C# Determine Duplicate in List [duplicate]

Question

Requirement: In an unsorted List, determine if a duplicate exists. The typical way I would do this is an n-squared nested loop. I'm wondering how others solve this. Is there an elegant, high performance method in Linq? Something generic that takes a lambda or a comparer would be nice.

Answer 1

Unless I'm missing something, then you should be able to get away with something simple using Distinct(). Granted it won't be the most complex implementation you could come up with, but it will tell you if any duplicates get removed:

var list = new List<string>();  // Fill the list  if(list.Count != list.Distinct().Count()) {      // Duplicates exist } 

Answer 2

According to Eric White's article on how to Find Duplicates using LINQ:

An easy way to find duplicates is to write a query that groups by the identifier, and then filter for groups that have more than one member. In the following example, we want to know that 4 and 3 are duplicates:

int[] listOfItems = new[] { 4, 2, 3, 1, 6, 4, 3 }; var duplicates = listOfItems     .GroupBy(i => i)     .Where(g => g.Count() > 1)     .Select(g => g.Key); foreach (var d in duplicates)     Console.WriteLine(d); // 4,3