Try
Algorithm Implementation/Strings/Longest common substring[
^]
From reference link :-
public int LongestCommonSubstring(string str1, string str2)
{
if (String.IsNullOrEmpty(str1) || String.IsNullOrEmpty(str2))
return 0;
int[,] num = new int[str1.Length, str2.Length];
int maxlen = 0;
for (int i = 0; i < str1.Length; i++)
{
for (int j = 0; j < str2.Length; j++)
{
if (str1[i] != str2[j])
num[i, j] = 0;
else
{
if ((i == 0) || (j == 0))
num[i, j] = 1;
else
num[i, j] = 1 + num[i - 1, j - 1];
if (num[i, j] > maxlen)
{
maxlen = num[i, j];
}
}
}
}
return maxlen;