using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        string input = "catfish";

        var mammals = new [] {"dog", "cat", "human", "dolphin"};

        var result = input.EqualsAny(mammals);

        Console.WriteLine(result);

    }

}

​

public static class StringExtensions {

    public static bool EqualsAny(this string str, IEnumerable<string> validStrings) {

        return validStrings.Any(v => v == str);

    }

}