Consider following class which stores the information about external urls.

1: class LinkInfo
2: {
3:     public int    Ord { get; set; }
4:     public string Url { get; set; }
5: }

In your application you somehow retrieve the list of such items:

1: List<LinkInfo> l = new List<LinkInfo>()
2: {
3:     new LinkInfo() { Ord = 1, Url = "" },
4:     new LinkInfo() { Ord = 2, Url = "test1" },
5:     new LinkInfo() { Ord = 3, Url = "test1" },
6:     new LinkInfo() { Ord = 4, Url = "test2" },
7:     new LinkInfo() { Ord = 5, Url = "" }
8: };

Your goal is to use Linq to filter this list in a special way:
    if the Url is empty - the item is always returned,
    if the Url is nonempty - only one item with such Url is returned.

In the above case the Linq expression should return:
1: {
2:     new LinkInfo() { Ord = 1, Url = "" },
3:     new LinkInfo() { Ord = 2, Url = "test1" },
4:     new LinkInfo() { Ord = 4, Url = "test2" },
5:     new LinkInfo() { Ord = 5, Url = "" }
6: };

or

1: {
2:     new LinkInfo() { Ord = 1, Url = "" },
3:     new LinkInfo() { Ord = 3, Url = "test1" },
4:     new LinkInfo() { Ord = 4, Url = "test2" },
5:     new LinkInfo() { Ord = 5, Url = "" }
6: }