Generic lists and Predicates

Recently I was working with a generic list containing my own Favorite class. My Favorite class contains an url and a title:

using System.Collections.Generic;

public class Favorites
{
  private List<Favorite> _favoriteList = new List<Favorite>();
}

One of the functions on my list is to remove all Favorites with a specific url. I quickly jumped to the help file and found the RemoveAll() function which takes a Predicate. A predicate is a generic delegate that takes the same type as the List<> and returns true if the parameter matches a statement:

public void DinosaurExample()
{
  List<string> dinosaurs = new List<string>();

  dinosaurs.Add("Compsognathus");
  dinosaurs.Add("Amargasaurus");
  dinosaurs.Add("Oviraptor");
  dinosaurs.RemoveAll(EndsWithSaurus));
}

private static bool EndsWithSaurus(String s)
{
  if ((s.Length > 5) &&
     (s.Substring(s.Length - 6).ToLower() == "saurus"))
  {
    return true;
  }
  else
  {
    return false;
  }
}

 The EndsWithSaurus(String s) is the predicate and the function returns true if the matching string ends with “saurus”, removing “Amargasaurus” from the list.

But hey! What if I don’t have a hard-coded match in my code (I rarely have)? The predicate cannot take any parameters. Also, it’s a Static member, polluting my class.

The solution is to create a “predicate class” that takes the expression to match as a parameter:

internal class FavoriteUrlMatch
{
  private string _url;
 
  public FavoriteUrlMatch(string url)
  {
    _url = url;
  }
   
  public bool Match(Favorite fav)
  {
    return fav.Url == _url;
  }
}

Then I can invoke my FavoriteUrlMatch class from my Favorites class:

public void Delete(string url)
{
  FavoriteUrlMatch match = new FavoriteUrlMatch(url);
  _favoriteList.RemoveAll(match.Match);
}

I am not very proud of this solution, but I cannot find a better one.

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