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tanthiamhuat
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Overwrite part of existing Binary File

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if I have a existing binary file of 20 bytes, and I want to replace (over-write) portion of that file, say 4 bytes, from index 10 to index 13, how could I do that?

I understand that FileMode.Append could be used, but it always append at the end, with offset 0.

I would appreciate if some code samples are shown.

thanks.

(x)
foamy
2499

You will probably need to read the file into memory, alter it and write the entire thing back

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last answered 2 years ago

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tick
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I'd use a BinaryWriter for this and call the Seek() method on the underlying stream to move to the 11th byte in the file. Here's some code:

using System;
using System.IO;
using System.Text;

class Test
{
    static void Main()
    {
        string fileName = "myfile.bin"; // or whatever
        Encoding enc = Encoding.ASCII;
       
        using(BinaryWriter bw = new BinaryWriter(File.Open(fileName, FileMode.Create), enc))
        {
           // write 20 bytes to file
           bw.Write(enc.GetBytes("abcdefghijklmnopqrst"));
        }

        using(BinaryWriter bw = new BinaryWriter(File.Open(fileName, FileMode.Open), enc))
        {
           // overwrite 4 bytes from 10 to 13 
           bw.BaseStream.Seek(10, SeekOrigin.Begin);
           bw.Write(enc.GetBytes("KLMN"));
        } 
   }
}

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tanthiamhuat
494

thanks, that is a neat solution. I find similar solution at http://www.neowin.net/forum/index.php?showtopic=609069

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