using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {       

        List<string> liste = new List<string>(){

            "alex", "JP", "toto","Fred"

        };

        var toto = liste.FirstOrDefault(x => Test(x)) ?? string.Empty;

        var toto2 = liste

                .Where(x => Test(x))

                .DefaultIfEmpty(string.Empty)

                .First();

    }

    public static bool Test(string texte){

        Console.WriteLine(texte);

        if(texte=="toto")

            return true;

        return false;

    }

}