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LINQ groupby search in contains

I have the following result:
var result = (from p1 in db.Table
select new ReportInform
{
DataValue = p1.DataValue,
SampleDate = p1.SampleDate
})
.Distinct()
.ToList();
// Next getting list of duplicate SampleDates
var duplicates = result.GroupBy(x => x.SampleDate)
.Where(g => g.Count() > 1)
.Select (x => x)
.ToList();
foreach (var r in result)
{
if (duplicates.Contains(r.SampleDate)) // get error here on incompatbility
{
r.SampleDate = r.SampleDate.Value.AddMilliseconds(index++);
}
}

Cannot convert from 'System.DateTime?' to 'System.Linq.IGrouping
That error is pretty clear but may not be at a first glance. As a programmer, you need to learn how to read, understand and make sense of compiler or runtime errors.
Anyhow it is complaining that it cannot convert DateTime? to System.Linq.IGrouping<System.DateTime, ReportInForm>. Why? Because this query returns an System.Linq.IGrouping<System.DateTime, ReportInForm>
var duplicates = result.GroupBy(x => x.SampleDate)
.Where(g => g.Count() > 1)
.Select (x => x)
.ToList();
The GroupBy method returns IGrouping<System.DateTime, ReportInForm> which has a Key and the Key is the thing you grouped by and a list of items in that group. You are grouping by SampleDate and checking if there are more than one items in that group and then selecting the group. Thus dulplicates has a list of IGrouping<System.DateTime, ReportInForm> and you are asking the runtime to check if it contains a DateTime? and it blows up at this line:
duplicates.Contains(r.SampleDate)
One way to fix this is: What you want to do is to select the key of that group. Thus do this:
.Select (x => x.Key)

If you are expecting duplicates to be of type List<DateTime?> then you meant to write this
.Select(x => x.Key)
instead of
.Select(x => x)

How to do in this in Linq C#

So far, I have this:
var v = Directory.EnumerateFiles(_strConfigurationFolder)
.GroupBy(x => GetReportName(Path.GetFileNameWithoutExtension(x)));
Configuration folder will contain pairs of files:
abc.json
abc-input.json
def.json
def-input.json
GetReportName() method strips off the "-input" and title cases the filename, so you end up with a grouping of:
Abc
abc.json
abc-input.json
Def
def.json
def-input.json
I have a ReportItem class that has a constructor (Name, str1, str2). I want to extend the Linq to create the ReportItems in a single statement, so really something like:
var v = Directory.EnumerateFiles(_strConfigurationFolder)
.GroupBy(x => GetReportName(Path.GetFileNameWithoutExtension(x)))
**.Select(x => new ReportItem(x.Key, x[0], x[1]));**
Obviously last line doesn't work because the grouping doesn't support array indexing like that. The item should be constructed as "Abc", "abc.json", "abc-input.json", etc.

If you know that each group of interest contains exactly two items, use First() to get the item at index 0, and Last() to get the item at index 1:
var v = Directory.EnumerateFiles(_strConfigurationFolder)
.GroupBy(x => GetReportName(Path.GetFileNameWithoutExtension(x)))
.Where(g => g.Count() == 2) // Make sure we have exactly two items
.Select(x => new ReportItem(x.Key, x.First(), x.Last()));

var v = Directory.EnumerateFiles(_strConfigurationFolder)
.GroupBy(x => GetReportName(Path.GetFileNameWithoutExtension(x))).Select(x => new ReportItem(x.Key, x.FirstOrDefault(), x.Skip(1).FirstOrDefault()));
But are you sure there will be exactly two items in each group? Maybe has it sence for ReportItem to accept IEnumerable, not just two strings?

c# and LINQ - convert IGrouping to List

I have the following code written to find common objects in a list of objects
https://dotnetfiddle.net/gCgNBf
..............................
var query = setOfPersons
.SelectMany(l => l.Select(l1 => l1))
.GroupBy(p => p.Id)
.Where(g => g.Count() == setOfPersons.Count);
After that, I need to convert "query" to a list of "Person" objects ( List ) to achieve something else.
I tried using "ToList()"... But it says:
" cannot convert IGrouping to a List ".
Can someone help me to fix it ?

Looking at your code it seems that what you are trying to achieve is to get the list of people that exist in each list. If so, you can use the following query:
var query = setOfPersons
.SelectMany(l => l.Select(l1 => l1))
.GroupBy(p => p.Id)
.Where(g => g.Count() == setOfPersons.Count)
.Select(x=>x.First()) // Select first person from the grouping - they all are identical
.ToList();
Console.WriteLine("These people appears in all set:");
foreach (var a in query)
{
Console.WriteLine("Id: {0} Name: {1}", a.Id, a.Name);
}
Here you select just a single item from each grouping, because they all are identical.

Remove every first element of grouped collection

I have a collection of elements and some of these elements are duplicating. I need to extract all records but only the first record if the record is one of a duplicate set.
I was able to group the elements and find all elements that have duplicates, but how to remove every first element of a group?
var records =
dbContext.Competitors
.GroupBy(x => x.Email)
.Select(x => new { Properties = x,
Count = x.Key.Count() })
.Where(x => x.Count > 1)
.ToList();
EDIT: Seems like it's impossible to accomplish this task with EF, because it fails to translate the desired linq expression to SQL. I'll be happy if someone offer different approach.

To exclude the first record from each email-address group with more than one entry, you could do this:
var records = dbContext.Competitors
.GroupBy(x => x.Email)
.SelectMany(x => (x.Count() == 1) ? x : x.OrderBy(t=>t).Skip(1))
.ToList();

This is the logic :
Group by a property > Select every Group > (Possibly) Sort that > Skip first one
This can be turned into some linq code like this :
//use SelectMany to flat the array
var x = list.GroupBy(g => g.Key).Select(grp => grp.Skip(1)).SelectMany(i => i);

Modifying an IEnumerable type

I have a a string IEnumerable type that I get from the below code.The var groups is an Enumerable type which has some string values. Say there are 4 values in groups and in the second position the value is just empty string "" .The question is how can I move it to the 4th ie the end position.I do not want to sort or change any order.Just move the empty "" value whereever it occurs to the last position.
List<Item> Items = somefunction();
var groups = Items.Select(g => g.Category).Distinct();

Simply order the results by their string value:
List<Item> Items = somefunction();
var groups = Items.Select(g => g.Category).Distinct().OrderByDescending(s => s);
Edit (following OP edit):
List<Item> Items = somefunction();
var groups = Items.Select(g => g.Category).Distinct();
groups = groups.Where(s => !String.IsNullOrEmpty(s))
.Concat(groups.Where(s => String.IsNullOrEmpty(s)));

You can't directly modify the IEnumerable<> instance, but you can create a new one:
var list = groups.Where(x => x != "").Concat(groups.Where(x => x == ""));
Note that in this query, groups is iterated twice. This is usually not a good practice for a deferred IEnumerable<>, so you should call ToList() after the Distinct() to eagerly evaluate your LINQ query:
var groups = Items.Select(g => g.Category).Distinct().ToList();
EDIT :
On second thought, there's a much easier way to do this:
var groups = Items.Select(g => g.Category).Distinct().OrderBy(x => x == "");
Note that this doesn't touch the order of the non-empty elements since OrderBy is stable.

var groups = Items.Select(g => g.Category).Distinct().OrderByDescending(s =>s);

I don't like my query but it should do the job. It selects all items which are not empty and unions it with the items which are empty.
var groups = Items.Select(g => g.Category).Distinct()
.Where(s => !string.IsNullOrEmpty(s))
.Union(Items.Select(g => g.Category).Distinct()
.Where(s => string.IsNullOrEmpty(s)));

Try something like
var temp = groups.Where(item => ! String.IsNullOrEmpty(item)).ToList<string>();
while (temp.Count < groups.Count) temp.Add("");