UpperCase in Tags

using System;

class Program
{
    public static string UpperCaseInTags(string text, string tag)
    {
        int count = 0;

        string TagStart = "<" + tag + ">";
        string TagEnd = "</" + tag + ">";
        while (text.IndexOf(TagStart) != -1)
        {
            int PosStart = text.IndexOf(TagStart);
            int PosEnd = text.IndexOf(TagEnd);
            string temp = text.Substring(PosStart + TagStart.Length, PosEnd - PosStart - TagStart.Length);
            text = text.Replace(TagStart + temp + TagEnd, temp.ToUpper());
        }
        return text;
    }

    static void Main()
    {
        string text = "We are living in a <upcase>yellow submarine</upcase>. We don't have <upcase>anything</upcase> else.";
        string tag = "upcase";
        Console.WriteLine(UpperCaseInTags(text, tag));
    }
}