如何读写大文件？

string dt = ReadData(filename,"GB2312");
dt += "rfsfsffs";
WriteData(filename,dt,"GB2312");
public string ReadData(string fn,string Ecd)//读文件
{
string rlt = "";
StreamReader Sr = new StreamReader(File.OpenRead(fn),Encoding.GetEncoding(Ecd));
rlt = Sr.ReadToEnd();
Sr.DiscardBufferedData();
Sr.Close();
return rlt; 
}public void WriteData(string fn,string Data,string Ecd)//写文件
{
StreamWriter Sw = new StreamWriter(fn,false,Encoding.GetEncoding(Ecd));
Sw.Write(Data);
Sw.Flush();
Sw.Close();
}