Using Linq to return a Comma separated string

Question

I have a class in my application

public class ProductInfo
{
  public int ProductId {get;set;}
  public int ProductType{get;set;}
}

I want to wr

Answer 1

var s = string.Join(",", products.Where(p => p.ProductType == someType)
                                 .Select(p => p.ProductId.ToString()));