How to make a comma separated string in list inside another list C#?

I have a list of objects (Bookings). Inside that I have a another list of objects (Products).

I need to get comma-separated product id's from this method

I have used for each and it's working fine

private static string GetProductIDs(List<BookingModel>? Bookings)
{

    List<long> ProductIDS = new List<long>();

    foreach (var booking in Bookings)
    {
        var products = booking?.Products ;
        if (products != null)
        {
            foreach (var product in products)
            {
                if (product.ProductID != null)
                {
                    ProductIDS.Add((long)product.ProductID);
                }

            }
        }   

    }
    return string.Join(",", ProductIDS.ToArray());

}

when I try the below code it's returning an error. could you please help me with the Best idea to do this?

var test = Bookings.SelectMany(c => c.Products.Where(p => p.ProductID != null)
    .Select(t => t.ProductID))
    .ToList();
               
return string.Join(",", test.ToArray());

Error:

I would move the final string.Join() out of the method. Change the method design to return an IEnumerable<long>, and then use that result to produce the final comma-separated string.

Additionally, we can't see this for certain, but I suspect at least part of the error is you have a nullable long?, instead of a simple long for ProductID. Otherwise there would be no point in checking whether ProductID was null, as primitive values like long can never be null.

So the new method looks like this:

private static IEnumerable<long> GetProductIDs(IEnumerable<BookingModel>? Bookings)
{
    if (Bookings == null) return Enumerable.Empty<long>();

    return Bookings.Where(b => b.products is object).
               SelectMany(b => b.products.
                                 Where(p => p.ProductID.HasValue).
                                 Select(p => p.ProductID.Value)
                         );
}

And we can call it like this:

var result = string.Join(",", GetProductIDs(MyBookingList));

You might further want to take care to enforce a string format for the long=>string conversion that will not use additional comma separators between the segments of the numbers, which would throw off your CSV data. You get good behavior by default, but people can change it. You want to be sure it will continue to work regardless of what happens with the localization settings on the machine:

var result = string.Join(",", GetProductIDs(MyBookingList).Select(n => n.ToString("###"));

Try

var test = Bookings.SelectMany(c => c.Products.Where(p => p.ProductID != null))
.Select(x => x.ProductID)
.ToArray();