While operating with collections, sometime we would like to perform Distinct on column, to achieve this we need to write the extension method.

public static class StaticDistinct
{
       public static IEnumerable<TSource> DistinctBy<TSource, TKey>
        (this IEnumerable<TSource> source, Func<TSource, TKey> keySelector)
        {
              HashSet<TKey> knownKeys = new HashSet<TKey>();
              foreach (TSource element in source)
               {
                    if (knownKeys.Add(keySelector(element)))
                    {
                         yield return element;
                    }
                }
          }
 }

// USAGE
var distinctUsers = users.DistinctBy(x => x.EmailId).ToList();

This example also demostrate what are extension methods and syntax of extension method.

Extension Methods:

#1) These are static methods, so these method should be surronded by static class

#2) These method signature would start this keyword. Let’s say you want to write extension method to string then the method would be some thing like below.

// DEFINE EXTENSION METHOD
 public static string ImExtensionMethod(this string value)
 {
   return "Hurray! I am EXTENSION method";
 }

//USAGE
  string whoAreYou = "Hello";
  whoAreYou.ImExtensionMethod();

By now you should be knowing why did we define DISTINCTBY as extension method

Happy Kooding…. Hope this helps!