Why does BinaryWriter prepend gibberish to the start of a stream? How do you avoid it?

Question

I\'m debugging some issues with writing pieces of an object to a file and I\'ve gotten down to the base case of just opening the file and writing \"TEST\" in it. I\'m doing

Answer 1

They are not byte-order marks but a length-prefix, according to MSDN:

public virtual void Write(string value);

Writes a length-prefixed string to [the] stream

And you will need that length-prefix if you ever want to read the string back from that point. See BinaryReader.ReadString().

Additional

Since it seems you actually want a File-Header checker

1. Is it a problem? You read the length-prefix back so as a type-check on the File it works OK

2. You can convert the string to a byte[] array, probably using Encoding.ASCII. But hen you have to either use a fixed (implied) length or... prefix it yourself. After reading the byte[] you can convert it to a string again.

3. If you had a lot of text to write you could even attach a TextWriter to the same stream. But be careful, the Writers want to close their streams. I wouldn't advice this in general, but it is good to know. Here too you will have to mark a Point where the other reader can take over (fixed header works OK).

Answer 2

As Henk pointed out in this answer, this is the length of the string (as a 32-bit int).

If you don't want this, you can either write "TEST" manually by writing the ASCII characters for each letter as bytes, or you could use:

System.Text.Encoding.UTF8.GetBytes("TEST")

And write the resulting array (which will NOT contain a length int)

Answer 3

What you're seeing is actually a 7 bit encoded integer, which is a kind of integer compression.
The BinaryWriter prepend the text with this so readers (i.e. BinaryReader) will know how long the written string is.

• BinaryWriter.Write7BitEncodedInt
• BinaryReader.Read7BitEncodedInt

You can read more about the implementation details of this at http://dpatrickcaldwell.blogspot.se/2011/09/7-bit-encoding-with-binarywriter-in-net.html.

Answer 4

You can save it as a UTF8 encoded byte array like this:

...

BinaryWriter w = new BinaryWriter(fs);

w.Write(UTF8Encoding.Default.GetBytes("test"));

...

Answer 5

Sounds like byte order marks.

http://en.wikipedia.org/wiki/Byte-order_mark

Perhaps you want to write the string as UTF-8.

Answer 6

That's a byte order mark, most likely. It's because the stream's encoding is set to Unicode.