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Retrieving non-duplicates from 2 Collections using LINQ

Background: I have two Collections of different types of objects with different name properties (both strings). Objects in Collection1 have a field called Name, objects in Collection2 have a field called Field.
I needed to compare these 2 properties, and get items from Collection1 where there is not a match in Collection2 based on that string property (Collection1 will always have a greater or equal number of items. All items should have a matching item by Name/Field in Collection2 when finished).
The question: I've found answers using Lists and they have helped me a little(for what it's worth, I'm using Collections). I did find this answer which appears to be working for me, however I would like to convert what I've done from query syntax (if that's what it's called?) to a LINQ query. See below:
//Query for results. This code is what I'm specifically trying to convert.
var result = (from item in Collection1
where !Collection2.Any(x => x.ColumnName == item.FieldName)
select item).ToList();
//** Remove items in result from Collection1**
//...
I'm really not at all familiar with either syntax (working on it), but I think I generally understand what this is doing. I'm struggling trying to convert this to LINQ syntax though and I'd like to learn both of these options rather than some sort of nested loop.
End goal after I remove the query results from Collection1: Collection1.Count == Collection2 and the following is true for each item in the collection: ItemFromCollection1.Name == SomeItemFromCollection2.Field (if that makes sense...)

You can convert this to LINQ methods like this:
var result = Collection1.Where(item => !Collection2.Any(x => x.ColumnName == item.FieldName))
.ToList();

Your first query is the opposite of what you asked for. It's finding records that don't have an equivalent. The following will return all records in Collection1 where there is an equivalent:
var results=Collection1.Where(c1=>!Collection2.Any(c2=>c2.Field==c1.Name));
Please note that this isn't the fastest approach, especially if there is a large number of records in collection2. You can find ways of speeding it up through HashSets or Lookups.

if you want to get a list of non duplicate values to be retained then do the following.
List<string> listNonDup = new List<String>{"6","1","2","4","6","5","1"};
var singles = listNonDup.GroupBy(n => n)
.Where(g => g.Count() == 1)
.Select(g => g.Key).ToList();
Yields: 2, 4, 5
if you want a list of all the duplicate values then you can do the opposite
var duplicatesxx = listNonDup.GroupBy(s => s)
.SelectMany(g => g.Skip(1)).ToList();

C# and LINQ - arbitrary statement instead of let

Let's say I'm doing a LINQ query like this (this is LINQ to Objects, BTW):
var rows =
from t in totals
let name = Utilities.GetName(t)
orderby name
select t;
So the GetName method just calculates a display name from a Total object and is a decent use of the let keyword. But let's say I have another method, Utilities.Sum() that applies some math on a Total object and sets some properties on it. I can use let to achieve this, like so:
var rows =
from t in totals
let unused = Utilities.Sum(t)
select t;
The thing that is weird here, is that Utilities.Sum() has to return a value, even if I don't use it. Is there a way to use it inside a LINQ statement if it returns void? I obviously can't do something like this:
var rows =
from t in totals
Utilities.Sum(t)
select t;
PS - I know this is probably not good practice to call a method with side effects in a LINQ expression. Just trying to understand LINQ syntax completely.

No, there is no LINQ method that performs an Action on all of the items in the IEnumerable<T>. It was very specifically left out because the designers actively didn't want it to be in there.

Answering the question
No, but you could cheat by creating a Func which just calls the intended method and spits out a random return value, bool for example:
Func<Total, bool> dummy = (total) =>
{
Utilities.Sum(total);
return true;
};
var rows = from t in totals
let unused = dummy(t)
select t;
But this is not a good idea - it's not particularly readable.
The let statement behind the scenes
What the above query will translate to is something similar to this:
var rows = totals.Select(t => new { t, unused = dummy(t) })
.Select(x => x.t);
So another option if you want to use method-syntax instead of query-syntax, what you could do is:
var rows = totals.Select(t =>
{
Utilities.Sum(t);
return t;
});
A little better, but still abusing LINQ.
... but what you should do
But I really see no reason not to just simply loop around totals separately:
foreach (var t in totals)
Utilities.Sum(t);

You should download the "Interactive Extensions" (NuGet Ix-Main) from Microsoft's Reactive Extensions team. It has a load of useful extensions. It'll let you do this:
var rows =
from t in totals.Do(x => Utilities.Sum(x))
select t;
It's there to allow side-effects on a traversed enumerable.

Please, read my comment to the question. The simplest way to achieve such of functionality is to use query like this:
var rows = from t in totals
group t by t.name into grp
select new
{
Name = t.Key,
Sum = grp.Sum()
};
Above query returns IEnumerable object.
For further information, please see: 101 LINQ Samples

Selecting several objects based on array of IDs

I have an array of ProgramIDs and would like to create a number of Select statements dynamically depending on how many ProgramIds there are.
For example:
var surveyProgramVar = surveyProgramRepository.Find().Where(x => x.ProgramId == resultsviewmodel.ProgramIds.FirstOrDefault());
This is an example of the select statement working with a single ProgramId.FirstOrDefault(). How do I create a list/array of SurveyProgramVars and select for each ProgramIds in the array?

It won't be necessarily optimal, but you might try:
var surveyProgramVar = surveyProgramRepository.Find()
.Where(x => resultsviewmodel.ProgramIds.Contains(x.ProgramId));

You could try something like:
var surveyProgramVar = surveyProgramRepository.Find().Where(x => resultsviewmodel.ProgramIds.Contains(x.ProgramId));
Tip: If the Find() method does a hit on a database, would be nice if you create a specific method to to a IN statment on the query. If you does not do this, it will take all records on a table and filter it in memory (linq to objects), which works but not very nice. Your code could be something like:
var surveyProgramVar = surveyProgramRepository.FindByProgramsId(resultsviewmodel.ProgramIds);

How can I set properties on all items from a linq query with values from another object that is also pulled from a query?

I have a query pulling from a database:
List<myClass> items = new List<myClass>(from i in context
select new myClass
{
A = i.A,
B = "", // i doesn't know this, this comes from elsewhere
C = i.C
}
I also have another query doing a similar thing:
List<myClass2> otherItems = new List<myClass2>(from j in context
select new myClass2
{
A = j.A, // A is the intersection, there will only be 1 A here but many A's in items
B = j.B
}
In reality these classes are much larger and query data that is separated not only by database but by server as well. Is it possible to use a LINQ query to populate the property B for all items where items.A intersect? All of the built in LINQ predicates appear only to do aggregates, selections or bool expressions.
In my brain I had something like this, but this is all off:
items.Where(x => x.B = (otherItems.Where(z => z.A == x.A).Single().B));
Or am I being ridiculous with trying to make this work in LINQ and should just abandon it in favor of a for loop where the actual setting becomes trivial? Because of deadlines I will be resorting to the for loop (and it's probably going to end up being a lot more readable in the long run anyway), but is it possible to do this? Would an extension method be necessary to add a special predicate to allow this?

LINQ is designed for querying. If you're trying to set things, you should definitely use a loop (probably foreach). That doesn't mean you won't be able to use LINQ as part of that loop, but you shouldn't be trying to apply a side-effect within LINQ itself.

Query the OtherItems first. Do a ToDictionary() on the result. Then, when querying the database, do this:
var items = from i in context
select new myClass
{ A = i.A,
B = otherItems[i.A],
C = i.C
}

Dynamic linq-to-sql that queries based on multiple keywords

I had been putting together a simple little search.
IEnumerable<Member> searchResults = (from m in members
where m.ScreenName.ToUpper().Contains(upperKeyword)
select m).AsEnumerable();
An then I realized this if the user typed in "keyword1 keyword2", this little query will always search for that exact string. So, I decided I should probably split keywords
string[] keywords = upperKeyword.split(' ');
and then I ran into an issue. I can't really do this:
IEnumerable<Member> searchResults = (from m in members
where m.ScreenName.ToUpper().Contains(keywords) // array of string
select m).AsEnumerable();
because .Contains() doesn't take array. How could I accomplish this?

Try this (untested):
IEnumerable<Member> searchResults = members.ToList().Where(m => keywords.Any(k => m.Summary.Contains(k)))
Edit
Added .ToList(), as I don't think LINQ will be able to convert the above into SQL, so we'll have to perform this in-memory.

For Exact Matches:
Try the inverse of what you have: where keywords.Contains(m.ScreenName)
For reference, Creating IN queries with LINQ-to-SQL
For Partial Matches:
string[] keywords = new[]{ ... };
var results = db.members.Where(m => keywords.Any(sn => m.ScreenName.Contains(sn)));
No compilation error here, but I don't have the data to test against.

I think you need to convert the array to a list. I'm just on my way out of the office, but I think this should work.
IEnumerable<Member> searchResults = (from m in members
where keywords.ToList().Contains(m.ScreenName.ToUpper()) // array of string
select m).AsEnumerable();