How to add an array of hash tables to an array powershell

Question

I'm trying to add an array of hashtables to an existing array of hastables in Powershell, adding a single item works fine but I can't seem to find out how to add multiple hashtables to an existing array.

For example I have the below;

$ToAdd = @()
$ToAdd += (@{ 
        id   = "1234";
        type = "Scope";
    })

I then want to be able to add the $ToAdd array of hashtables to the object below ($helloBody.helloAccess.AccessRequired)

  $HelloBody = [ordered] @{
    "helloAccess" = @(
        @{
            hello = "hello"
            accessRequired = @(
                @{
                    id   = "random guid" 
                    type = "item 123"   
                }
                
                
            )
                              
        }
    )
    
}

How is it possible to add an array of hashtables into another array?

When I try $helloBody.helloAccess.AccessRequired += $ToAdd, I get the following error:

A hash table can only be added to another hash table.

mklement0 · Accepted Answer · 2022-10-05 15:21:16Z

3

In order to update the .accessRequired entry of your nested hashtable, you must avoid member-access enumeration:

In your case, this means that you must access the first element of the array stored in .helloAccess explicitly:

$helloBody.helloAccess[0].AccessRequired += $ToAdd

Omitting [0] doesn't work, because member-access enumeration (regrettably) unwraps a single-element array instead of reporting it as such.

In concrete terms:

$helloBody.helloAccess performs member-access enumeration on the single-element array stored in that property, and returns the AccessRequired entry from each element.
The one and only .AccessRequired entry is itself a single-element array, and member-access enumeration unwraps that single-element array, yielding its only element directly, which is a hashtable.
+= / + on a hashtable only works if the operand is also a hashtable, in which case the entries are merged; using any other type as the operand causes the error you saw (the error message is a bit confusing, but that is what it is trying to tell you).

Note:

Member-access enumeration unwraps single-element arrays, because it emulates the behavior of the ForEach-Object cmdlet in the pipeline, which, as cmdlets generally do, emits objects to the pipeline one by one by enumerating them, so that Write-Output -NoEnumerate @('foo') | ForEach-Object { $_ } yields the same as 'foo' | ForEach-Object { $_ }: A single-element array is treated the same as its one and only element, and the array wrapper is lost.
- For reasons of backward compatibility this behavior is unlikely to change; see GitHub issue #6802 for a discussion.
More generally, note that you can only use member-access enumeration for getting property values, not for setting them:
- For instance, the following fails:
```
# ERROR: "The property 'foo' cannot be found on this object..."
@(@{ foo = 1 }, @{ foo = 2 }).foo = 42
```
  - However, due to the unwrapping behavior, if the result happens to be only a single object, updating a property of that single object happens to work (which is why your assignment worked in principle, but failed due to an incompatible operand):
```
# OK, but only because the single-element array wrapper
# is effectively ignored.
@(@{ foo = @{ bar = 1 } }).foo.bar = 42
```
- Preventing setting is deemed by-design, to prevent inadvertent changes; either way, the error message is confusing - see GitHub issue #5271 for a discussion and a workaround.

edited Oct 5, 2022 at 15:21

answered Oct 5, 2022 at 14:36

mklement0

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2 Comments

Russeller Over a year ago

You're a legend, I must admit I do feel a little embarrassed now as i've been doing a lot of searching around for this and none have worked as of yet. Very good to know that's now working perfectly and great to know for the future! Thanks a lot!

mklement0 Over a year ago

@Russeller, glad to hear it helped - please see my update, which explains the behavior in more detail and provides background information. No need to feel embarrassed at all - this is tricky stuff, both for newcomers and experienced users (even though I'm familiar with the underlying problem, it took me a while to spot it in your code).

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