做的是：

using (FileStream fsSource = new FileStream(pathSource,
        FileMode.Open, FileAccess.Read))
    {

        // Read the source file into a byte array.
        int numBytesToRead = // Your amount to read at a time
        byte[] bytes = new byte[numBytesToRead];

        int numBytesRead = 0;
        while (numBytesToRead > 0)
        {
            // Read may return anything from 0 to numBytesToRead.
            int n = fsSource.Read(bytes, numBytesRead, numBytesToRead);

            // Break when the end of the file is reached.
            if (n == 0)
                break;

            // Do here what you want to do with the bytes read (convert to string using Encoding.YourEncoding.GetString())
        }
    }