I want understand .Net 6 Core StreamReader properties

seb 0
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I need to calculate MD5 for a file.

private string GetMD5(string file)
{
  using var md5 = MD5.Create();
  using var stream = new StreamReader(file);
  return (BitConverter.ToString(md5.ComputeHash(stream.BaseStream)).Replace("-", string.Empty)).ToLower();
}

private string GetMD5_V2(string file)
{
  using var md5 = MD5.Create();
  using var stream = new StreamReader(file);
  **_ = stream.EndOfStream;**
  return (BitConverter.ToString(md5.ComputeHash(stream.BaseStream)).Replace("-", string.Empty)).ToLower();
}

test()
{
  var fichier = "myFile.txt";
  var md5_1 = GetMD5(fichier);
  var md5_2 = GetMD5_V2(fichier);
}

When I run this code md5_1 and md5_2 is different. I not understand why when I read the propertie stream.EndOfStream this change the result of stream.BaseStream?

streamreader .net c#
2 Answers

StreamReader.EndOfStream consumes some bytes from the underlying stream.

Source

Specifically, it will read enough bytes to fill its own internal buffer. Since your MD5 is operating on that underlying stream and not on the StreamReader, the bytes sitting in that buffer are no longer available to the MD5 calculation.

Matt Thomas0

Querying the EndOfStream property of a freshly initialized StreamReader reads some bytes from the underlying stream. See the source code of this property's getter (link):

public bool EndOfStream
{
    get
    {
        ThrowIfDisposed();
        CheckAsyncTaskInProgress();
 
        if (_charPos < _charLen)
        {
            return false;
        }
 
        // This may block on pipes!
        int numRead = ReadBuffer();
        return numRead == 0;
    }
}

On a freshly instantiated StreamReader, the value of both _charPos and _charLen is zero, leading to the EndOfStream getter invoking ReadBuffer().

ReadBuffer() reading from the underlying stream will then advance the read/write position of that stream, hence the MD5 instance then consuming only the remaining bytes from the stream beginning from the now advanced stream read/write position. Which then in turn yields a different MD5 hash compared to calculating the MD5 hash over the entire stream.

MySkullCaveIsADarkPlace0
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