Returns all string permutations using the Heap's algorithm (recursive).

GetPermutationsRecursive.cs

char[] chars = ['a', 'b', 'c', 'd'];
GetPermutationsRecursive(chars.Length, chars).Dump();

static IEnumerable<string> GetPermutationsRecursive(int take, char[] chars)
{
    if (take is 1)
    {
        yield return new string(chars);
    }
    else
    {
        var lastIndex = take - 1;
        foreach (var item in GetPermutationsRecursive(lastIndex, chars))
            yield return item;

        for (var i = 0; i < lastIndex; i++)
        {
            if (take % 2 == 0)
                (chars[i], chars[lastIndex]) = (chars[lastIndex], chars[i]);
            else
                (chars[0], chars[lastIndex]) = (chars[lastIndex], chars[0]);

            foreach (var item in GetPermutationsRecursive(lastIndex, chars))
                yield return item;
        }
    }
}