C# How to write to a byte array starting from index x

Question

I have a custom protocol to send and receive messages over TCP like the following:

The first 4 bytes is the message type, the following 4 bytes is the length of the message and the rest of the buffer is containing the message itself.

 private byte[] CreateMessage(int mtype,string data)
 {

        byte[] buffer = new byte[4 + 4 + data.Length];

       //write mtype, data.Length, and data to buffer

        return buffer;

 }

I want to write mtype to the first 4 bytes of buffer and then data.Length to next 4 bytes and then the data. I am coming from golang world and we do that like the following:

buf := make([]byte, 4+4+len(data))
binary.LittleEndian.PutUint32(buf[0:], uint32(mtype))
binary.LittleEndian.PutUint32(buf[4:], uint32(len(data)))

So basically you're asking how to convert an int to a byte[4] with some certain order to the bytes? — Caius Jard
– Caius Jard, Commented Feb 28, 2022 at 19:49
Perhaps just use a BinaryWriter, and write the ints to the network stream; they'll end up as byte sequences on the wire .. — Caius Jard
– Caius Jard, Commented Feb 28, 2022 at 19:54
@CaiusJard in all seriousness: in something like 18 years specialising in .NET IO code (serializers, network code, etc): I have yet to find a scenario where BinaryWriter is the correct answer to any requirement — Marc Gravell
– Marc Gravell, Commented Feb 28, 2022 at 22:44

Marc Gravell · Accepted Answer · 2022-02-28 22:43:16Z

1

Span<byte> span = buffer;
BinaryPrimitives.WriteUInt32LittleEndian(span, type);
BinaryPrimitives.WriteUInt32LittleEndian(span.Slice(4), (uint)len);
// etc

A span is sort of like an array, and you can create a span from an array..but a span can be sliced internally. Not all APIs work with spans, but those that do ... sweet.

answered Feb 28, 2022 at 22:43

Marc Gravell

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