private void button1_Click(object sender, EventArgs e)
{
    FileStream vFileStream = new FileStream(@"c:\temp\temp.txt",
        FileMode.Open, FileAccess.Read, FileShare.ReadWrite);
    StreamReader vStreamReader = new StreamReader(vFileStream, Encoding.Default);
    Console.WriteLine(vStreamReader.ReadToEnd());
}
private FileStream writeStream = new FileStream(@"c:\temp\temp.txt",
    FileMode.Append, FileAccess.Write, FileShare.ReadWrite);

private StreamWriter streamWriter;
private void button2_Click(object sender, EventArgs e)
{                    
    if (streamWriter == null)
        streamWriter = new StreamWriter(writeStream, Encoding.Default);
    streamWriter.WriteLine("Zswang路过");
    streamWriter.Flush();
}