you have in the Db and go a bit different route.
If I were you, I'd make a query (for each of the lists, presuming it's not Db bound) so that tracks are sorted in the first place (and sort on whatever is used to compare them, usually),
then enumerate in parallel (using enumerators), comparing other things in the process (like in that link).
that's likely the most efficient way.
if/when it comes from database, optimize at the 'source' - i.e. fetch data already sorted and filtered as much as you can. And basically, build an SQL first, or inspect the returned SQL from the linq query (let me know if you need the link).

How do I apply the LINQ to SQL Distinct() operator to a List<T>?

I have a serious(it's getting me crazy) problem with LINQ to SQL. I am developing an ASP.NET MVC3 application using c# and Razor in Visual Studio 2010.
I have two database tables, Product and Categories:
Product(Prod_Id[primary key], other attributes)
Categories((Dept_Id, Prod_Id) [primary keys], other attributes)
Obviously Prod_Id in Categories is a foreign key. Both classes are mapped using the Entity Framework (EF). I do not mention the context of the application for simplicity.
In Categories there are multiple rows containing the Prod_Id. I want to make a projection of all Distinct Prod_Id in Categories. I did it using plain (T)SQL in SQL Server MGMT Studio according to this (really simple) query:
SELECT DISTINCT Prod_Id
FROM Categories
and the result is correct. Now I need to make this query in my application so I used:
var query = _StoreDB.Categories.Select(m => m.Prod_Id).Distinct();
I go to check the result of my query by using:
query.Select(m => m.Prod_Id);
or
foreach(var item in query)
{
item.Prod_Id;
//other instructions
}
and it does not work. First of all the Intellisense when I attempt to write query.Select(m => m. or item.shows just suggestions about methods (such as Equals, etc...) and not properties. I thought that maybe there was something wrong with Intellisense (I guess most of you many times hoped that Intellisense was wrong :-D) but when I launch the application I receive an error at runtime.
Before giving your answer keep in mind that;
I checked many forums, I tried the normal LINQ to SQL (without using lambdas) but it does not work. The fact that it works in (T)SQL means that there is something wrong with the LINQ to SQL instruction (other queries in my application work perfectly).
For application related reasons, I used a List<T> variable instead of _StoreDB.Categories and I thought that was the problem. If you can offer me a solution without using a List<T> is appreciated as well.

This line:
var query = _StoreDB.Categories.Select(m => m.Prod_Id).Distinct();
Your LINQ query most likely returns IEnumerable... of ints (judging by Select(m => m.Prod_Id)). You have list of integers, not list of entity objects. Try to print them and see what you got.

Calling _StoreDB.Categories.Select(m => m.Prod_Id) means that query will contain Prod_Id values only, not the entire entity. It would be roughly equivalent to this SQL, which selects only one column (instead of the entire row):
SELECT Prod_Id FROM Categories;
So when you iterate through query using foreach (var item in query), the type of item is probably int (or whatever your Prod_Id column is), not your entity. That's why Intellisense doesn't show the entity properties that you expect when you type "item."...
If you want all of the columns in Categories to be included in query, you don't even need to use .Select(m => m). You can just do this:
var query = _StoreDB.Categories.Distinct();
Note that if you don't explicitly pass an IEqualityComparer<T> to Distinct(), EqualityComparer<T>.Default will be used (which may or may not behave the way you want it to, depending on the type of T, whether or not it implements System.IEquatable<T>, etc.).
For more info on getting Distinct to work in situations similar to yours, take a look at this question or this question and the related discussions.

As has been explained by the other answers, the error that the OP ran into was because the result of his code was a collection of ints, not a collection of Categories.
What hasn't been answered was his question about how to use the collection of ints in a join or something in order to get at some useful data. I will attempt to do that here.
Now, I'm not really sure why the OP wanted to get a distinct list of Prod_Ids from Categories, rather than just getting the Prod_Ids from Projects. Perhaps he wanted to find out what Products are related to one or more Categories, thus any uncategorized Products would be excluded from the results. I'll assume this is the case and that the desired result is a collection of distinct Products that have associated Categories. I'll first answer the question about what to do with the Prod_Ids first, and then offer some alternatives.
We can take the collection of Prod_Ids exactly as they were created in the question as a query:
var query = _StoreDB.Categories.Select(m => m.Prod_Id).Distinct();
Then we would use join, like so:
var products = query.Join(_StoreDB.Products, id => id, p => p.Prod_Id,
(id,p) => p);
This takes the query, joins it with the Products table, specifies the keys to use, and finally says to return the Product entity from each matching set. Because we know that the Prod_Ids in query are unique (because of Distinct()) and the Prod_Ids in Products are unique (by definition because it is the primary key), we know that the results will be unique without having to call Distinct().
Now, the above will get the desired results, but it's definitely not the cleanest or simplest way to do it. If the Category entities are defined with a relational property that returns the related record from Products (which would likely be called Product), the simplest way to do what we're trying to do would be the following:
var products = _StoreDB.Categories.Select(c => c.Product).Distinct();
This gets the Product from each Category and returns a distinct collection of them.
If the Category entity doesn't have the Product relational property, then we can go back to using the Join function to get our Products.
var products = _StoreDB.Categories.Join(_StoreDB.Products, c => c.Prod_Id,
p => p.Prod_Id, (c,p) => p).Distinct();
Finally, if we aren't just wanting a simple collection of Products, then some more though would have to go into this and perhaps the simplest thing would be to handle that when iterating through the Products. Another example would be for getting a count for the number of Categories each Product belongs to. If that's the case, I would reverse the logic and start with Products, like so:
var productsWithCount = _StoreDB.Products.Select(p => new { Product = p,
NumberOfCategories = _StoreDB.Categories.Count(c => c.Prod_Id == p.Prod_Id)});
This would result in a collection of anonymous typed objects that reference the Product and the NumberOfCategories related to that Product. If we still needed to exclude any uncatorized Products, we could append .Where(r => r.NumberOfCategories > 0) before the semicolon. Of course, if the Product entity is defined with a relational property for the related Categories, you wouldn't need this because you could just take any Product and do the following:
int NumberOfCategories = product.Categories.Count();
Anyway, sorry for rambling on. I hope this proves helpful to anyone else that runs into a similar issue. ;)

Linq2Entities, many to many and dynamic where clause

I'm fairly new to Linq and struggling using dynamic where over a many to many relationship.
Database tables are like so:
Products <-> Products_SubCategories <-> SubCategories
with Products_SubCategories being a link table.
My full linq statement is
db.Products.Where("it.SubCategories.SubCategoryID = 2")
.Include("SubCategories")
.OrderBy(searchOrderBy)
.Skip(currentPage * pageSize)
.Take(pageSize)
.ToList()
.ForEach(p => AddResultItem(items, p));
So ignoring everything bar the Where() I'm just trying to pull out all products which are linked to sub category ID 2, this fails with
To extract properties out of collections, you must use a sub-query to iterate over the collection., near multipart identifier, line 8, column 1.
I think using the SQL-esque syntax I can do a subquery as per this link. However I'm not sure how to do that in the lambda / chaining syntax.
This is the start of a search function and I would like to build up the where string dynamically, as I have with the searchOrderBy string to avoid a large SELECT CASE. Products is linked to another table via a link table that I will need to include once I understand how to do this example.
Any help would be much appreciated!
Thanks

This is wrong:
db.Products.Where("it.SubCategories.SubCategoryID = 2")
SubCategories is a list. It does not have a property called SubCategoryID. Rather, it contains a group of entities which each have a property called SubCategoryID. That's a critical distinction.
When you run into a situation where you don't know how to proceed in there are multiple problems, it is good to break the problem down into several, smaller problems.
Let's start by removing the dynamic query. It will be easier to solve the problem with a non-dynamic query. Once you've done that, you can go back and make it dynamic again.
So start by using the non-dynamic syntax. Type something like this in Visual Studio, and see what IntelliSense does for you:
db.Products.Where(p => p.SubCategories.
You will quickly see that there is no SubCategoryID property. Instead, you will see a bunch of LINQ API methods for working with lists. If you know LINQ well, you will recognize that the Any() method is what you want here:
db.Products.Where(p => p.SubCategories.Any(sc => sc.SubCategoryID == 2))
Go ahead and run that query. Does it work? If so, you can move ahead to making it dynamic. I'm no ESQL expert, but I'd start with something along the lines of:
db.Products.Where("EXISTS(SELECT SC FROM it.SubCategories AS SC WHERE SC.SubCategoryID = 2");
As an aside, I use MS Dynamic Query ("Dynamic LINQ") for this sort of thing rather than Query Builder, as it's more testable.

It worked for me.
db.Products.Where("SubCategories.Any(SubCategoryID = 2)")

Using [0] or First() with LINQ OrderBy

If I have a structure like this
Albums
- Album
- Discs
- Tracks
and I want to order a collection of albums by the title of the first track on the first disc.
Is there something similar to the following I could do (keeping in mind I need to use the OrderBy extension method that accepts a string)?
albums.OrderBy("Discs[0].Tracks[0].Title")
I need to be able to sort using a string expression thus the need to use the OrderBy method i.e. albums.OrderBy("Track[0].Title"). The reason for this is our custom framework uses a sort expression (e.g. "Title") passed back from a GridView which is looked up in a dictionary (e.g. "Track[0].Title") to get the correct order by clause. That is, the field and direction of sorting is dynamically determined at runtime.
or
albums.OrderBy("Discs.First().Tracks.First().Title")

Untested, but how about:
var query = from album in albums
let disc = album.Discs.First()
let track = disc.Tracks.First()
orderby track.Title
select album;

LINQ has two ways to query "from . in .." and Lambda expressions. They way you were almost writing it looked Lambda-ish. Here would be the Lambda expression:
albums.OrderBy(a=>a.Discs.First().Tracks.First().Title)
I used variable 'a' to indicate album but you can use any variable, this is identical to the first expression:
albums.OrderBy(album=>album.Discs.First().Tracks.First().Title)
or you can use the from obj in obj form as mention in the other answers.

How about this, in order to satisfy your need for an initial query that does not perform the sorting? This uses anonymous types to store the album information, plus the name of the first track so you can sort on it later.
var query = from album in albums
let disc = album.Discs.First()
let track = disc.Tracks.First()
select new { Album = album, FirstTrack = track.Title };
var sortedQuery = from album in query
order by album.FirstTrack
select album.Album;

Sorry people,
It looks like the OrderBy method that I am asking about and trying to use is specific to the ORM (genom-e) that we are using and is not reflected on the .net Queryable or IEnumerable classes (unlike the majority of genom-e's LINQ functionality). There is no OrderBy overload that accepts a string in .net, this is specific to genom-e.
Those of you using .net encountering a similar problem should probably give either of the above two answers a try.