decompress and get size of file in c# (bzip2)

Question

Can anyone spot the error why it gives me the wrong value every time, never the correct one?

private long Main(string bzip2FilePath)
{
    long totalUncompressedSize = 0;

    try
    {
        
        var compressedDataByteArray = File.ReadAllBytes(bzip2FilePath);

       
        using (var mstream = new MemoryStream(compressedDataByteArray))
        
        using (var unzipstream = new BZip2InputStream(mstream))
       
        using (var reader = new StreamReader(unzipstream))
        {
       
            char[] buffer = new char[4096];
            int bytesRead; 
            while ((bytesRead = reader.Read(buffer, 0, buffer.Length)) > 0)
            {
                totalUncompressedSize += bytesRead;
            }
        }
    }
    catch (Exception ex)
    {
        MessageBox.Show($"Error calculating BZip2 uncompressed size: {ex.Message}");
    }

    return totalUncompressedSize;
}

Answer 1

I think the problem is that StreamReader reads characters, not bytes, and thus size calculations might be wrong because of conversions in the encoding. You instead need to use a BinaryReader and read bytes.

What i can suggest is that you may use this:

using (var reader = new BinaryReader(unzipstream))

instead of this: using (var reader = new StreamReader(unzipstream))

So, as a result, you may also want to update the associated types in your code too: like this:

byte[] buffer = new byte[4096];

and the while loop :

while ((bytesRead = reader.Read(buffer, 0, buffer.Length)) > 0)
        {
            totalUncompressedSize += bytesRead;
        }

Answer 2

Unclear why you are using a StreamReader at all. You just need to get the number of bytes, not characters. You also don't need to cache the whole file in a byte array.

using var file = File.Open(bzip2FilePath);
using var unzipstream = new BZip2InputStream(file);
if (unzipstream.CanSeek)   // grab the length if we can
    return unzipstream.Length;

// otherwise loop the stream
var totalBytes = 0;
var buffer = new byte[8192];
int bytesRead; 
while ((bytesRead = reader.Read(buffer, 0, buffer.Length)) > 0)
{
    totalBytes += bytesRead;
}
return totalBytes;