Convert List of bytes to memorystream without using ToArray()

Question

How can I save my list<byte> to MemoryStream() without using ToArray() or creating new array ?

This is my current method:

public Packet(List<byte> data)
{
    // Create new stream from data buffer
    using (Stream stream = new MemoryStream(data.ToArray()))
    {
        using (BinaryReader reader = new BinaryReader(stream))
        {
            Length = reader.ReadInt16();
            pID = reader.ReadByte();
            Result = reader.ReadByte();
            Message = reader.ReadString();
            ID = reader.ReadInt32();
        }
    }
}

Why is this so bad? The answers posted give you alternatives but I doubt they are any more efficient — Matt Wilko
– Matt Wilko, Commented Aug 10, 2016 at 9:38

usr · Accepted Answer · 2016-08-10 09:46:28Z

2

The ToArray solution is the most efficient solution possible using documented APIs. MemoryStream will not copy the array. It will just store it. So the only copy is in List<T>.ToArray().

If you want to avoid that copy you need to pry List<T> open using reflection and access the backing array. I advise against that.

Instead, use a collection that allows you to obtain the backing array using legal means. Write your own, or use a MemoryStream in the first place.

A List<T> is not the most efficient way to move around bytes anyway. Storing them is fine, moving them usually has more overhead. For example, adding items bytewise will be far slower than a memcpy.

answered Aug 10, 2016 at 9:46

usr

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4 Comments

Cheng Chen Over a year ago

If possible, use a byte[] from the very beginning instead of List<byte>.

Roope Järvenpää Over a year ago

I was using byte[] , but i need dynamic arrays so list is more efficient than using array.resize multiple times.

Roope Järvenpää Over a year ago

@usr , thank you - this MemoryStream will not copy the array. It will just store it. answers to my question perfect.

usr Over a year ago

@RoopeJärvenpää you can implement your own list that grows an array by a factor of two. That handles the growth and the idea to never copy the array contents.

Lloyd · Accepted Answer · 2016-08-10 09:37:37Z

0

What about something like:

public Packet(List<byte> data)
{
    using (Stream stream = new MemoryStream())
    {
        // Loop list and write out bytes
        foreach(byte b in data)
            stream.WriteByte(b);

        // Reset stream position ready for read
        stream.Seek(0, SeekOrigin.Begin);

        using (BinaryReader reader = new BinaryReader(stream))
        {
            Length = reader.ReadInt16();
            pID = reader.ReadByte();
            Result = reader.ReadByte();
            Message = reader.ReadString();
            ID = reader.ReadInt32();
        }
    }
}

But why do you have a list in the first place? Can't you pass it into the method as a byte[] to start with? It'd be interesting to see how you populate that list.

answered Aug 10, 2016 at 9:37

Lloyd

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1 Comment

Roope Järvenpää Over a year ago

Well, i used Array.resize when i used byte[] to handle TCP packet streams. That caused my program to overwrite new array with previous arrays data - that could be fixed if i added thread sleep just after array creation. AND what i read on internet, they say list is most efficient way to have dynamic array. And that is what i need.

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