C#•14mo ago

❔ ✅ Best way to just get not null values from a collection of possibly null values

I've got a List<MyType?>, and I want to filter out all null values and get a List<MyType back. Which of these is preferable (and more idiomatic) for this, or is there a 3rd better way:

List<MyType> validTypes = possiblyNullTypes.Where(item => item is not null).Cast<MyType>().ToList();
List<MyType> validTypes = possiblyNullTypes.Where(item => item is not null).Select(item => item!).ToList();

List<MyType> validTypes = possiblyNullTypes.Where(item => item is not null).Cast<MyType>().ToList();
List<MyType> validTypes = possiblyNullTypes.Where(item => item is not null).Select(item => item!).ToList();

Thanks

20 Replies

Henkypenky•14mo ago

just

var x = list.Where(p => p is not null).ToList();

var x = list.Where(p => p is not null).ToList();

should suffice

Hugh•14mo ago

If I do that, x is still a List<MyType?>, though, even though I know that there aren't any null values in it And then when I iterate over it later on, my IDE will still want me to check for null

Henkypenky•14mo ago

ah okay then define the type explicitly and cast

Hugh•14mo ago

So Cast<MyType>() over Select(item => item!)

Thinker•14mo ago

Important distinction, is MyType a struct?

Hugh•14mo ago

it's a class Good point

Thinker•14mo ago

right

MODiX•14mo ago

Henkypenky#4865

REPL Result: Success

List<int?> list = new() { 1, 2, 3, null, 5, 6, 7, null };

var x = list.Where(x => x is not null).Cast<int>().ToList();

foreach (var item in x)
{
Console.WriteLine(item);
}

List<int?> list = new() { 1, 2, 3, null, 5, 6, 7, null };

var x = list.Where(x => x is not null).Cast<int>().ToList();

foreach (var item in x)
{
Console.WriteLine(item);
}

Console Output

Compile: 711.182ms | Execution: 130.866ms | React with ❌ to remove this embed.

Thinker•14mo ago

Then you can do what @Henkypenky said will work and you can safely ignore any null warnings.

Henkypenky•14mo ago

yeah Cast is ...

Doombox•14mo ago

ToList also allows you to explicitly set the type, .Where(x => x is not null)!.ToList<string>(); would work with the null forgiveness just thrown in there

Thinker•14mo ago

as long as you slap a ! at the end to signify that you know it's valid and null-safe

Doombox•14mo ago

var strings = new List<string?>();
var res = strings.Where(x => x is not null).ToList<string>();

var strings = new List<string?>();
var res = strings.Where(x => x is not null).ToList<string>();

that will give you a nullability warning but yeah you can just forgive it so, dunno if I'd bother with the cast as well when ToList allows you to specify the type, but that's up to you always bump into this, so weird that the static analysis can't detect this with any reliability catshrug

Henkypenky•14mo ago

it's also a semantics thing right, cause the type is declared as nullable, you just filtered the collection, you know nothing is null, but yeah as Task says the analyzers don't always get it right, they have to cover a lot of basis

Doombox•14mo ago

just one of those annoying little nullability quirks you learn to live with

Henkypenky•14mo ago

indeed

Hugh•14mo ago

Thanks all - this has been helpful How about if I didn't specifically want a List<MyType>, but just wanted to get an IEnumerable<MyType> instead? yeah - it certainly seems that this should be able to be dealt with

Doombox•14mo ago

just .Cast<Type>(); without the ToList() call at all

Hugh•14mo ago

👍 Thanks - appreciate the help here

Accord•14mo ago

Was this issue resolved? If so, run /close - otherwise I will mark this as stale and this post will be archived until there is new activity.

Gaming

Programming

❔ ✅ Best way to just get not null values from a collection of possibly null values