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Linq get Distinct ToDictionary

need help to only select/get distinct entries based on i.Code.
There are duplicates and thus I'm getting an error in my expression "An item with the same key has already been added."
var myDictionary = dbContext.myDbTable
.Where(i => i.shoesize>= 4)
.OrderBy(i => i.Code)
.ToDictionary(i => i.Code, i => i);
Have tried to use Select and/or Distinct in different combinations and also by themselves but am still getting the same error
var myDictionary= dbContext.myDbTable
.Where(i => i.shoesize>= 4)
.OrderBy(i => i.Code)
//.Select(i => i)
//.Distinct()
.ToDictionary(i => i.Code, i => i);
Can anybody help? C#
UPDATE: If there are multiple objects with the same code I only want to add the first object(with that particular code) to myDictionary.

You can group by Code and select the first item from each group (which is equivalent to distinct):
var myDictionary = dbContext.myDbTable
.Where(i => i.shoesize >= 4) // filter
.GroupBy(x => x.Code) // group by Code
.Select(g => g.First()) // select 1st item from each group
.ToDictionary(i => i.Code, i => i);
You don't need the OrderBy since Dictionarys represent an unordered collection. If you need an ordered dictionary you could use SortedDictionary.

It sounds to me that what you are looking for is .DistinctBy() (available in .NET 6), which lets you specify which property to distinct the elements in your collection by:
var myDictionary= dbContext.myDbTable
.Where(i => i.shoesize>= 4)
.DistinctBy(i => i.Code)
.ToDictionary(i => i.Code, i => i);

By dividing it and creating a list first it worked as compared to when it was all bundled up into one linq, guess the First() needed it to be in a list before being able to make it into a dict.
var firstLinq = dbContext.myDbTable
.Where(i => i.shoesize>= 4)
.ToList();
then
var finalLinq = fromConcurWithDuplicates
.GroupBy(i => i.Code)
.Select(i => i.First())
.ToList()
.ToDictionary(i => i.Code, i => i);

Cannot access groupBy data Linq C#

Inside method I have a list that contains grouped data:
var listofData = _context.DBONE.where(x => x.Id==3 && x.Status!=0)
.GroupBy(x => new { x.Name, x.Class })
.Select(q => new { Result = q.ToList() }).ToList();
if (methodParam == 10)
{
data = listofData.Where(x => FunctionCheck(---CANNOT ACCESS THE FIELDS FROM GROUP DATA TO PASS AS PARAMETERS---) == 10).ToList();
}
And this is the function that will receive 2 parameter from the grouped data:
private int FunctionCheck(int id, string name)
{...}
But, I cannot access the desired field inside 'listofData'. I can access only in case the listofData is not using groupBy().

If I understand what you are trying to do correctly, you are able to access your data but you are actually creating a "list of a list"
Watch my example, I think I have reproduced your scenario here:
As you can see, I then have a "result" which contains a list of users where Id == 3. The problem is that you create a new anonymous object with a props that is a list. So if you try the last thing you see in my image above, I think you will be able to access your rows.

The reason is that after your GroupBy call, the result is of a grouping type - every item of your list is an Enumerable of the original item, so you would have to operate on that grouping in a following manner:
// Groups such that all items in that group pass your check
listofData
.Where(group => group.All(item => FunctionCheck(item.Id, item.Name) == 10))
.ToList();
// Groups where at least one item matches
listofData
.Where(group => group.Any(item => FunctionCheck(item.Id, item.Name) == 10))
.ToList();
The desired outcome is not really clear from the question but this is the step you are likely missing.
Another approach which might be potentially useful is pre-filter the colleciton of items before grouping them:
var listOfGroupedDatas = _context.DBONE
.Where(x => x.Id ==3 && x.Status != 0 && FunctionCheck(item.Id, item.Name) == 10)
.GroupBy(x => new { x.Name, x.Class })
.ToList();
// This will result in a list of groupings in which all items pass your check

I think you want to call SelectMany to project into one dimensional array.
var listofData = _context.DBONE.where(x => x.Id==3 && x.Status!=0)
.GroupBy(x => new { x.Name, x.Class })
.SelectMany(q => q.ToList()).ToList();

C# LINQ getting duplicates from a list

I have a list of objects that have a string, and int and another int.
I want to be able to create a list of all the objects that have a duplicate string.
Here is what I have so far:
MyObject duplicates = allMyObjects.GroupBy(a => a.MyString)
.Where(a => a.Count() > 1)
.ToList();
The error I am getting is that I cannot implicitly convert the type System.Collections.Generic.List<string, MyObject> to MyObject

var duplicates = allMyObjects.GroupBy(a => a.MyString)
.Where(a => a.Count() > 1)
.SelectMany(g=>g)
.ToList();

you need to write
List<MyObject> duplicates = allMyObjects.GroupBy(a => a.MyString)
.Where(a => a.Count() > 1)
.ToList();

You could use ToLookup to make a nice data structure with all the info you need
var objectsByString = allMyObjects.ToLookup(o => o.MyString);
This will return a Lookup<string, MyObject>. You can get the duplicate strings using:
var duplicateStrings = objectsByString.Where(l => l.Count()>1).Select(l => l.Key);
which will return a IEnumerable<string> with the duplicate strings. And, for each duplicate you can access the actual objects that have duplicates using something like this:
string duplicateKey = duplicateStrings.First();
var duplicateObjects = objectsByString[duplicateKey]
which returns a IEnumerable<MyObject> with the items that have that string.

There are several problem, the first is a List-of-MyObject cannot be assigned to MyObject, so let's use var to ignore this for a second.
var duplicates = allMyObjects.GroupBy(a => a.MyString)
.Where(a => a.Count() > 1)
.ToList();
Now, the type of duplicates is List<IGrouping<string, MyObject>> (despite the incorrectly reported error message). Whoops, gotta get rid of (or write to code to account for) the groups!
var duplicates = allMyObjects.GroupBy(a => a.MyString)
.Where(a => a.Count() > 1)
.SelectMany(g => g)
.ToList();
Now the type of duplicates is List<MyObject>, after having selected every ("selected many") object from every group with more than one item. Better, but this still isn't an MyObject. Well, that's fine: fix the declared type of the variable (that var was previously automatically doing)..
List<MyObject> duplicates = /* same as before */;
Or leave var to do it's thing and if an IEnumerable<MyObject> is fine, simply omit the ToList:
var duplicates = allMyObjects.GroupBy(a => a.MyString)
.Where(a => a.Count() > 1)
.SelectMany(g => g);
Go forth and iterate thy duplicates!

C# LINQ get count to dictionary

I got a LINQ query with Entity Framework (EF) and getting a list of items. Now I want to create a dictionary with the incrementing index of the item and the item itself.
I have it like this:
result = context
.Items
.Where(b => !b.Deleted)
.OrderBy(b => b.Name)
.ToDictionary(COUNTHERE, b => b.Name)
So the dictionary have to look like this:
1: "item1"
2: "item2"
3: "item5"
4: "item10"
5: "item100"

I think what you need is to have the item name as the key instead the count as the key, because if there is two items that have the same count, it will throw exception that the key has been added.
Then you can Use GroupBy before ToDictionary so that you can count it.
result = context
.Items
.Where(b => !b.Deleted)
.GroupBy(x => x.Name)
.ToDictionary(g => g.Key, g => g.Count())
.OrderBy(g => g.Key);
based on your updated comment, then what you need is
result = context
.Items
.Where(b => !b.Deleted)
.OrderBy(b => b.Name)
.AsEnumerable()
.Select((v,i) => new { i, v.Name })
.ToDictionary(g => g.i + 1, g => g.Name);
Note that you need to add AsEnumerable so that the Select clause works as linq to object (Select that accept index is not supported in L2S).

Just use:
int count = 0;
var result = context
.Items
.Where(b => !b.Deleted)
.OrderBy(b => b.Name)
.ToDictionary(b => ++count, b => b.Name);

An alternative way of achieving this is:
var sortedNames = context
.Items
.Where(b => !b.Deleted)
.Select(b => b.Name)
.OrderBy(b => b.Name)
.ToArray();
result = Enumerable.Range(1, sortedNames.Length)
.ToDictionary(i => i, i => sortedNames[i - 1]);
To get zero-based numbering, use Enumerable.Range(0, sortedNames.Length) and sortedNames[i] instead.

How to retrieve the specific indexes from array?

I have this array :
Point[] arr = samples.pointsArray;
I using this row to retrieve all elements that satisfy condition:
var maxXCol = arr.Where( p => maxX.X == p.X );
Any idea how to modify row above, to get only the indexes of these elements?
Thank you in advance!

EDIT
Use the version of Select that takes both the index and the object and create an anonymous object with the object and index inside it. It would look like this:
someEnumerable.Select((obj, idx) => new {Item = obj, Index = idx})
You'll need to do this before you use Where so that the original index remains intact after the filter operation.
In the following operations you can use the item like so:
x => x.Item
and the index like so:
x => x.Index

var maxXCol = arr.Select((p, inx) => new { p,inx})
.Where(y => maxX.X == y.p.X)
.Select(z => z.inx);

You may select the value first with the index in anonymous type, later you filter it with your condition and then select the index.
var result = arr.Select((g, index) => new { g, index })
.Where(r => maxX.X == r.X)
.Select(t => t.index);

You can use Select overload which takes an index, and project that index together with the original row. Then take only the index for the result collection.
var maxXCol = arr
.Select((p, index) => new { Item = p, Index = index })
.Where(p => maxX.X == p.Item.X)
.Select(x => x.Index);

Try this:
arr.Select((e,i)=>new{index=i, value=e}).Where(ei=>ei.value.X==maxX.X).Select(ei=>ei.index);

var maxXCol = arr
.Select((a, b) => new { b, a })
.Where(p => maxX.X == p.a.X)
.Select(i=>i.b);