C#•5mo ago

List, Collection Initializer and Capacity

Hi! Does creating a new List with new List<T> { new T() } (or any number of elements > 0) starts with the proper capacity in order to avoid resizes or does it start at 0 and then grows to add the elements and I should be using new List<T>(1) { new T() } (for instance) instead? Thanks for helping me figure this out!

43 Replies

Unknown User•5mo ago

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ikalou.OP•5mo ago

I think the docs say new List<T> starts at 0 (instead of 4)?

MODiX•5mo ago

TeBeCo

REPL Result: Success

(new List<int>()).Capacity

(new List<int>()).Capacity

Result: int

Compile: 204.168ms | Execution: 17.922ms | React with ❌ to remove this embed.

Unknown User•5mo ago

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MODiX•5mo ago

TeBeCo

REPL Result: Success

(new List<int>() {1}).Capacity

(new List<int>() {1}).Capacity

Result: int

Compile: 268.383ms | Execution: 25.126ms | React with ❌ to remove this embed.

Unknown User•5mo ago

Message Not Public

MODiX•5mo ago

TeBeCo

REPL Result: Success

(new List<int>(3) {1}).Capacity

(new List<int>(3) {1}).Capacity

Result: int

Compile: 234.006ms | Execution: 27.054ms | React with ❌ to remove this embed.

Unknown User•5mo ago

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SleepWellPupper•5mo ago

var l = new List<T>()
{
  new T(),
  new T(),
  new T()
}

var l = new List<T>()
{
  new T(),
  new T(),
  new T()
}

gets turned into

var l = new List<T>();
l.Add(new T());
l.Add(new T());
l.Add(new T());

var l = new List<T>();
l.Add(new T());
l.Add(new T());
l.Add(new T());

To answer your question

MODiX•5mo ago

TeBeCo

REPL Result: Success

var l = new List<int>();
l.Add(1);
l.Capacity

var l = new List<int>();
l.Add(1);
l.Capacity

Result: int

Compile: 338.572ms | Execution: 21.330ms | React with ❌ to remove this embed.

SleepWellPupper•5mo ago

So it will grow accordingly

Unknown User•5mo ago

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ikalou.OP•5mo ago

My question is‚ when using a collection initializer with n elements‚ is new List<T>() {...} be as efficient as new List<T>(n) {...}. It would seem logical but I can't find the information nor do I know how to test it.

Unknown User•5mo ago

Message Not Public

MODiX•5mo ago

TeBeCo

REPL Result: Success

(new List<int>() {1}).Capacity

(new List<int>() {1}).Capacity

Result: int

Quoted by

<@689473681302224947> from #List, Collection Initializer and Capacity (click here)

Compile: 268.383ms | Execution: 25.126ms | React with ❌ to remove this embed.

Unknown User•5mo ago

Message Not Public

MODiX•5mo ago

TeBeCo

REPL Result: Success

(new List<int>() {1,2,3,4,5}).Capacity

(new List<int>() {1,2,3,4,5}).Capacity

Result: int

Compile: 235.112ms | Execution: 25.023ms | React with ❌ to remove this embed.

Unknown User•5mo ago

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reflectronic•5mo ago

Unknown User•5mo ago

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SleepWellPupper•5mo ago

https://sharplab.io/#v2:C4LgTgrgdgNAJiA1AHwAICYAMBYAUBgRj1UwAJUCAWAbjzwDcBDMUgG1IF5SoBTAd1IAZAJYBnYAB5hUYAD4AFAEo8AbzylSBGOtLptuDQGY8AX1r4CATnmsAdAGFGAB0YBjYcACei6kA===

SharpLab

C#/VB/F# compiler playground.

reflectronic•5mo ago

if you wrote new List<T>() the the parameterless constructor is what you get

SleepWellPupper•5mo ago

Dropdown on the right "run" can be switched to "C#" to show the lowered code

SleepWellPupper•5mo ago

namely:

ikalou.OP•5mo ago

Thanks everyone. So I should specify the capacity manually in the constructor to accommodate for the number of elements if the collection initializer if the code is in a hot path.

Unknown User•5mo ago

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reflectronic•5mo ago

you should use a collection expression

ikalou.OP•5mo ago

I was thinking maybe when using a collection initializer the compiler (or whatever component has this job) might have set the capacity implicit to minimize operations

reflectronic•5mo ago

it is meant to address this problem, among other things List<int> x = [1, 2, 3];

ikalou.OP•5mo ago

But SleepWellPupper's screenshot shows that this is not the case

reflectronic•5mo ago

that’s because that’s not a collection expression

ikalou.OP•5mo ago

I see. I though that's what it was called.

reflectronic•5mo ago

that’s a collection initializer a collection expression is this

MODiX•5mo ago

reflectronic

sharplab.io (click here)

List<int> l =
[
  1,
  2,
  3
];
Console.WriteLine(l.Capacity);

List<int> l =
[
  1,
  2,
  3
];
Console.WriteLine(l.Capacity);

React with ❌ to remove this embed.

ikalou.OP•5mo ago

I'm using Unity and I dont think it has support for collection expressions unfortunately.

SleepWellPupper•5mo ago

So use the language version preview/latest

Petris•5mo ago

note that it's different on main of roslyn

SleepWellPupper•5mo ago

how does it differ?

Petris•5mo ago

you can check it there yourself

SleepWellPupper•5mo ago

wowza that is noice thanks for the hint :)

Aaron•5mo ago

you can't unity bundles their own Roslyn stuck on a subset of C#9

SleepWellPupper•5mo ago

yuck sorry to hear

Petris•5mo ago

iirc it has roslyn 3.9

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List, Collection Initializer and Capacity