Find matching substring from two strings

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;

void CompareTwoSTring()
      {
          string[] Arrayvalue1 = new string[100];
          string[] Arrayvalue2 = new string[100];
          string values=null;
          string firsValue = "1234567";
          string secondValue = "12476";

          for (int i = 0; i < firsValue.Length; i++)
          {
              Arrayvalue1[i] = firsValue[i].ToString();
              for (int j = 0; j < secondValue.Length; j++)
              {
                  Arrayvalue2[j] = secondValue[j].ToString();
                  if (Arrayvalue1[i].ToString() == Arrayvalue2[j].ToString())
                  {
                      values = values + Arrayvalue1[i];

                  }
              }
          }

          MessageBox.Show(values);

      }