C # extension methods: a simple but useful example

What are extension methods? This is an opportunity to expand the functionality of an existing class by adding a static method to it. Here is a simple example that at the same time remains very useful to me.

 namespace SampleNS
 {
 public static class CommonUtil
 {
     public static string ListToString (this IList list)
     {
         StringBuilder result = new StringBuilder ("");
        
         if (list.Count> 0)
         {
             result.Append (list [0] .ToString ());
             for (int i = 1; i <list.Count; i ++)
                 result.AppendFormat (", {0}", list [i] .ToString ());                
         }
         return result.ToString ();
     }
 }
 }

What does this code do? It extends all classes that implement the IList interface with the ListToString method, which allows you to get a comma-separated list of list elements as a string. To extend the functionality of the standard interface, all that is needed now is to connect the SampleNS namespace. (Comment: thanks to the very useful remark of easyman , the example was rewritten using the StringBuilder class)

Here is an example of use:

     var _list = DataContextORM.ExecuteQuery <string> ("Select name from products"). ToList ();
     string result = _list.ListToString ();

This example will receive in _list a list of names of all products, and then assign a string with a comma-separated list of names to the result variable.

PS: transferred from a personal blog