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 have a class and list:
public class className
{
public string firstParam { get; set; }
public string secondParam { get; set; }
}
public static List<className> listName = new List<className>();
The list includes (for example):
Apple Banana
Corn Celery
Corn Celery
Corn Grapes
Raisins Pork
I am trying to edit the list (or create a new list) to get:
Apple Banana
Corn Celery
Corn Grapes
Raisins Pork
I have tried:
var listNoDupes = listName.Distinct();
And:
IEnumerable<className> listNoDupes = listName.Distinct();
But both return the list in the same condition as before, with duplicates.
You need to override/implement Equals() and GetHashCode(), right now you are listing distinct instances and they are correctly ALL distinct/unique from each other.
The problem you are running into is the identity of the objects is not what you think. Your intuition is telling you that the identity is the combination of firstParam and secondParam. What truly is happening is each distinct instance of className has its own identity that does not rely on the implementation of the object. You will need to override the methods provided via System.Object, mainly Equals and GetHashCode although you might get away with not overriding GetHashCode (this will be needed for hash sets to work properly.)
If your class only contains those two fields then instead of implementing Equals and GetHashCode You can also do:
var listNoDupes = listName.GroupBy(r => new { r.firstParam, r.secondParam })
.Select(grp => grp.First())
.ToList();
Or you can get an IEnumerable<T> back like:
IEnumerable<className> listNoDupes =
listName
.GroupBy(r => new { r.firstParam, r.secondParam })
.Select(grp => grp.First());
The code above would group on the properties firstParam and secondParam, later grp.First would return you a single item from the group and you will end up a single item from each group, (no duplicates)
There is the third possibility - use Distinct method version that takes IEqualityComparer. Unfortunately, C# does not support creating anonymous, temporary implementations of interfaces. We can create helper class and extension:
public static class IEnumerableExtensions
{
public class LambdaEqualityComparer<T> : IEqualityComparer<T>
{
private Func<T, T, bool> comparer;
private Func<T, int> hash;
public LambdaEqualityComparer(Func<T, T, bool> comparer,
Func<T, int> hash)
{
this.comparer = comparer;
this.hash = hash;
}
public bool Equals(T x, T y)
{
return comparer(x, y);
}
public int GetHashCode(T x)
{
return hash(x);
}
}
public static IEnumerable<T> Distinct<T>(this IEnumerable<T> elems,
Func<T, T, bool> comparer,
Func<T, int> hash)
{
return elems.Distinct(new LambdaEqualityComparer<T>(comparer, hash));
}
}
and then we can provide lambdas for Distinct method:
var filteredList = myList.Distinct((x, y) => x.firstParam == y.firstParam &&
x.secondParam == y.secondParam,
x => 17 * x.firstParam.GetHashCode() + x.secondParam.GetHashCode());
This allows you to distinct objects on single shot, without implementing Equals and GetHashCode. If, for example, there is a single place in the project, where you are calling such Distinct, this is probably enough to use this extension. If, on the other hand, identity of the className objects is a concept that spans through many methods and classes, for sure it will be better to define simply Equals and GetHashCode.
I'm trying to build a generic GroupBy Method, I guess it should be something like this
var result = GenericGroupBy<List<DTO>>(dataList, g=>g.Id);
public object GenericGroupBy<T>(object data, Func<T, bool> groupByExpression)
{
return ((List<T>)data).GroupBy(groupByExpression);
}
But I can not make it work.
How to pass expression like g=>g.Id?
Currently there are two problems:
Your method expects a Func<T, bool> and I suspect g => g.Id fails that because your Id property isn't a bool
You're currently specifying List<DTO> as the type argument, when I suspect you really want just DTO.
Given your comments, this will work:
var result = GenericGroupBy<DTO>(dataList, g => g.Id);
public object GenericGroupBy<T>(object data, Func<T, int> groupByExpression)
{
return ((List<T>)data).GroupBy(groupByExpression);
}
... but I'd make it a bit more general unless you always want to group by int:
var result = GenericGroupBy<DTO, int>(dataList, g => g.Id);
public object GenericGroupBy<TElement, TKey>
(object data, Func<TElement, TKey> groupByExpression)
{
return ((IEnumerable<TElement>)data).GroupBy(groupByExpression);
}
Note how I've also changed the cast from List<T> to IEnumerable<T> - you don't need it to be a List<T>, so why cast to that?
I have a Dictionary which I want to filter by different conditions, e.g.
IDictionary<string, string> result = collection.Where(r => r.Value == null).ToDictionary(r => r.Key, r => r.Value);
I would like to pass the Where clause as a parameter to a method that performs the actual filtering, e.g.
private static IDictionary<T1, T2> Filter<T1, T2>(Func<IDictionary<T1, T2>, IDictionary<T1, T2>> exp, IDictionary<T1, T2> col)
{
return col.Where(exp).ToDictionary<T1, T2>(r => r.Key, r => r.Value);
}
This does not compile, though.
I have tried to call this method by using
Func<IDictionary<string, string>, IDictionary<string, string>> expression = r => r.Value == null;
var result = Filter<string, string>(expression, collection);
What am I doing wrong?
Where wants a Func<TSource, bool>, in your case Func<KeyValuePair<TKey, TValue>, bool>.
Furthermore, your return type of the method is incorrect. It should use T1 and T2 instead of string. Additionally, it is better to use descriptive names for the generic parameters. Instead of T1 and T2 I use the same names as the dictionary - TKey and TValue:
private static IDictionary<TKey, TValue> Filter<TKey, TValue>(
Func<KeyValuePair<TKey, TValue>, bool> exp, IDictionary<TKey, TValue> col)
{
return col.Where(exp).ToDictionary(r => r.Key, r => r.Value);
}
If you look at the constructor for the Where extension method you will see
Func<KeyValuePair<string, string>, bool>
So this is what you need to filter by, try this extension method.
public static class Extensions
{
public static IDictionairy<TKey, TValue> Filter<TKey, TValue>(this IDictionary<TKey, TValue> source, Func<KeyValuePair<TKey, TValue>, bool> filterDelegate)
{
return source.Where(filterDelegate).ToDictionary(x => x.Key, x => x.Value);
}
}
Call as
IDictionary<string, string> dictionairy = new Dictionary<string, string>();
var result = dictionairy.Filter((x => x.Key == "YourValue"));
I would like to create an extension method that will allow me to call ToSerializableDictionary(p => p.ID) instead of .ToDictionary(p => p.ID) in the following LINQ context. Though I'm not sure what class i'm supposed to be making an extension method for to replace ToDictionary<T>.
response.attributes = (
from granuleGroup in groups
let granuleRow = granuleGroup.First().First()
select new USDAttributes()
{
id = (int)granuleRow["id"],
...
attributes =
(
...
).ToDictionary(p => p.ID) <--** LINE IN QUESTION **
}
).ToList();
My SerializableDictionary class taken from here is so that I may serialize dictionary objects in my webservice to return hash tables that play nice with JSON.
Initially I was creating an extension method for IDictionary so I can do something like this: ...).ToDictionary(p => p.ID).ToSerializableDictionary(); But this has been a complete failure because it's my first time creating extension methods and I don't know what I'm doing.
public static class CollectionExtensions
{
public static SerializableDictionary<string, object> ToSerializableDictionary(this IDictionary<string,object> sequence)
{
SerializableDictionary<string, object> sDic = new SerializableDictionary<string, object>();
foreach (var item in sequence)
{
}
return sDic;
}
}
public static SerializableDictionary<TKey, T> ToSerializableDictionary<TKey, T>(this IEnumerable<T> seq, Func<T, TKey> keySelector)
{
var dict = new SerializableDictionary<TKey, T>();
foreach(T item in seq)
{
TKey key = keySelector(item);
dict.Add(key, item);
}
return dict;
}
Actually the class you provided has a handy constructor for doing this, so you can actually do
attributes = new SerializableDictionary( (
...
).ToDictionary(p => p.ID) );
But here you go with the extension method (again using that constructor):
public static partial class Extension {
public static SerializableDictionary<T, Q> ToSerializableDictionary(
this IDictionary<T, Q> d) {
return new SerializableDictionary(d);
}
}
Lee's response is the correct answer but just to offer another approach you could try this slightly terser version:
public static SerializableDictionary<TKey, T> ToSerializableDictionary<TKey, T>(this IEnumerable<T> seq, Func<T, TKey> keySelector)
{
var dict = seq.ToDictionary(keySelector);
//since SerializableDictionary can accept an IDictionary
return new SerializableDictionary<TKey, T>(dict);
}
Personally however I'd consider an even simpler approach and use JSON.Net for this task - it works perfectly, is ridiculously simple to use and is incredibly fast. I believe Microsoft have even switched to using JSON.Net in MVC3 (or perhaps 4?) for these reasons. Heartily recommended