how to check null before linq concat list c#

Question

User-1256377279 posted

Hi Guys,

I have 3 list object and want to join using concat, but how to check nulls before it concat.

Below is my code.

  nonmandat.RelatedAuthorities = nonmandat.RelatedPersons.Concat(nonmandat.RelatedFamilies.DefaultIfEmpty())
                                                                        .Concat(nonmandat.RelatedCorporateBodies.DefaultIfEmpty())
                                                                         .ToList();

From above code if RelatedPersons or RelatedFamilies or RelatedCorporateBodies is null how to avoid concating.

Thanks,

Shabbir

Answer 1

User753101303 posted

Hi,

You are sure it happens ? It's common to use lists which are not null but empty. I expect concatenating  empty lists should work (it should just left the source list unchanged).

I wonder if the problem couldn't be that you are using mistalkenly DefaulfEmpty. For example with :

var a = new List<string> { "a", "b" };
var b = new List<string>();

var c = a.Concat(b); will be the same than a that is "a","b"

var c = a.Concat(b.DefaultIfEmpty()); is "a","b", null because with DefaultIfEmpty you asked to return a singleton list with the default value for its element type (and the default value for a string is null)

var c = a.Concat(b.DefaultIfEmpty("z")); would return the "a", "b", "z" list.


So in short I wonder if you don't want to ,just use :

nonmandat.RelatedPersons.Concat(nonmandat.RelatedFamilies).Concat(nonmandat.RelatedCorporateBodies).ToList();