Combine 2 integers and store it in a ulong

Question

I am having a Dictionary<int,ulong>, where I want to store StudentId and his/her registered courses (which is guaranteed to be 2).

Now, As you can see, instead of saving 2 courseids into a List of Integers, i want to store them as ulong as ulong occupy 64 bits and int occupy 32 bits.

So my question is, how can i combine these 2 integer ids and store them into a ulong variable. I've tried with some Bitwise operation and shifting but unable to figure it out.

Answer 1

"Packing" the data of two ints into 64 bits can be accomplished without ulong, for example like this:

Dictionary<int,ValueTuple<int,int>>

ValueTuple<int,int> takes exactly as much space as ulong, but it lets you access individual ints through its properties.

If you must use ulong, here is one approach that lets you pack and unpack ints:

private static ulong Combine(int a, int b) {
    uint ua = (uint)a;
    ulong ub = (uint)b;
    return ub <<32 | ua;
}
private static void Decombine(ulong c, out int a, out int b) {
    a = (int)(c & 0xFFFFFFFFUL);
    b = (int)(c >> 32);
}

Demo.