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Check if a specific value appears more than once in a table with a LINQ query

I tried with this but it is not what I want :
var result = myList
.GetQueryable()
.GroupBy(g => g.Name)
.Where(w => w.Name == "myName")
.AnyAsync(a => a.Count() > 1);
This should return true if it finds more then 1 record with the name "myName" and false if at most one name "myName" is found.
I get compile time error in the Where clause. But I need to check for a specific value.
Any idea?

There's no need in GroupBy (which will group the entire myList) and in Count() (if we have, say, 123456789 "myName" items then Count() will count them all and only then compare with 1):
var result = myList
.Where(item => item.Name == "myName") // items of interest only
.Skip(1) // Skip the first one
.Any(); // do we have second?
we filter items of interest: Where, Skip the very first of them and check if we have second one.

bool result = (myList.Where(item => item.Name == "myName").Count() > 1)

var differentNames = myList.Select(l => l.name == "myName")
.Distinct()
.Count();
var res = differentNames >= 2;

IQueryable where clause generated from list that needs to be OR'd together

I have a list that I want to generate a query from. I need to get back the items that match each entry in my list and the list uses two values to match against the database. Manually created code would be like this pattern...
from x in Context.Items
where (x.Prop1 == 5 && x.Prop2 == "Foo") ||
(x.Prop1 == 2 && x.Prop2 == "Bar") ||
(x.Prop1 == 9 && x.Prop2 == "Etc")
select x
If I only wanted to compare a single property I would just use the 'list.Contains(x => x.Prop1)' approach but I need to compare on two values and not one. Any ideas?

There is a few easy ways to do it and there are better answers if you search for it. This is my version based on, if the "list" model contains Prop1 and Prop2.
First Solution - This gets Prop1 and Prop2 into their own lists.
var getProp1List = list.Select(i => i.Prop1).ToList();
var getProp2List = list.Select(i => i.Prop2).ToList();
var results = queryList.Where(i => getProp1List.Contains(i.Prop1) && getProp2List.Contains(i.Prop2)).ToList();
Second Solution - This selects multiple objects into one list.
var getSearchTerms = list.Select(i => new { i.Prop1, i.Prop2 }).ToList();
var results = queryList.Where(i => getSearchTerms.Select(x=>x.Prop1).Contains(i.Prop1) && getSearchTerms.Select(x => x.Prop2).Contains(i.Prop2)).ToList();
Third Solution - Simplest - Uses gets the lists of data using select from original list.
var results = queryList.Where(i => list.Select(x=>x.Prop1).Contains(i.Prop1) &&
list.Select(x => x.Prop2).Contains(i.Prop2)).ToList();
And that's if I have, understood the question properly!!

linq - find item in list within multiple lists

I have a highly nested class, and trying to find a single item buried deep within. The following gives me an error "Can't convert type match to bool', although I don't see why it thinks I'm trying to return a boolean.
var match = community.TeamLeagues
.Where(x => x.Seasons
.Where(y => y.Divisions
.Where(z => z.Matches
.Where(a => a.Id == "1234").FirstOrDefault())));

Where by itself returns a (deferred) enumerable of items and cannot as such be used as a condition by the outer Where. What you probably want to do is to use Contains(), Any() or All() inside the outer Wheres that will return the result you're looking for.
Something like this might be what you're after:
var match = community.TeamLeagues.Where(t =>
t.Seasons.Any(
s => s.Divisions.Any(
d => d.Matches.Any(
m => m.Id == "1234")
)));

The Where method needs to evaluate an expression that returns a bool. Your nested Wheres are not doing that - the only Where that is, is the last one a => a.Id == "1234", all the other expressions are returning an IEnumerable.

z.Matches.Where(a => a.Id == "1234").FirstOrDefault() returns a object of type Match(your collection item type of the IEnumerable Matches) (or null), no boolean value. I guess you need to check if there are entires in matches that have a Id 1234. Use Any to evaluate a condition:
var match = community.TeamLeagues.Where(x =>
x.Seasons.Any(y =>
y.Divisions.Any(z =>
z.Matches.Any(a => a.Id == "1234")
)));
[items.Where(x => x.Id == 4).Any() is the same as items.Any(x => x.Id == 4)]
This returns you all TeamLeagues which contain a Season which contain a Division which contain a Match which has a element with the id 1234.

To make it simple you can also use the Matches table directly and using a ViewModel you can represent your view.
like:
var MyViewModel = (from l in Mathes
where l.Id == "1234"
select new MyViewModel
{
Id = l.Id,
MatchName = l.Name,
}).ToList();

Couldn't get it working with linq, but works with query syntax.
var leagueMatch = (from teamLeague in community.TeamLeagues
from season in teamLeague.Seasons
from division in season.Divisions
from match in division.Matches.Where(x => x.Id == "1234")
select match).FirstOrDefault();

Lambda Expression to search with conditions and return only 1 latest result

I have a List that contains Supplier data and I would like to search it by using SupplierID, non-active supplier and only 1 latest result.
So I've got:
List<Supplier> filteredList = this.toList();
filteredList.OrderByDescending(m => m.ModifiedDatetime).FirstOrDefault();
filteredList.Where(f => (f.Active == false && f.FieldId == SupplierFieldID))
.ToList<Supplier>();
But I can't make this work; please help.

You need to chain your LINQ expressions, like this:
var filteredList = unfilteredData
.Where(f => f.Active == false && f.FieldId == SupplierFieldID)
.OrderByDescending(m => m.ModifiedDatetime)
.FirstOrDefault();
You do not need a ToList(), because you need a single item, not a list; this is what FirstOrDefault() does. If you need the last item, you need to order by the reverse of your original ordering condition. For example, if you would like the entry with the latest modified date, you need to order by descending (as you did).

You can do this in one statement, chaining together the LINQ operators:
var filteredList = myList.Where(f => f.Active == false && f.FieldId == SupplierFieldID)
.OrderByDescending(m => m.ModifiedDatetime)
.Take(1);
or as #Preston Guillot suggested, the even shorter form:
var filteredList = unfilteredData
.OrderByDescending(m => m.ModifiedDatetime)
.FirstOrDefault(f => f.Active == false && f.FieldId == SupplierFieldID);

Ternary operator with Linq-to-SQL causing problems?

I have the following Linq-to-SQL statement:
return db.Photos.SingleOrDefault(p => p.PhotoID == id
&& includePending ? true : p.Live);
For includePending I am passing false. In the database, "Live" is true for all but 2 photos.
However instead of returning one photo as expected, it returns ALL photos in the database except for 2! PhotoID is a primary key (thus can only be true for one item) and boolean logic states that FALSE AND TRUE = FALSE. So what's going on here? Why is it ignoring the p.PhotoID == id portion of my query?

I don't remember the full precedence rules by heart, but your condition is equivalent to:
p => (p.PhotoID == id && includePending) ? true : p.Live
whereas you want:
p => p.PhotoID == id && (includePending ? true : p.Live)
Just use the latter form to make it explicit, or even change it to not use the conditional:
p => p.PhotoID == id && (includePending || p.Live)
which I'd argue is simpler. I would suggest that in situations like this you use bracketing to make the logic clearer even when the precedence rules work in your favour.
You could even use two where clauses:
.Where(p => p.PhotoID == id)
.Where(p => includePending || p.live)
or even conditionalise the second:
var query = ...
.Where(p => p.PhotoID == id);
if (!includePending)
{
query = query.Where(p => p.live);
}