TryGetNonEnumeratedCount

public static class EnumerableExtensions
{
    public static bool TryGetNonEnumeratedCount<T>(this IEnumerable<T> enumerable, out int count)
    {
        count = 0;
        if (enumerable is ICollection<T> list)
        {
            count = list.Count;
            return true;
        }
        return false;
    }
}

static void A(IEnumerable<int> data)
{
    if (data.TryGetNonEnumeratedCount(out var count))
    {
        // We will get the count without enumeration.
    }
    else
    {
        // fallback
    }
}