Array.GetValue() returns new instance of an struct object

Question

I have an unknown array of struct type. When I'm trying to get some index I get a NEW instance of this object:

var inst = ((SomeStruct)((Array)arrOfSomeStruct).GetValue(0));

Now inst is a different instance from arrOfSomeStruct[0] .

What is the best or the fastest way to get the original instance of the array item, and NOT by using [ ] operators?

What are you trying to do that requires the source struct? — Enigmativity
– Enigmativity, Commented Aug 9, 2016 at 6:57
What are you really trying to achieve? As soon as you assign a struct value to a variable, as in var inst2 = inst1;, the value is copied, so a new instance is created. As soon as you unbox a struct value that has been residing in an object box, as in var inst = (SomeStruct)obj;, a new value/instance is created. As soon as you implicitly box a struct value, as in public object GetValue() { /* ... */ return someStruct /* boxing! */; }, a new instance is created in a new box. Is your struct mutable or immutable since you care about all this? — Jeppe Stig Nielsen
– Jeppe Stig Nielsen, Commented Aug 9, 2016 at 11:56
var inst = orig_arr.GetValue(0); inst.x = 5; // Now orig_arr.x != inst.x — Guy P
– Guy P, Commented Aug 10, 2016 at 11:12

Suren Srapyan · Accepted Answer · 2016-08-09 06:53:12Z

4

You can't get the original if it is a value type. Every Value Type will be copied if it is returned from the method or passed into the method.

To work as you want you must use a reference type(class)

answered Aug 9, 2016 at 6:53

Suren Srapyan

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Community · Accepted Answer · 2017-05-23 12:22:53Z

2

This is default behavior of interaction with a struct. It is copied, not referenced. This is the same for every exchange with a struct, not just your GetValue method (also this[index] for example).

As Jeppe Stig Nielsen commented:

... for an array of value types, .GetValue(0) returns a reference to a boxed copy of the struct value. On the other hand [0] is the original struct value. If you assign to a variable, then a copy is made, as in var valueCopy = arr[0];. However, that is mostly relevant for mutable value types. If the struct has a member that mutates the struct value, then arr[0].MutateValue(); will mutate the original. Of course valueCopy.MutateValue(); will change the copy.

The best option you have is to make the struct an object, ie. move to a class.

In some extreme cases, the use of unsafe code is a last resort:

unsafe
{
    Point p = new Point(1, 2);
    Point*[] points = new Point*[] { &p };

    Point* p2 = points[0];

    p2->X = 2; // this changes both p and p2
}

edited May 23, 2017 at 12:22

CommunityBot

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answered Aug 9, 2016 at 6:53

Patrick Hofman

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1 Comment

Jeppe Stig Nielsen Over a year ago

To clarify, for an array of value types, .GetValue(0) returns a reference to a boxed copy of the struct value. On the other hand [0] is the original struct value. If you assign to a variable, then a copy is made, as in var valueCopy = arr[0];. However, that is mostly relevant for mutable value types. If the struct has a member that mutates the struct value, then arr[0].MutateValue(); will mutate the original. Of course valueCopy.MutateValue(); will change the copy.

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