Quick Tip: BinaryWriter's Write(string) overload prepends the binary data with the length of the string

If you’re like me and you’re using the .NET Framework’s BinaryWriter class to write bytes to a stream, you might call the overload that takes a string as its parameter and expect to get the sequence of characters that is that string written to the stream as a series of bytes according to the encoding set in the BinaryWriter instance.

In other words, I was expecting the BinaryWriter‘s Write(string) implementation to look something like this pseudocode:

public void Write(string text)
{
     this.buffer.Append(this.encoding.GetBytes(text));
}

After all, when you call Write() with an int, you get four bytes written. If I call Write() with the string “TEST” and the BinaryWriter is using the ASCII encoding, I’d expect four bytes to be written. But instead it writes five.

That’s because the BinaryWriter writes the length of the string in one byte and then calls GetBytes() to output the string data.

Now that I think about it, this makes perfect sense: strings in the .NET framework are typically not thought of as being null-terminated, they’ve got a length, and in order for the BinaryReader‘s Read(string) method to work, it’ll need to be able to know the length of the string to determine how many bytes to read.

In my case, I was writing data to an Epson TM-T88III receipt printer, and given the structure of the commands that the printer expects, it doesn’t need or want the length of the string in this way. Because I didn’t read the MSDN documentation closely, I was left scratching my head as to why weird characters were showing up or characters were being omitted in my output.

The workaround is to just call GetBytes() myself and pass the byte buffer to the BinaryWriter‘s Write(byte[]) overload:

// "bw" is an instance of a BinaryWriter
// "barcode" is a string
 
bw.Write(AsciiControlChars.GroupSeparator);
bw.Write('k');
bw.Write((byte)4);
bw.Write('*');
bw.Write(Encoding.ASCII.GetBytes(barcode)); // NOT bw.Write(barcode);
bw.Write('*');
bw.Write(AsciiControlChars.Null);

Hope this saves someone from a forehead-slapping moment.