Convert byte array to array segments of a certain length

if speed isn't a concern

var bytes = new byte[640 * 6];

for (var i = 0; i <= bytes.Length; i+=640)
{
   var chunk = bytes.Skip(i).Take(640).ToArray();
   ...
}

Alternatively you could use

• Span.Slice Method
• Buffer.BlockCopy(Array, Int32, Array, Int32, Int32) Method

Span

Span<byte> bytes = arr; // Implicit cast from T[] to Span<T>

...

slicedBytes = bytes.Slice(i, 640);

BlockCopy

Note this will probably be the fastest of the 3

var chunk = new byte[640]
Buffer.BlockCopy(bytes, i, chunk, 0, 640);

If you truly want to make new arrays from each 640 byte chunk, then you're looking for .Skip and .Take

Here's a working example (and a repl of the example) that I hacked together.

using System;
using System.Linq;
using System.Text;
using System.Collections;
using System.Collections.Generic;

class MainClass {
public static void Main (string[] args) {
        // mock up a byte array from something
        var seedString = String.Join("", Enumerable.Range(0, 1024).Select(x => x.ToString()));
        var byteArrayInput = Encoding.ASCII.GetBytes(seedString);

        var skip = 0;
        var take = 640;
        var total = byteArrayInput.Length;

        var output = new List<byte[]>();

        while (skip + take < total) {
            output.Add(byteArrayInput.Skip(skip).Take(take).ToArray());
            skip += take;
        }

        output.ForEach(c => Console.WriteLine($"chunk: {BitConverter.ToString(c)}"));
    }
}

It's really probably better to actually use the ArraySegment properly --unless this is an assignment to learn LINQ extensions.

You can write a generic helper method like this:

public static IEnumerable<T[]> AsBatches<T>(T[] input, int n)
{
    for (int i = 0, r = input.Length; r >= n; r -= n, i += n)
    {
        var result = new T[n];
        Array.Copy(input, i, result, 0, n);
        yield return result;
    }
}

Then you can use it in a foreach loop:

byte[] byteArray = new byte[123456];

foreach (var batch in AsBatches(byteArray, 640))
{
    Console.WriteLine(batch.Length); // Do something with the batch.
}

Or if you want a list of batches just do this:

List<byte[]> listOfBatches = AsBatches(byteArray, 640).ToList();

If you want to get fancy you could make it an extension method, but this is only recommended if you will be using it a lot (don't make an extension method for something you'll only be calling in one place!).

Here I've changed the name to InChunksOf() to make it more readable:

public static class ArrayExt
{
    public static IEnumerable<T[]> InChunksOf<T>(this T[] input, int n)
    {
        for (int i = 0, r = input.Length; r >= n; r -= n, i += n)
        {
            var result = new T[n];
            Array.Copy(input, i, result, 0, n);
            yield return result;
        }
    }
}

Which you could use like this:

byte[] byteArray = new byte[123456];

// ... initialise byteArray[], then:

var listOfChunks = byteArray.InChunksOf(640).ToList();

[EDIT] Corrected loop terminator from r > n to r >= n.