c# 替換原有文件內容

//實現替換原有文件內容
//Seek(0, SeekOrigin.Begin)將指針指向文件的開始，才能從文件的開頭,但是此時如果新寫入的流長度小於原有文件長度，則會保留原文件多余的信息
using (FileStream fileStream = new FileStream(file, FileMode.Open, FileAccess.Write, FileShare.Write))
{
    fileStream.Seek(0, SeekOrigin.Begin);
    using (StreamWriter streamWriter = new StreamWriter(fileStream))
    {
        streamWriter.Write("this is clear file");
        fileStream.Flush();
        streamWriter.Close();
    }
    fileStream.Close();
}

//調用Stream的SetLength這個方法，將長度設為0，可以實現清空原有文件
using (FileStream fileStream = new FileStream(file, FileMode.Open, FileAccess.Write, FileShare.Write))
{
    fileStream.SetLength(0);
    using (StreamWriter streamWriter = new StreamWriter(fileStream))
    {
        streamWriter.Write("this is clear file");
        fileStream.Flush();
        streamWriter.Close();
    }
    fileStream.Close();
}

//如果文件已存在則清空，從Truncate打開的文件中讀取將引發異常
using (FileStream fileStream = new FileStream(file, FileMode.Truncate, FileAccess.Write, FileShare.Write))
{
    using (StreamWriter streamWriter = new StreamWriter(fileStream))
    {
        streamWriter.Write("this is");
        fileStream.Flush();
        streamWriter.Close();
    }
    fileStream.Close();
}