C#•2y ago

❔ Updating empty object in a list

Hi, is there any way to update specific empty property (e.g. of int type) of an object in a List?

64 Replies

TheRanger•2y ago

u have a List<object> or something?

Lucas.mrozikplOP•2y ago

(1) I create an empty object in a List. (2) I want to update a property in that empty object.

TheRanger•2y ago

@Lucas.mrozikpl

Lucas.mrozikplOP•2y ago

in my class I have: public ObservableCollection<Day> ActivitiesPerDay { get; set; } = new(); in a method somewhere I created an empty object there ActivitiesPerDay.Add( new Day() { }); now I want to update chosen object's property if the object had filled property I would have updated it easily

TheRanger•2y ago

lets say u want to choose the first object

Lucas.mrozikplOP•2y ago

yes

TheRanger•2y ago

ActivitiesPerDay[0].YourProperty = YourValue;

ActivitiesPerDay[0].YourProperty = YourValue;

Lucas.mrozikplOP•2y ago

was that simple? 🙂

TheRanger•2y ago

yes, its called Indexer

Lucas.mrozikplOP•2y ago

I was looking into some LINQ methods but I could not find anything useful

TheRanger•2y ago

ActivitiesPerDay.First().YourProperty = YourValue;

ActivitiesPerDay.First().YourProperty = YourValue;

First is a LINQ Method

Lucas.mrozikplOP•2y ago

let me use it then! Thank you!

TheRanger•2y ago

i mean u can use indexers as it can be slower with LINQ

Lucas.mrozikplOP•2y ago

ahh I see

Anton•2y ago

it will only work with reference types (list indexer is not a ref getter, right?)

TheRanger•2y ago

Anton•2y ago

well if it's a get set property, it won't work with structs if it's a ref get, it will

TheRanger•2y ago

what exactly wont work with structs?

Anton•2y ago

the indexer will make a copy

TheRanger•2y ago

copy of what?

Anton•2y ago

hence the item in the array won't be modified

TheRanger•2y ago

no it wont

Anton•2y ago

copy of the item in the array yes it will

TheRanger•2y ago

it copies if u try to assign it to a variable but here im directly accessing the value

Anton•2y ago

how do I run code here? i forgot

TheRanger•2y ago

$eval

MODiX•2y ago

To compile C# code in Discord, use !eval Example:

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!eval for(int i = 0; i < 4; i++) Console.WriteLine(i);

Please don't try breaking the REPL service. Also, remember to do so in #bot-spam! To compile C# code in Discord, use !eval Example:

!eval for(int i = 0; i < 4; i++) Console.WriteLine(i);

!eval for(int i = 0; i < 4; i++) Console.WriteLine(i);

Please don't try breaking the REPL service. Also, remember to do so in #bot-spam!

MODiX•2y ago

AntonC#3545

REPL Result: Failure

$eval
struct Item
{
    public int value;
}
class A
{
    Item item;
    public Item this[int index]
    {
        get => item;
        set => item = value;
    }
}

A a = new A();
a[0].value = 5;
a[0].value

$eval
struct Item
{
    public int value;
}
class A
{
    Item item;
    public Item this[int index]
    {
        get => item;
        set => item = value;
    }
}

A a = new A();
a[0].value = 5;
a[0].value

Exception: CompilationErrorException

- Unexpected character '$'
- ; expected
- The name 'eval' does not exist in the current context
- Cannot modify the return value of 'A.this[int]' because it is not a variable

- Unexpected character '$'
- ; expected
- The name 'eval' does not exist in the current context
- Cannot modify the return value of 'A.this[int]' because it is not a variable

Compile: 582.411ms | Execution: 0.000ms | React with ❌ to remove this embed.

MODiX•2y ago

AntonC#3545

REPL Result: Failure

struct Item
{
    public int value;
}
class A
{
    Item item;
    public Item this[int index]
    {
        get => item;
        set => item = value;
    }
}

A a = new A();
a[0].value = 5;
a[0].value

struct Item
{
    public int value;
}
class A
{
    Item item;
    public Item this[int index]
    {
        get => item;
        set => item = value;
    }
}

A a = new A();
a[0].value = 5;
a[0].value

Exception: CompilationErrorException

- Cannot modify the return value of 'A.this[int]' because it is not a variable

- Cannot modify the return value of 'A.this[int]' because it is not a variable

Compile: 544.023ms | Execution: 0.000ms | React with ❌ to remove this embed.

Anton•2y ago

see, it doesn't even work with a simple get set property it will work with ref get what's different about lists?

MODiX•2y ago

AntonC#3545

REPL Result: Failure

public struct A
{
    public int value;
}
var l = new List<A>();
l.Add(new A());
a[0].value = 5;
a[0].value

public struct A
{
    public int value;
}
var l = new List<A>();
l.Add(new A());
a[0].value = 5;
a[0].value

Exception: CompilationErrorException

- The name 'a' does not exist in the current context
- The name 'a' does not exist in the current context

- The name 'a' does not exist in the current context
- The name 'a' does not exist in the current context

Compile: 563.141ms | Execution: 0.000ms | React with ❌ to remove this embed.

MODiX•2y ago

AntonC#3545

REPL Result: Failure

public struct A
{
    public int value;
}
var l = new List<A>();
l.Add(new A());
l[0].value = 5;
l[0].value

public struct A
{
    public int value;
}
var l = new List<A>();
l.Add(new A());
l[0].value = 5;
l[0].value

Exception: CompilationErrorException

- Cannot modify the return value of 'List<A>.this[int]' because it is not a variable

- Cannot modify the return value of 'List<A>.this[int]' because it is not a variable

Compile: 605.340ms | Execution: 0.000ms | React with ❌ to remove this embed.

Anton•2y ago

same here

TheRanger•2y ago

u said array here

Anton•2y ago

sorry I meant list array indexer is ref get

MODiX•2y ago

AntonC#3545

REPL Result: Success

struct Item
{
    public int value;
}
class A
{
    Item item;
    public ref Item this[int index]
    {
        get => ref item;
    }
}

A a = new A();
a[0].value = 5;
a[0].value

struct Item
{
    public int value;
}
class A
{
    Item item;
    public ref Item this[int index]
    {
        get => ref item;
    }
}

A a = new A();
a[0].value = 5;
a[0].value

Result: int

Compile: 552.809ms | Execution: 49.238ms | React with ❌ to remove this embed.

Anton•2y ago

so like this

TheRanger•2y ago

i dont think array has an indexer, its just works directly

Anton•2y ago

well it does do bounds checking and it's a class so idk about that even if it's some internal special thing, it acts just like a ref indexer

TheRanger•2y ago

     public object? GetValue(int index)
        {
            if (Rank != 1)
                ThrowHelper.ThrowArgumentException(ExceptionResource.Arg_Need1DArray);
 
            return InternalGetValue(GetFlattenedIndex(index));
        }

     public object? GetValue(int index)
        {
            if (Rank != 1)
                ThrowHelper.ThrowArgumentException(ExceptionResource.Arg_Need1DArray);
 
            return InternalGetValue(GetFlattenedIndex(index));
        }

in class Array atleast no ref here i cant find T[] class, probably a built in thing

Anton•2y ago

well yeah this one gets you a copy of the value

TheRanger•2y ago

why would array's getter be ref anyway

Anton•2y ago

wdym?

TheRanger•2y ago

Anton•2y ago

wdym why?

TheRanger•2y ago

its just a getter whats the point of it to have ref

MODiX•2y ago

AntonC#3545

REPL Result: Failure

struct Item
{
    public int value;
}

Item[] a = new Item[1];
ref var i = ref a[0];
i.value = 1;
a[0]

struct Item
{
    public int value;
}

Item[] a = new Item[1];
ref var i = ref a[0];
i.value = 1;
a[0]

Exception: CompilationErrorException

- Syntax error, '(' expected
- Identifier expected
- Identifier expected
- ) expected
- The type or namespace name 'a' could not be found (are you missing a using directive or an assembly reference?)
- Array size cannot be specified in a variable declaration (try initializing with a 'new' expression)
- The contextual keyword 'var' may only appear within a local variable declaration or in script code
- 'i(ref a[])' must declare a body because it is not marked abstract, extern, or partial
- 'i(ref a[])' is a method, which is not valid in the given context

- Syntax error, '(' expected
- Identifier expected
- Identifier expected
- ) expected
- The type or namespace name 'a' could not be found (are you missing a using directive or an assembly reference?)
- Array size cannot be specified in a variable declaration (try initializing with a 'new' expression)
- The contextual keyword 'var' may only appear within a local variable declaration or in script code
- 'i(ref a[])' must declare a body because it is not marked abstract, extern, or partial
- 'i(ref a[])' is a method, which is not valid in the given context

Compile: 577.046ms | Execution: 0.000ms | React with ❌ to remove this embed.

TheRanger•2y ago

in that code u added ref manually

Anton•2y ago

so what that's the same with custom ref indexers

TheRanger•2y ago

doesnt mean array's getter is ref

Anton•2y ago

to assign to a ref variable you need to type ref both for the variable type and for the expression

TheRanger•2y ago

just like how ref is placed here

Anton•2y ago

well if it weren't ref you couldn't assign like this ref var item = ref a[index] but you can

TheRanger•2y ago

this is arraysegment atleast

      public T this[int index]
        {
            get
            {
                if ((uint)index >= (uint)_count)
                {
                    ThrowHelper.ThrowArgumentOutOfRange_IndexMustBeLessException();
                }
 
                return _array![_offset + index];
            }
            set
            {
                if ((uint)index >= (uint)_count)
                {
                    ThrowHelper.ThrowArgumentOutOfRange_IndexMustBeLessException();
                }
 
                _array![_offset + index] = value;
            }
        }

      public T this[int index]
        {
            get
            {
                if ((uint)index >= (uint)_count)
                {
                    ThrowHelper.ThrowArgumentOutOfRange_IndexMustBeLessException();
                }
 
                return _array![_offset + index];
            }
            set
            {
                if ((uint)index >= (uint)_count)
                {
                    ThrowHelper.ThrowArgumentOutOfRange_IndexMustBeLessException();
                }
 
                _array![_offset + index] = value;
            }
        }

no ref here

Anton•2y ago

That means you can't do this trick with array segment you can't assign to a ref variable if the indexer is not ref

TheRanger•2y ago

the ref var i = ref a[0]; trick?

Anton•2y ago

yeah

TheRanger•2y ago

let me test

Anton•2y ago

It's C# 8 semantics i think

TheRanger•2y ago

well T[] i cant find it in source code probably magic, lol

Anton•2y ago

yeah it's got to be some internal thing but the fact is a fact that the indexer acts like a ref get

TheRanger•2y ago

this sadens me

MODiX•2y ago

TheRanger#3357

REPL Result: Failure

Array array = new int[1];
Console.WriteLine(array[0]);

Array array = new int[1];
Console.WriteLine(array[0]);

Exception: CompilationErrorException

- Cannot apply indexing with [] to an expression of type 'Array'

- Cannot apply indexing with [] to an expression of type 'Array'

Compile: 570.442ms | Execution: 0.000ms | React with ❌ to remove this embed.

Accord•2y 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.

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❔ Updating empty object in a list