I am playing with LINQ to learn about it, but I can't figure out how to use Distinct when I do not have a simple list (a simple list of integers is pretty easy to do, this is not the question). What I if want to use Distinct on a List<TElement> on one or more properties of the TElement?
Example: If an object is Person, with property Id. How can I get all Person and use Distinct on them with the property Id of the object?
Person1: Id=1, Name="Test1"
Person2: Id=1, Name="Test1"
Person3: Id=2, Name="Test2"
How can I get just Person1 and Person3? Is that possible?
If it's not possible with LINQ, what would be the best way to have a list of Person depending on some of its properties?
What if I want to obtain a distinct list based on one or more properties?
Simple! You want to group them and pick a winner out of the group.
List<Person> distinctPeople = allPeople
.GroupBy(p => p.PersonId)
.Select(g => g.First())
.ToList();
If you want to define groups on multiple properties, here's how:
List<Person> distinctPeople = allPeople
.GroupBy(p => new {p.PersonId, p.FavoriteColor} )
.Select(g => g.First())
.ToList();
Note: Certain query providers are unable to resolve that each group must have at least one element, and that First is the appropriate method to call in that situation. If you find yourself working with such a query provider, FirstOrDefault may help get your query through the query provider.
Note2: Consider this answer for an EF Core (prior to EF Core 6) compatible approach. https://stackoverflow.com/a/66529949/8155
EDIT: This is now part of MoreLINQ.
What you need is a "distinct-by" effectively. I don't believe it's part of LINQ as it stands, although it's fairly easy to write:
public static IEnumerable<TSource> DistinctBy<TSource, TKey>
(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector)
{
HashSet<TKey> seenKeys = new HashSet<TKey>();
foreach (TSource element in source)
{
if (seenKeys.Add(keySelector(element)))
{
yield return element;
}
}
}
So to find the distinct values using just the Id property, you could use:
var query = people.DistinctBy(p => p.Id);
And to use multiple properties, you can use anonymous types, which implement equality appropriately:
var query = people.DistinctBy(p => new { p.Id, p.Name });
Untested, but it should work (and it now at least compiles).
It assumes the default comparer for the keys though - if you want to pass in an equality comparer, just pass it on to the HashSet constructor.
Use:
List<Person> pList = new List<Person>();
/* Fill list */
var result = pList.Where(p => p.Name != null).GroupBy(p => p.Id)
.Select(grp => grp.FirstOrDefault());
The where helps you filter the entries (could be more complex) and the groupby and select perform the distinct function.
You could also use query syntax if you want it to look all LINQ-like:
var uniquePeople = from p in people
group p by new {p.ID} //or group by new {p.ID, p.Name, p.Whatever}
into mygroup
select mygroup.FirstOrDefault();
I think it is enough:
list.Select(s => s.MyField).Distinct();
Solution first group by your fields then select FirstOrDefault item.
List<Person> distinctPeople = allPeople
.GroupBy(p => p.PersonId)
.Select(g => g.FirstOrDefault())
.ToList();
Starting with .NET 6, there is new solution using the new DistinctBy() extension in Linq, so we can do:
var distinctPersonsById = personList.DistinctBy(x => x.Id);
The signature of the DistinctBy method:
// Returns distinct elements from a sequence according to a specified
// key selector function.
public static IEnumerable<TSource> DistinctBy<TSource, TKey> (
this IEnumerable<TSource> source,
Func<TSource, TKey> keySelector);
You can do this with the standard Linq.ToLookup(). This will create a collection of values for each unique key. Just select the first item in the collection
Persons.ToLookup(p => p.Id).Select(coll => coll.First());
The following code is functionally equivalent to Jon Skeet's answer.
Tested on .NET 4.5, should work on any earlier version of LINQ.
public static IEnumerable<TSource> DistinctBy<TSource, TKey>(
this IEnumerable<TSource> source, Func<TSource, TKey> keySelector)
{
HashSet<TKey> seenKeys = new HashSet<TKey>();
return source.Where(element => seenKeys.Add(keySelector(element)));
}
Incidentially, check out Jon Skeet's latest version of DistinctBy.cs on Google Code.
Update 2022-04-03
Based on an comment by Andrew McClement, best to take John Skeet's answer over this one.
I've written an article that explains how to extend the Distinct function so that you can do as follows:
var people = new List<Person>();
people.Add(new Person(1, "a", "b"));
people.Add(new Person(2, "c", "d"));
people.Add(new Person(1, "a", "b"));
foreach (var person in people.Distinct(p => p.ID))
// Do stuff with unique list here.
Here's the article (now in the Web Archive): Extending LINQ - Specifying a Property in the Distinct Function
Personally I use the following class:
public class LambdaEqualityComparer<TSource, TDest> :
IEqualityComparer<TSource>
{
private Func<TSource, TDest> _selector;
public LambdaEqualityComparer(Func<TSource, TDest> selector)
{
_selector = selector;
}
public bool Equals(TSource obj, TSource other)
{
return _selector(obj).Equals(_selector(other));
}
public int GetHashCode(TSource obj)
{
return _selector(obj).GetHashCode();
}
}
Then, an extension method:
public static IEnumerable<TSource> Distinct<TSource, TCompare>(
this IEnumerable<TSource> source, Func<TSource, TCompare> selector)
{
return source.Distinct(new LambdaEqualityComparer<TSource, TCompare>(selector));
}
Finally, the intended usage:
var dates = new List<DateTime>() { /* ... */ }
var distinctYears = dates.Distinct(date => date.Year);
The advantage I found using this approach is the re-usage of LambdaEqualityComparer class for other methods that accept an IEqualityComparer. (Oh, and I leave the yield stuff to the original LINQ implementation...)
You can use DistinctBy() for getting Distinct records by an object property. Just add the following statement before using it:
using Microsoft.Ajax.Utilities;
and then use it like following:
var listToReturn = responseList.DistinctBy(x => x.Index).ToList();
where 'Index' is the property on which i want the data to be distinct.
You can do it (albeit not lightning-quickly) like so:
people.Where(p => !people.Any(q => (p != q && p.Id == q.Id)));
That is, "select all people where there isn't another different person in the list with the same ID."
Mind you, in your example, that would just select person 3. I'm not sure how to tell which you want, out of the previous two.
In case you need a Distinct method on multiple properties, you can check out my PowerfulExtensions library. Currently it's in a very young stage, but already you can use methods like Distinct, Union, Intersect, Except on any number of properties;
This is how you use it:
using PowerfulExtensions.Linq;
...
var distinct = myArray.Distinct(x => x.A, x => x.B);
When we faced such a task in our project we defined a small API to compose comparators.
So, the use case was like this:
var wordComparer = KeyEqualityComparer.Null<Word>().
ThenBy(item => item.Text).
ThenBy(item => item.LangID);
...
source.Select(...).Distinct(wordComparer);
And API itself looks like this:
using System;
using System.Collections;
using System.Collections.Generic;
public static class KeyEqualityComparer
{
public static IEqualityComparer<T> Null<T>()
{
return null;
}
public static IEqualityComparer<T> EqualityComparerBy<T, K>(
this IEnumerable<T> source,
Func<T, K> keyFunc)
{
return new KeyEqualityComparer<T, K>(keyFunc);
}
public static KeyEqualityComparer<T, K> ThenBy<T, K>(
this IEqualityComparer<T> equalityComparer,
Func<T, K> keyFunc)
{
return new KeyEqualityComparer<T, K>(keyFunc, equalityComparer);
}
}
public struct KeyEqualityComparer<T, K>: IEqualityComparer<T>
{
public KeyEqualityComparer(
Func<T, K> keyFunc,
IEqualityComparer<T> equalityComparer = null)
{
KeyFunc = keyFunc;
EqualityComparer = equalityComparer;
}
public bool Equals(T x, T y)
{
return ((EqualityComparer == null) || EqualityComparer.Equals(x, y)) &&
EqualityComparer<K>.Default.Equals(KeyFunc(x), KeyFunc(y));
}
public int GetHashCode(T obj)
{
var hash = EqualityComparer<K>.Default.GetHashCode(KeyFunc(obj));
if (EqualityComparer != null)
{
var hash2 = EqualityComparer.GetHashCode(obj);
hash ^= (hash2 << 5) + hash2;
}
return hash;
}
public readonly Func<T, K> KeyFunc;
public readonly IEqualityComparer<T> EqualityComparer;
}
More details is on our site: IEqualityComparer in LINQ.
If you don't want to add the MoreLinq library to your project just to get the DistinctBy functionality then you can get the same end result using the overload of Linq's Distinct method that takes in an IEqualityComparer argument.
You begin by creating a generic custom equality comparer class that uses lambda syntax to perform custom comparison of two instances of a generic class:
public class CustomEqualityComparer<T> : IEqualityComparer<T>
{
Func<T, T, bool> _comparison;
Func<T, int> _hashCodeFactory;
public CustomEqualityComparer(Func<T, T, bool> comparison, Func<T, int> hashCodeFactory)
{
_comparison = comparison;
_hashCodeFactory = hashCodeFactory;
}
public bool Equals(T x, T y)
{
return _comparison(x, y);
}
public int GetHashCode(T obj)
{
return _hashCodeFactory(obj);
}
}
Then in your main code you use it like so:
Func<Person, Person, bool> areEqual = (p1, p2) => int.Equals(p1.Id, p2.Id);
Func<Person, int> getHashCode = (p) => p.Id.GetHashCode();
var query = people.Distinct(new CustomEqualityComparer<Person>(areEqual, getHashCode));
Voila! :)
The above assumes the following:
Property Person.Id is of type int
The people collection does not contain any null elements
If the collection could contain nulls then simply rewrite the lambdas to check for null, e.g.:
Func<Person, Person, bool> areEqual = (p1, p2) =>
{
return (p1 != null && p2 != null) ? int.Equals(p1.Id, p2.Id) : false;
};
EDIT
This approach is similar to the one in Vladimir Nesterovsky's answer but simpler.
It is also similar to the one in Joel's answer but allows for complex comparison logic involving multiple properties.
However, if your objects can only ever differ by Id then another user gave the correct answer that all you need to do is override the default implementations of GetHashCode() and Equals() in your Person class and then just use the out-of-the-box Distinct() method of Linq to filter out any duplicates.
Override Equals(object obj) and GetHashCode() methods:
class Person
{
public int Id { get; set; }
public int Name { get; set; }
public override bool Equals(object obj)
{
return ((Person)obj).Id == Id;
// or:
// var o = (Person)obj;
// return o.Id == Id && o.Name == Name;
}
public override int GetHashCode()
{
return Id.GetHashCode();
}
}
and then just call:
List<Person> distinctList = new[] { person1, person2, person3 }.Distinct().ToList();
The best way to do this that will be compatible with other .NET versions is to override Equals and GetHash to handle this (see Stack Overflow question This code returns distinct values. However, what I want is to return a strongly typed collection as opposed to an anonymous type), but if you need something that is generic throughout your code, the solutions in this article are great.
List<Person>lst=new List<Person>
var result1 = lst.OrderByDescending(a => a.ID).Select(a =>new Player {ID=a.ID,Name=a.Name} ).Distinct();
You should be able to override Equals on person to actually do Equals on Person.id. This ought to result in the behavior you're after.
If you use old .NET version, where the extension method is not built-in, then you may define your own extension method:
public static class EnumerableExtensions
{
public static IEnumerable<T> DistinctBy<T, TKey>(this IEnumerable<T> enumerable, Func<T, TKey> keySelector)
{
return enumerable.GroupBy(keySelector).Select(grp => grp.First());
}
}
Example of usage:
var personsDist = persons.DistinctBy(item => item.Name);
Definitely not the most efficient but for those, who are looking for a short and simple answer:
list.Select(x => x.Id).Distinct().Select(x => list.First(y => x == y.Id)).ToList();
Please give a try with below code.
var Item = GetAll().GroupBy(x => x .Id).ToList();
I would like to make a sorting extension method which will take a Generic Collection and sort it using one or more keys. The keys will be properties of the collection's containing objects.
A sample LINQ query with 3 keys looks like this.
studentResults.OrderBy(x => x.CG).ThenBy(x => x.Student.Roll)
.ThenBy(x => x.Student.Name).ToList();
I have already found something which can do this with one key.
public static List<TSource> OrderByAsListOrNull<TSource, TKey>(
this ICollection<TSource> collection, Func<TSource,TKey> keySelector)
{
if (collection != null && collection.Count > 0) {
return collection
.OrderBy(x => keySelector(x))
.ToList();
}
return null;
}
I thought of using IEnumerable<Func<TSource, TKey> keySelector>, but I cannot call the function like that.
So, how may I implement a method of this kind?
In theory, you could build a multi-levelled sort extension, which diffentiates between the initial OrderBy and the subsequent ThenBys for secondary, tertiary sorting tiebreakers. Since by taking multiple order functions, each of which could reference a different type, you'll need to soften the projected type (I've used object, below).
public static class Extensions
{
public static IEnumerable<T> MyOrderBy<T>(
this IEnumerable<T> source,
params Func<T, object>[] orders)
{
Debug.Assert(orders.Length > 0);
var sortQuery = source.OrderBy(orders[0]);
foreach(var order in orders.Skip(1))
{
sortQuery = sortQuery.ThenBy(order);
}
return sortQuery;
}
}
public class Poco
{
public string Name {get; set;}
public int Number {get; set;}
}
void Main()
{
var items = new []{
new Poco{Name = "Zebra", Number = 99},
new Poco{Name = "Apple", Number = 123}};
foreach(var poco in items.MyOrderBy(i => i.Number, i => i.Name))
{
Console.WriteLine(poco.Name);
}
}
The problem with this (as with your original function) is that you'll probably want to order by descending at some point. Although for numeric sort functions this could be hacked by passing a *-1, it's going to be really difficult to do this for an arbitrary type
// Hack : Order a numeric descending
item => item.Number * -1
For me, I would just stay with Linq's sorting extensions, and not try to abstract them in any way!
Let's say I have the following object:
public class MyObject
{
public string MyValue { get; set; }
}
And in another class I have a list of these objects:
public class MyClass
{
private List<MyObject> _list;
public MyClass(List<MyObject> myObjects)
{
_list = myObjects;
}
public bool AllUniqueValues()
{
...
}
}
I want to check if all MyObjects in the list have an unique (non-duplicated) Value. When I use the following it works:
public bool AllUnique()
{
return _list.All(x => _list.Count(y => String.Equals(y.Value, x.Value)) == 1);
}
But I have the feeling this can be done easier / more elegant. So, my question, is there a better / more elegant approach to check if all MyObjects have a non-duplicated Value, and if so, how?
I find this quite elegant:
public static class EnumerableExtensions
{
public static bool AllUnique<TSource, TResult>(this IEnumerable<TSource> enumerable,
Func<TSource, TResult> selector)
{
var uniques = new HashSet<TResult>();
return enumerable.All(item => uniques.Add(selector(item)));
}
}
And now your code becomes:
var allUnique = _list.AllUnique(i => i.MyValue);
One of many way to do it:
return !_list.GroupBy(c=>c.MyValue).Any(c=>c.Count() > 1);
At least it is a little bit more clear.
The most elegant way of solving this is using a set data structure. An unordered collection of unique elements. In .NET, you need to use HashSet<T>.
You can either override Equals and GetHashCode of MyObject to provide what equality means in your case, or implement an IEqualityComparer<T>.
If you instantiate HashSet<T> and you don't provide an IEqualityComparer<T> implementation, then it will use your overrides, otherwise it will use the whole implementation. Usually you implement equality comparers if there're more than a meaning of equality for the same object.
I might still need an ordered collection of elements
If you still need to store your objects in order, you can both store the elements in both the HashSet<T> and List<T> in parallel. What you get with HashSet<T> is a practically O(1) access to your items when you need check if an item exists, get one or perform some supported operations in the collection, since it's a hashed collection, it won't need to iterate it entirely to find the element.
There are many ways to do it, but personally, I'd do the following:
public bool AllUnique()
{
return _list.GroupBy(x => x.MyValue).Count() == _list.Count();
}
I need to get a case-insensitive list from the results of an entity framework query. I have the following:
var myList = myEF.GroupBy(e => new { e.Code, e.Description })
.Select(e => e.First())
.ToList();
That gives me a distinct list, but it's case-sensitive. I need case-insensitive.
I figured I should be able to do something like this:
var myList = myEF.GroupBy(e => new { e.Code, e.Description }, StringComparer.InvariantCultureIgnoreCase)
.Select(e => e.First())
.ToList();
But that doesn't seem to want to work with the anonymous object.
Adding .ToLower or .ToLowerInvariant doesn't seem to work either. Nor does using Distinct(StringComparer.InvariantCultureIgnoreCase) instead of the GroupBy.
Seems there ought to be an easy way to do this, but I'm not finding it.
I tried various methods you said and all of them did fail. I got this working however:
var distinct = list.Distinct(new CaseInsensitiveComparer());
public class CaseInsensitiveComparer : IEqualityComparer<A>
{
public bool Equals(A x, A y)
{
return x.Code.Equals(y.Code, StringComparison.OrdinalIgnoreCase) &&
x.Description.Equals(y.Description, StringComparison.OrdinalIgnoreCase);
}
public int GetHashCode(A obj)
{
return obj.Code.ToLowerInvariant().GetHashCode();
}
}
Feel free to tweak to your needs.
Fiddle
You will likely need 2 passes for this query.
First query for the conversion to lowercase, second one for the grouping.
Only escapes would be that you could extract in uppercase immediately from Entity Framework.
Or with additional EF/LINQ features that are still under developement :(
Messy, but it works:
private class StructuralTupleComparer<T>: IEqualityComparer<Tuple<T, T>>{
private IEqualityComparer<T> _cmp;
public StructuralTupleComparer(IEqualityComparer<T> cmp){
this._cmp = cmp
}
public bool Equals(Tuple<T, T> t1, Tuple<T, T> t2)
{
return _cmp(t1.Item1, t2.Item1) && _cmp(t1.Item2, t2.Item2);
}
public int GetHashCode(Tuple<T, T> t)
{
return _cmp.GetHashCode(t.Item1) ^ _cmp.GetHashCode(t.Item2)
}
}
and then
var myList = myEF.GroupBy(e => new Tuple<String, String>(e.Code, e.Description),
new StructuralTupleComparer(StringComparer.InvariantCultureIgnoreCase))
.Select(e => e.First())
.ToList();
and hope that in some point in the glorious future there will be a static IGrouping GroupBy<T, U>(this IEnumerable<T> src, Func<T, U> groupingprojection, Func<U, bool> equalitytester) extension method (for Great Justice)
I have a class
public class Foo
{
public int ID { get; set; }
}
and I've implemented a LinqEqualityComparer to allow dynamic IEqualityComparer tests for the Except extenion method.
public class LinqEqualityComparer<T> : IEqualityComparer<T>
{
protected Func<T, T, bool> Comparison { get; set; }
public LinqEqualityComparer(Func<T, T, bool> comparison)
{
Comparison = comparison;
}
public bool Equals(T x, T y)
{
return Comparison(x, y);
}
public int GetHashCode(T obj)
{
return obj.GetHashCode();
}
}
I've created the following code to test it:
IEnumerable<Foo> settings = new Foo[]
{
new Foo{ID = 1},
new Foo{ID = 2}
};
IEnumerable<Foo> currentSettings = new Foo[]
{
new Foo{ID = 1},
new Foo{ID = 2},
new Foo{ID = 3}
};
IEqualityComparer<Foo> comparer = new LinqEqualityComparer<Foo>((x, y) => x.ID == y.ID);
IEnumerable<Foo> missing = currentSettings.Except(settings, comparer);
However Foos 1,2 and 3 are all present in the 'missing' variable.
Why does this LinqEqualityComparer not work?
Because your equality comparer does not implement GetHashCode correctly. The GetHashCode implementation must produce the same code for elements that compare equal. This does not happen here because the equality comparison is customized while the hash codes are not generated accordingly.
To make this work you would need to do one of two things:
Make the comparer accept the hash code implementation as an additional argument, i.e. x => x.ID.GetHashCode() and forward to that. This is easiest and what you should do in practice.
Modify GetHashCode in such a way that it is an aggregate function of the hash codes of the properties that take part in the comparison (here that is the ID property) -- a straight xor of the individual hash codes would work (even though it might not be optiomal).
That leaves you with the problem of how to detect which
properties are compared. To be able to answer that question automatically you would need to accept an expression tree instead of a delegate for the comparison, i.e. an Expression<Func<T, T, bool>> and then visit the expression tree to determine what to do. That's bound to not be easy going.