FirstOrDefault

FirstOrDefault finds the first matching value or returns a fallback default.

FirstOrDefaultExample.cs

using System;
using System.Linq;

class Program
{
    static void Main()
    {
        string prefix = ;
        string[] codes = { "ax", "by", "bz" };
        string match = codes.FirstOrDefault(code => code.StartsWith(prefix)) ?? "none";

        Console.WriteLine($"prefix={prefix}");
        Console.WriteLine($"match={match}");
    }
}

using System;
using System.Linq;

class Program
{
    static void Main()
    {
        string prefix = ;
        string[] codes = { "ax", "by", "bz" };
        string match = codes.FirstOrDefault(code => code.StartsWith(prefix)) ?? "none";

        Console.WriteLine($"prefix={prefix}");
        Console.WriteLine($"match={match}");
    }
}

using System;
using System.Linq;

class Program
{
    static void Main()
    {
        string prefix = ;
        string[] codes = { "ax", "by", "bz" };
        string match = codes.FirstOrDefault(code => code.StartsWith(prefix)) ?? "none";

        Console.WriteLine($"prefix={prefix}");
        Console.WriteLine($"match={match}");
    }
}

FirstOrDefault `FirstOrDefault` returns the first match or a default value when none matches.