C#•4mo ago

Making a List<T> from an instance of T without knowing T

Is that possible? The idea is to make a List of T if the the object in question is T

34 Replies

Angius•4mo ago

Could you show some example? Not sure how would you have an instance of T without knowing what the T is

im not sure what you are looking for exactly, but this is the answer I assume you would want something like

public static List<T> MakeListFrom<T>(){
    return new List<T>();
}

public static List<T> MakeListFrom<T>(){
    return new List<T>();
}

Since i assume you are working with generics? if its not what you are looking for, then please can you provide more details?

{sus}OP•4mo ago

Hey folks, already resolved this earlier with some changes in the approach. But used the following at some point:

        public static IList CreateListFromType(Type elementType)
        {
            if (elementType == null)
                throw new ArgumentNullException(nameof(elementType));

            Type listType = typeof(List<>).MakeGenericType(elementType);

            return (IList)Activator.CreateInstance(listType);
        }

        public static IList CreateListFromType(Type elementType)
        {
            if (elementType == null)
                throw new ArgumentNullException(nameof(elementType));

            Type listType = typeof(List<>).MakeGenericType(elementType);

            return (IList)Activator.CreateInstance(listType);
        }

Izagawd•4mo ago

i see. so you wanted to make it from reflection.

{sus}OP•4mo ago

That's what I ended up with.

Izagawd•4mo ago

though I have a question. what areas would you need to make a list of type T via reflection? I want to see if theres a better way can I see an example of its use case in your code?

Angius•4mo ago

I'm also curious what prompted you to resort to reflections

{sus}OP•4mo ago

My initial problem is that at some point in the code I end up with a:

Box boxA = new Box();

Box boxA = new Box();

But the consuming class for this object has something like:

class Warehouse{

List<Box> boxes;

}

class Warehouse{

List<Box> boxes;

}

So, the idea was to automatically convert a Box() class if it's a singular object to List<Box> without knowing the type (Say this code is on a baseclass instead of each object So what I did instead, is to make a List<Box> in the first place, instead of dealing with singular boxes. And then at some point, I have to do:

List<IBox> boxes

List<IBox> boxes

where boxes can be a List<Box> object. Hence the reflection.

Angius•4mo ago

Uh,

class Warehouse<TBox>
{
    public TBox SingleBox { get; set; }
    public List<TBox> AllBoxes { get; set; }
}

class Warehouse<TBox>
{
    public TBox SingleBox { get; set; }
    public List<TBox> AllBoxes { get; set; }
}

Izagawd•4mo ago

yh this works. unless its not what you are looking for?

{sus}OP•4mo ago

It's somewhat what I ended up with a similar approach.

Angius•4mo ago

Imma be honest I don't fully understand what you're talking about With "converting a box class if it's singular" and stuff

{sus}OP•4mo ago

Its okay. HAhaha

Izagawd•4mo ago

what do you intend to do with the boxes

{sus}OP•4mo ago

All good. Think of it as forcing a singular result to a list instead.

Angius•4mo ago

Then... just have a list? And add to that list? No clue what you mean

Izagawd•4mo ago

you want to make one result into a list? why? what way? is there something im missing

Angius•4mo ago

"Forcing a singular result to a list" is just making the result be a list Like, instead of return new Box() you do return new List<Box>(){new Box()}

{sus}OP•4mo ago

Yes. This is what I ended up with. On initial development, the data being queried was a singular result, but the consuming object of that data expects a list of those objects. Therefore the incompatiblity

Angius•4mo ago

No idea why reflections are involved

{sus}OP•4mo ago

So I had the idea of "what if I convert this on the fly" etc.

Angius•4mo ago

If the objects expects a list give it a list...? Like ???

{sus}OP•4mo ago

That's what I ended up doing actually hahaha

Izagawd•4mo ago

why cant you just do

queryResult.FirstOrDefault();

queryResult.FirstOrDefault();

Angius•4mo ago

So am I to assume you threw away that reflections code? Good

{sus}OP•4mo ago

it's supposed to be a lazy autoresolver, where if it sees that the object is just a singular object BUT the List expecting is composed of the object's type, then make a new list of the object's type so it would return a List. Get me? haha say:

List<IBox> boxes = [Do convert single box to List<Box>]

List<IBox> boxes = [Do convert single box to List<Box>]

Angius•4mo ago

So you wanted, what, List<IBox> boxes = new Box() to be valid? Because you can just List<IBox> boxes = [new Box()]

{sus}OP•4mo ago

WAIT WHAT Alright, ill check that out.

Izagawd•4mo ago

u need to have c# 12 for this though

{sus}OP•4mo ago

Ohno

MODiX•4mo ago

Angius

REPL Result: Success

List<int> ints = [1];
ints

List<int> ints = [1];
ints

Result: List<int>

[
  1
]

[
  1
]

Compile: 282.492ms | Execution: 27.961ms | React with ❌ to remove this embed.

{sus}OP•4mo ago

Interesting Will check that out, see if it fits.

Angius•4mo ago

In slightly older versions you'd use target-type new

List<int> ints = new(){ 1 };

List<int> ints = new(){ 1 };

Izagawd•4mo ago

it doesnt only work for list. it works for every type that implements from IEnumerable<T>

int[] a = [1,2,3,4];
HashSet<SomeClass> b = [new SomeClass(), new SomeClass()]

int[] a = [1,2,3,4];
HashSet<SomeClass> b = [new SomeClass(), new SomeClass()]

you can also plop it in a method

public void SomeMethod(List<int> idk){}



SomeMethod([1,2,3,4]);

public void SomeMethod(List<int> idk){}



SomeMethod([1,2,3,4]);

removing the need to type new() makes it feel 10x better for some reason :kekw:

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Making a List<T> from an instance of T without knowing T