using System;
using System.Linq;
public class Program
{
public static void Main()
{
var path = "file.txt";
var lines = new string[]{"123456789", "bbbb"};
// lines = File.ReadAllLines(path);
string FindSubString(string substring) => lines.FirstOrDefault(x => x.StartsWith(substring));
var res = FindSubString("1");
Console.WriteLine(res);
}
}