这是读取文本文件中的一个极其低效的方式，更何况是大的。如果你只需要一个传球，更换或添加单个字符，你应该使用的StreamReader 如果您只需要向前看一个字符，你只需要维护一个中间状态，是这样的：

That's an extremely inefficient way to read a text file, let alone a large one. If you only need one pass, replacing or adding individual characters, you should use a StreamReader. If you only need one character of lookahead you only need to maintain a single intermediate state, something like:

enum ReadState
{
    Start,
    SawOpen
}


using (var sr = new StreamReader(@"path\to\clinic.txt"))
using (var sw = new StreamWriter(@"path\to\output.txt"))
{
    var rs = ReadState.Start;
    while (true)
    {
        var r = sr.Read();
        if (r < 0)
        {
            if (rs == ReadState.SawOpen)
                sw.Write('<');
            break;
        }

        char c = (char) r;
        if ((c == '\r') || (c == '\n'))
            continue;

        if (rs == ReadState.SawOpen)
        {
            if (c == 'C')
                sw.WriteLine();

            sw.Write('<');
            rs = ReadState.Start;
        }

        if (c == '<')
        {
            rs = ReadState.SawOpen;
            continue;
        }

        sw.Write(c);
    }
}