C# Batch Replace with encoding

Here is the code I wrote for batch replace with GB2312 encoding:-

var dir = new DirectoryInfo(@"D:\hgb");
string strFileName;
string content;
var encoding = Encoding.GetEncoding(936);

foreach (var f in dir.GetFiles("*.htm", SearchOption.AllDirectories))
{
strFileName = f.DirectoryName + @"\" + f.Name;
using (StreamReader reader = new StreamReader(strFileName, encoding))
{
content = reader.ReadToEnd();
content = Regex.Replace(content, "font size=-1", "font size=48");
}




StreamWriter writer = new StreamWriter(strFileName, false, encoding);
writer.Write(content);
writer.Close();
}