Checking a TXT Doc against anther TXT Doc

Question

I will do what I can to get a snipit of the txt files on here for you to look at. most of them are class and it will take a bit of work for me to do that. The idea is to take a txt file with say a list of over 500 places run them against a list of 50 places and pull out the ones that match. As I said I have tried many diffrent ways, well all the ones I know, and I can not seem to get it to work right. I am trying to take the following code and have it do the above action. Does that help.

InPutBox = Input.Text;
    int x = 1;
    var lines = Input.Text.Split(new string[] { Environment.NewLine },StringSplitOptions.None);

    for (var index = 0; index < lines.Length; index++)
    {

        var line = lines[index];

        do
        {
            x++;
            System.Console.WriteLine("'{0}'", InPutBox);
            bool test1 = InPutBox.StartsWith("TAG");
            bool test2 = InPutBox.EndsWith("TAG");
            if (test1 && test2)
            {
                int first = InPutBox.IndexOf("TAG");
                int last = InPutBox.LastIndexOf("TAG");
                InPutBox = InPutBox.Substring(first, last - first);
            }
        }
        while (x < 50);
        System.Console.WriteLine("'{0}'", line);
        if ((line.StartsWith("TAG") && line.EndsWith("TAG")))
        {
            MessageBox.Show("These are errors in line" + index + ": " + line);
            break;
        }

    }