longestMatch.cs

void Main()
{
	Console.WriteLine(
		LongestMatch("a8dgk1ndflg0s84142kd","093841tgpngh044flg0s84142123rbbdja7")
	);	
}

string LongestMatch(string small, string large)
{
	string currLargest = "";

	if (small.Length >= large.Length)
		(small, large) = (large, small);
		
	for (int i=0; i < large.Length; i++) {
		string tmp = "";
		int largePos = i;
		for (int j=0; j < small.Length; j++) {
			if (largePos > large.Length-1)
				continue;
				
			if (small[j]==large[largePos]) {
				tmp += small[j];
				largePos++;
			} else {
				if (currLargest.Length < tmp.Length) {
					currLargest = tmp;
				}
				tmp = "";
			}
		}
	}
	return currLargest;
}