FileStream類操作的是字節和節節數組，而stream類操作的是字符數據。字符數據易於使用，但是有些操作，比如隨機文件訪問(訪問文件中間某點的數據)，就必須由FileStream對象執行。
             FielStream類維護內部文件指針，該指針指向文件中進行下一次讀寫操作的位置。
             FileStream只能處理原始字節(raw byte),處理原去吧字節的功能使用FileStream類可以用於處理任何數據文件。
             通過讀取字節數據，FileStream對像可以用於讀取圖像和聲音。代價是FileStream類不能將數據直接讀入字符串
             此指針可以修改用Seek方法，它有兩個參數：
             第一個參數：規定文件指針以字節為參數的移動距離
             第二個參數：規定開始計算的起始位置

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        byte[] byDataValue = new byte[200];
        char[] charDataValue = new char[200];

        ///
        /// FilesStream讀取用法

        ///
        private void FilesStreamReadFile()
        {
            try
            {
                FileStream fsFile = new FileStream(@"d:\log.cs", FileMode.Open);
                //文件指針移到文件的135個字節
                fsFile.Seek(135, SeekOrigin.Begin);
                //將接下來的字節讀到Array中
                fsFile.Read(byDataValue, 0, 200);
            }
            catch (Exception e)
            {
                throw e;
            }
            //將字節轉換成字符
            Decoder dc = Encoding.UTF8.GetDecoder();
            //字節數組轉換成字符數組，便於顯示
            dc.GetChars(byDataValue, 0, byDataValue.Length, charDataValue, 0);

        }
----------------------------------------------------------------------------------------------
        ///
        /// FilesStream寫入用法
        ///
        private void FileStreamWriteFile()
        {
            FileStream fsFile = new FileStream(@"D:\log.txt", FileMode.Create);
            charDataValue="This is test string".ToCharArray();
            byDataValue=new byte[charDataValue.Length];

            //將字符數組轉換成字節數組
            Encoder ec=Encoding.UTF8.GetEncoder();
            ec.GetBytes(charDataValue,0,charDataValue.Length,byDataValue,0,true);

            //將指針設定起始位置
            fsFile.Seek(0,SeekOrigin.Begin);
            //寫入文件
            fsFile.Write(byDataValue,0,byDataValue.Length);
        }

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        ///
        /// StreamWriter寫入方法
        ///
        private void StreamWriterMetod()
        {
            try
            {
                FileStream fsFile = new FileStream(@"d:\log.txt",FileMode.OpenOrCreate);
                StreamWriter swWriter = new StreamWriter(fsFile);
                //寫入數據
                swWriter.WriteLine("Hello Wrold.");
                swWriter.WriteLine("It is now {0}", DateTime.Now.ToLongDateString());   
                swWriter.Close();
            }
            catch (Exception e)
            {
                throw e;
            }
        }

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        ///
        /// StreamWriter讀取方法
        ///
        private void StreamReaderMetod()
        {
            string sLine = "";
            try
            {
                FileStream fsFile = new FileStream(@"d:\log.txt", FileMode.Open);
                StreamReader srReader = new StreamReader(fsFile);
                //讀取文件(讀取大文件時，最好不要用此方法)
                sLine = srReader.ReadToEnd();
                txbValue.Text = sLine;
                srReader.Close();
            }
            catch (Exception e)
            {
                throw e;
            }
        }

----------------------------------------------------------------------------------
        ///
        /// StreamWriter讀取方法2
        ///
        private void StreamReaderMetodTwo()
        {
            try
            {
                FileStream fsFile = new FileStream(@"d:\log.txt", FileMode.Open);
                /*
                 * 讀取數據最簡單的方法是Read()。此方法將流的下一個字符作為正整數值返回，
                 * 如果達到了流的結尾處，則返回-1。
                 */
                StreamReader srReader = new StreamReader(fsFile);
                int iChar;
                iChar = srReader.Read();
                while (iChar != -1)
                {
                    txbValue.Text += (Convert.ToChar(iChar));
                    iChar = srReader.Read();
                }
                srReader.Close();
            }
            catch (Exception e)
            {
                throw e;
            }
        }

<h1>----------------------------------------------------------------------
</h1><h1>FileStream Open File [C#]</h1>

This example shows how to open files for reading or writing, how to load and save files using FileStream in C#. To open file create instance of FileStream class with FileMode and FileAccess enumerations as parameters.

<h2>FileStream typical use</h2>

This is typical code when opening file using FileStream. It's important to always close the stream. If you don't close the stream it can take a minute to be file again accessible (it will wait to garbage collector to free the FileStream instance and close the file).

[C#]

using System.IO;

FileStream fileStream = new FileStream(@"c:\file.txt", FileMode.Open);
try
{
 // read from file or write to file
}
finally
{
 fileStream.Close();
}
--------------------------------

<h2>Open file examples</h2>

Following examples show the most common cases how to open a file for reading or writing or how to create a file.

[C#] Open existing file for read and write.

FileStream fileStream = new FileStream(@"c:\file.txt", FileMode.Open);

[C#] Open existing file for reading.

FileStream fileStream = new FileStream(@"c:\file.txt", FileMode.Open, FileAccess.Read);

[C#] Open existing file for writing.

FileStream fileStream = new FileStream(@"c:\file.txt", FileMode.Open, FileAccess.Write);

[C#] Open file for writing (with seek to end), if the file doesn't exist create it.

FileStream fileStream = new FileStream(@"c:\file.txt", FileMode.Append);

[C#] Create new file and open it for read and write, if the file exists overwrite it.

FileStream fileStream = new FileStream(@"c:\file.txt", FileMode.Create);

[C#] Create new file and open it for read and write, if the file exists throw exception.

FileStream fileStream = new FileStream(@"c:\file.txt", FileMode.CreateNew);

reference:
http://www.csharp-examples.net/filestream-open-file/
http://www.cnblogs.com/scottckt/archive/2008/07/01/1233523.html
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