C#替换文件中特定字符串，按照原来的编码格式保存

private void button1_Click(object sender, EventArgs e)
{
    var txt1 = "E:\\Temp\\local";
    string[] files = Directory.GetFiles(txt1, "*.txt");
    
    foreach (string file in files)
    {
        execFile(file);
    }
}

void execFile(string file)
{
    int i = 0;
    var eType = EncodingType.GetType(file);
    var lines = File.ReadAllLines(file, eType);
    StreamWriter sw = new StreamWriter(file, false, eType);
    while(i<lines.Length)
    {
        sw.WriteLine(lines[i].Replace("开始", "kaizhan"));
        i++;
    }
    sw.Close();
}