How to get distinct instance from a list by Lambda or LINQ
Solution 1
Both Marc's and dahlbyk's answers seems to work very well. I have a much simpler solution though. Instead of using Distinct
, you can use GroupBy
. It goes like this:
var listDistinct
= list.GroupBy(
i => i.value1,
(key, group) => group.First()
).ToArray();
Notice that I've passed two functions to the GroupBy()
. The first is a key selector. The second gets only one item from each group. From your question, I assumed First()
was the right one. You can write a different one, if you want to. You can try Last()
to see what I mean.
I ran a test with the following input:
var list = new [] {
new { value1 = "ABC", objT = 0 },
new { value1 = "ABC", objT = 1 },
new { value1 = "123", objT = 2 },
new { value1 = "123", objT = 3 },
new { value1 = "FOO", objT = 4 },
new { value1 = "BAR", objT = 5 },
new { value1 = "BAR", objT = 6 },
new { value1 = "BAR", objT = 7 },
new { value1 = "UGH", objT = 8 },
};
The result was:
//{ value1 = ABC, objT = 0 }
//{ value1 = 123, objT = 2 }
//{ value1 = FOO, objT = 4 }
//{ value1 = BAR, objT = 5 }
//{ value1 = UGH, objT = 8 }
I haven't tested it for performance. I believe that this solution is probably a little bit slower than one that uses Distinct
. Despite this disadvantage, there are two great advantages: simplicity and flexibility. Usually, it better to favor simplicity over optimization, but it really depends on the problem you're trying to solve.
Solution 2
Hmm... I'd probably write a custom IEqualityComparer<T>
so that I can use:
var listDistinct = list.Distinct(comparer).ToList();
and write the comparer via LINQ....
Possibly a bit overkill, but reusable, at least:
Usage first:
static class Program {
static void Main() {
var data = new[] {
new { Foo = 1,Bar = "a"}, new { Foo = 2,Bar = "b"}, new {Foo = 1, Bar = "c"}
};
foreach (var item in data.DistinctBy(x => x.Foo))
Console.WriteLine(item.Bar);
}
}
}
With utility methods:
public static class ProjectionComparer
{
public static IEnumerable<TSource> DistinctBy<TSource,TValue>(
this IEnumerable<TSource> source,
Func<TSource, TValue> selector)
{
var comparer = ProjectionComparer<TSource>.CompareBy<TValue>(
selector, EqualityComparer<TValue>.Default);
return new HashSet<TSource>(source, comparer);
}
}
public static class ProjectionComparer<TSource>
{
public static IEqualityComparer<TSource> CompareBy<TValue>(
Func<TSource, TValue> selector)
{
return CompareBy<TValue>(selector, EqualityComparer<TValue>.Default);
}
public static IEqualityComparer<TSource> CompareBy<TValue>(
Func<TSource, TValue> selector,
IEqualityComparer<TValue> comparer)
{
return new ComparerImpl<TValue>(selector, comparer);
}
sealed class ComparerImpl<TValue> : IEqualityComparer<TSource>
{
private readonly Func<TSource, TValue> selector;
private readonly IEqualityComparer<TValue> comparer;
public ComparerImpl(
Func<TSource, TValue> selector,
IEqualityComparer<TValue> comparer)
{
if (selector == null) throw new ArgumentNullException("selector");
if (comparer == null) throw new ArgumentNullException("comparer");
this.selector = selector;
this.comparer = comparer;
}
bool IEqualityComparer<TSource>.Equals(TSource x, TSource y)
{
if (x == null && y == null) return true;
if (x == null || y == null) return false;
return comparer.Equals(selector(x), selector(y));
}
int IEqualityComparer<TSource>.GetHashCode(TSource obj)
{
return obj == null ? 0 : comparer.GetHashCode(selector(obj));
}
}
}
Solution 3
You can use this extension method:
IEnumerable<MyClass> distinctList = sourceList.DistinctBy(x => x.value1);
public static IEnumerable<TSource> DistinctBy<TSource, TKey>(
this IEnumerable<TSource> source,
Func<TSource, TKey> keySelector)
{
var knownKeys = new HashSet<TKey>();
return source.Where(element => knownKeys.Add(keySelector(element)));
}
Solution 4
Check out Enumerable.Distinct(), which can accept an IEqualityComparer:
class MyClassComparer<T> : IEqualityComparer<MyClass<T>>
{
// Products are equal if their names and product numbers are equal.
public bool Equals(MyClass<T> x, MyClass<T>y)
{
// Check whether the compared objects reference the same data.
if (Object.ReferenceEquals(x, y)) return true;
// Check whether any of the compared objects is null.
if (Object.ReferenceEquals(x, null) || Object.ReferenceEquals(y, null))
return false;
// Check whether the products' properties are equal.
return x.value1 == y.value1;
}
// If Equals() returns true for a pair of objects,
// GetHashCode must return the same value for these objects.
public int GetHashCode(MyClass<T> x)
{
// Check whether the object is null.
if (Object.ReferenceEquals(x, null)) return 0;
// Get the hash code for the Name field if it is not null.
return (x.value1 ?? "").GetHashCode();
}
}
Your code snippet could look like this:
List<MyClass<T>> list;
...
List<MyClass<T>> listDistinct = list.Distinct(new MyClassComparer<T>).ToList();
Solution 5
This will be more simple...
var distinctList = list.GroupBy(l => l.value1, (key, c) => l.FirstOrDefault());
David.Chu.ca
Updated on August 23, 2020Comments
-
David.Chu.ca over 3 years
I have a class like this:
class MyClass<T> { public string value1 { get; set; } public T objT { get; set; } }
and a list of this class. I would like to use .net 3.5 lambda or linq to get a list of MyClass by distinct value1. I guess this is possible and much simpler than the way in .net 2.0 to cache a list like this:
List<MyClass<T>> list; ... List<MyClass<T>> listDistinct = new List<MyClass<T>>(); foreach (MyClass<T> instance in list) { // some code to check if listDistinct does contain obj with intance.Value1 // then listDistinct.Add(instance); }
What is the lambda or LINQ way to do it?