CODEWITHSUNDEEP
c#unicode
ahpPer
ahpPer

Reputation: 23

How can I replace a unicode string in a binary file?

I've been trying to get my program to replace unicode in a binary file. The user would input what to find, and the program would find and replace it with a specific string if it can find it.

I've searched around, but there's nothing I can find to my specifics, what I would like would be something like:

string text = File.ReadAllText(path, Encoding.Unicode);
text = text.Replace(userInput, specificString);
File.WriteAllText(path, text);

but anything that works in a similar manner should suffice. Using that results in a file that is larger and unusable, though.

I use:

int var = File.ReadAllText(path, Encoding.Unicode).Contains(userInput) ? 1 : 0;
if (var == 1)
{
    //Missing Part
}

for checking if the file contains the user inputted string, if it matters.

Upvotes: 2

Views: 2253

Answers (1)

Peter Duniho
Peter Duniho

Reputation: 70652

This can work only in very limited situations. Unfortunately, you haven't offered enough details as to the nature of the binary file for anyone to know if this will work in your situation or not. There are a practically endless variety of binary file formats out there, at least some of which would be rendered invalid if you modify a single byte, many more of which could be rendered invalid if the file length changes (i.e. data after your insertion point is no longer where it is expected to be).

Of course, many binary files are also either encrypted, compressed, or both. In such cases, even if you do by some miracle find the text you're looking for, it probably doesn't actually represent that text, and modifying it will render the file unusable.

All that said, for the sake of argument let's assume your scenario doesn't have any of these problems and it's perfectly okay to just completely replace some text found in the middle of the file with some entirely different text.

Note that we also need to make an assumption about the text encoding. Text can be represented in a wide variety of ways, and you will need to use the correct encoding not just to find the text, but also to ensure the replacement text will be valid. For the sake of argument, let's say your text is encoded as UTF8.

Now we have everything we need:

void ReplaceTextInFile(string fileName, string oldText, string newText)
{
    byte[] fileBytes = File.ReadAllBytes(fileName),
        oldBytes = Encoding.UTF8.GetBytes(oldText),
        newBytes = Encoding.UTF8.GetBytes(newText);

    int index = IndexOfBytes(fileBytes, oldBytes);

    if (index < 0)
    {
        // Text was not found
        return;
    }

    byte[] newFileBytes =
        new byte[fileBytes.Length + newBytes.Length - oldBytes.Length];

    Buffer.BlockCopy(fileBytes, 0, newFileBytes, 0, index);
    Buffer.BlockCopy(newBytes, 0, newFileBytes, index, newBytes.Length);
    Buffer.BlockCopy(fileBytes, index + oldBytes.Length,
        newFileBytes, index + newBytes.Length,
        fileBytes.Length - index - oldBytes.Length);

    File.WriteAllBytes(filename, newFileBytes);
}

int IndexOfBytes(byte[] searchBuffer, byte[] bytesToFind)
{
    for (int i = 0; i < searchBuffer.Length - bytesToFind.Length; i++)
    {
        bool success = true;

        for (int j = 0; j < bytesToFind.Length; j++)
        {
            if (searchBuffer[i + j] != bytesToFind[j])
            {
                success = false;
                break;
            }
        }

        if (success)
        {
            return i;
        }
    }

    return -1;
}

Notes:

  • The above is destructive. You may want to run it only on a copy of the file, or prefer to modify the code so that it takes an addition parameter specifying the new file to which the modification should be written.
  • This implementation does everything in-memory. This is much more convenient, but if you are dealing with large files, and especially if you are on a 32-bit platform, you may find you need to process the file in smaller chunks.

Upvotes: 5

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