using System;

using System.Collections.Generic;

public class Program

{

    static void Main()

    {

        var strings = new List<(string part, string full)> { ("abcdefgh", "cdefghijklmno"), ("abcdefgh", "efghijklmno"), ("aaabbb", "abbbfffxx"), ("not inside", "abcfgh") };

        AddLongRandomStrings(strings, 10, 100000, 10000);

​

        var sw = System.Diagnostics.Stopwatch.StartNew();

        foreach (var x in strings)

        {

            string result = RemovePartFromStart(x.full, x.part, out string prefixFound);

            if(prefixFound != null)

            {

                Console.WriteLine($"Found <{prefixFound}>:" + result);

            }

            else

            {

                Console.WriteLine("Not found");

            }

        }

        Console.WriteLine();

        TimeSpan elapsed = sw.Elapsed;

        Console.WriteLine(elapsed);

    }  

​

    public static string RemovePartFromStart(string full, string part, out string prefixFound)

    {

        for (int i = 0; i < part.Length; i++)

        {

            string part2 = part.Substring(i);

            if (full.StartsWith(part2))

            {

                prefixFound = part2;

                return full.Substring(part2.Length);

            }

        }

​

        prefixFound = null;

        return full;

    }

​

    private static void AddLongRandomStrings(List<(string part, string full)> strings, int count, int fullStringLength, int partStringLength)

    {

        const string chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";

        Random r = new Random();

​

        for (int i = 0; i < count; i++)

        {

            char[] randomFullCharArray = new char[fullStringLength];

            for (int j = 0; j < randomFullCharArray.Length; j++)

            {

                randomFullCharArray[j] = chars[r.Next(chars.Length)];

            }

            char[] randomPartCharArray = new char[partStringLength];

            for (int j = 0; j < randomPartCharArray.Length; j++)

            {

                randomPartCharArray[j] = chars[r.Next(chars.Length)];

            }

​

            strings.Add((new string(randomPartCharArray), new string(randomFullCharArray)));

        }

    }

}