Question

With enum under .net the biggest number you can use is ULong.
This mean a maximum of 64 flags.

What would be the alternative when you need more than 64 flags?

Edit

Sorry, I forgot to add this, alternative that would still work with bitwise operations at least these one; and and or.

using Josh Einstein suggestion, I came up with this, does it make sense?

class bitArrayFlag
{
    private const int flagSize = 255; //allow X numbers of flags

    public BitArray flag1;
    public BitArray flag2;
    public BitArray flagN;

    public bitArrayFlag()
    {
        int flagPos = 0;
        bool[] flagBit = new bool[flagSize];

        flagBit[flagPos] = true;
        flag1 = new BitArray(flagBit);

        flagBit[flagPos] = false;
        flagPos += 1;
        flagBit[flagPos] = true;
        flag2 = new BitArray(flagBit);

        //...
        //...
        //...

        flagBit[flagPos] = false;
        flagPos += 1;
        flagBit[flagPos] = true;
        flagN = new BitArray(flagBit);
    }
}

Answer 1

You could then switch to using a BitArray. You would lose all the “features” of an enum such as the default string formatting and parsing ability. A BitArray would be fundamentally similar to having a bunch of boolean fields except that the storage is much more efficient.

Indeed as Jeff says in the comments with that many independent bit states it seems like an Enum is the wrong solution anyway. A BitArray may be much more suitable for your particular scenario.