C#

string openTag = "<upcase>", closeTag = "</upcase>";

string str = "We are living in a <upcase>yellow submarine</upcase>.";

int startIndex = str.IndexOf(openTag);
int endIndex = str.IndexOf(closeTag) + closeTag.Length;

string subStr = str.Substring(startIndex, (endIndex - startIndex));

subStr = subStr.Replace(openTag, "");
subStr = subStr.Replace(closeTag, "");

subStr = subStr.ToUpper();

str = str.Remove(startIndex, (endIndex - startIndex));

str = str.Insert(startIndex, subStr);

private string upperCaseTags(string str, string openTag, string closeTag)
{
    int startIndex;
    while ((startIndex = str.IndexOf(openTag)) != -1)
    {
        int endIndex;
        if ((endIndex = str.IndexOf(closeTag) + closeTag.Length) != -1)
        {
            string subStr = str.Substring(startIndex, (endIndex - startIndex));

            subStr = subStr.Replace(openTag, "").Replace(closeTag, "").ToUpper();

            str = str.Remove(startIndex, (endIndex - startIndex));

            str = str.Insert(startIndex, subStr);
        }
    }
    return str;
}

static void Main(string[] args)
       {
           string str= "19 5456 65";
           string key = "546";
           String encode = Encode(str, key);
           String decode = Encode(encode, key);
           Console.Write(str + "\n" + encode+"\n"+decode);
       }
       static string Encode(string str,string key)
       {
           char []keyChar = key.ToCharArray();
           char[] strChar = str.ToCharArray(); int a = 0;
           for (int i = 0; i < strChar.Length;i++ )
           {
               strChar[i] = (char)(strChar[i] | keyChar[a]);
               a++;
               if (a>keyChar.Length-1)
               {
                   a = 0;
               }
           }
           str = new string(strChar);
           return str;
       }

string filepath = Server.MapPath(Request.ApplicationPath + "//");
string filename = Path.GetFileName(filepath);
FileStream fs = new FileStream(filepath, FileMode.Open, FileAccess.Read);

string filepath = Server.MapPath(Request.ApplicationPath + "//");
string filename = Path.GetFileName(filepath);
FileStream fs = new FileStream(filepath <code>+ filename</code>, FileMode.Open, FileAccess.Read);