Faster alternative to nested loops?

As a reminder: you probably do not need this kind of code while developing your own solution. This can and should only used in very specific situations. Readability is often more important than speed.

You can use the properties of a struct and allocate the structure in advance. I cut off some levels in the sample below, but I’m sure you’ll be able to figure out the specifics. Runs about 5-6 times faster than the original (release mode).

The block:

struct ByteBlock
{
    public byte A;
    public byte B;
    public byte C;
    public byte D;
    public byte E;
}

The loop:

var data = new ByteBlock[2*3*4*3*4];
var counter = 0;

var bytes = new ByteBlock();

for (byte a = 0; a < 2; a++)
{
    bytes.A = a;
    for (byte b = 0; b < 3; b++)
    {
        bytes.B = b;
        for (byte c = 0; c < 4; c++)
        {
            bytes.C = c;
            for (byte d = 0; d < 3; d++)
            {
                bytes.D = d;
                for (byte e = 0; e < 4; e++)
                {
                    bytes.E = e;
                    data[counter++] = bytes;
                }
            }
        }
    }
}

It’s faster because it doesn’t allocate a new list every time you add it to the list. Also since it’s creating this list, it needs a reference to every other value (a,b,c,d,e). You can assume each value is only modified once inside the loop, so we can optimize it to do so (data locality).

Also read the comments for side-effects.

Edited the answer to use an T[] instead of a List<T>.