How to compare two text files and get the match percentage

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C#

How to compare the contents of two text files and get the match percentage.

I compared two strings and was able to get the total percentage match.
ex:

string s1 = Manju is good guy;
string s2 = Manju is good guy;

The percentage is 100%
The code I have used is.

protected void Button1_Click(object sender, EventArgs e)
      {
          int count = 0;
          string[] str1 = TextBox1.Text.Split(' ');
          string[] str2 = TextBox2.Text.Split(' ');
          for (int i = 0; i < str1.Length; i++)
          {
              for (int j = 0; j <str2.length;>              {
                  if (str1[i] == str2[j])
                  { 
                      count++;
                  }
              }
          }
           double per=0.0;
           if (count <= str1.Length)
               per = (count * 100) / str1.Length;
           else
               per = 100;
          Response.Write(per.ToString());

But the problem is, when I give the following data.

string s1 = Manju is good guy;
string s2 = Manju is bad guy;

The percentage is 75%

The meaning is totally different, but the matching is of 80%
Is there any algorithm or codes where in I can compare the word and also the meaning or something like that.
Please help me out.

Thanks in advance.

Update:
My bad. I said 80% because, my string was Manju is a good guy. So, the mistake while posting the problem. Heh.