Adding a hash-table to a hash-table - powershell

Question

I'm trying to add a hash-table to a hash-table using powershell. However, Im getting the following error:
Item has already been added. Key in dictionary: 'Dev' Key being added: 'Dev'
Here's my code:
$colors = #("black","white","yellow","blue")
$Applications=#{}
Foreach ($i in $colors)
{
$Applications += #{
Colour = $i
Prod = 'SrvProd05'
QA = 'SrvQA02'
Dev = 'SrvDev12'
}
}
What am I doing wrong?

Answer 1

I think what you want is something more like this:
$colors = #("black","white","yellow","blue")
$Applications=#{}
Foreach ($i in $colors)
{
$Applications[$i] = #{
Colour = $i
Prod = 'SrvProd05'
QA = 'SrvQA02'
Dev = 'SrvDev12'
}
}
I will also point out that Hashtables often need to be handled defensively. Each key must be unique but values do not need to be. Here is the typical method of handling that:
$colors = #("black","white","yellow","blue")
$Applications=#{}
Foreach ($i in $colors)
{
if($Applications.ContainsKey($i)){
#Do things here if there is already an entry for this key
}else{
$Applications[$i] = #{
Colour = $i
Prod = 'SrvProd05'
QA = 'SrvQA02'
Dev = 'SrvDev12'
}
}
}

Answer 2

EBGreen's helpful answer offers a solution for what you likely meant to do.
To complement that with an explanation of why your code failed:
When you use + to "add" two hashtables, their entries are merged: in other words: the entries of the RHS are added to the LHS hashtable.
(Technically, a new instance is created with the merged entries.)
However - by sensible design - merging is only performed if the hashtables have no keys in common; otherwise, you'll get the error message you saw, complaining about duplicate keys.
If this safeguard weren't in place, you would lose data if the values associated with duplicate entries differ.
Since your loop repeatedly tried to merge a hashtable with the same keys directly into an existing hashtable, your 2nd loop iteration invariably failed.
You can verify this more simply:
$Applications = #{} # create empty hashtable.
# Merge a hashtable literal into $Applications.
# This works fine, because the two hashtables have no keys in common.
$Applications += #{ first = 1; second = 2 }
# $Application now contains the following: #{ first = 1; second = 2 }
# If you now try to add a hashtable with the same set of keys again,
# the operation invariably fails due to duplicate keys.
$Applications += #{ first = 10; second = 20 } # FAILS
# By contrast, adding a hashtable with unique keys works fine:
$Applications += #{ third = 3; fourth = 4 } # OK
# $Application now contains: #{ first = 1; second = 2; third = 3; fourth = 4 }

Answer 3

I was looking for a way to do some sort of "list of enum" and since I'm fairly new to PowerShell, the only thing I could think of is "HashTable of HashTables!".
Could not find a way to do it all "inline", and I got some "can't add a null key to a hashtable" error. After some experimentation I ended up with this, working exactly as I wanted :
$Enums = #{}
$Enums.Frequency = #{
Daily = "daily"
Weekly = "weekly"
Monthly = "monthly"
}
$Enums.SomethinElse = #{
Thing1 = "dflskdjf"
Thing2 = 12
Thing3 = $null
}
You can then use it simply
$Enums.SomethinElse.Thing2
or (if you add it to a Global Variable)
$Global:Enums.Frequency.Daily
It's not exactly the same as the example provided but directly in line with the topic, so I hope it helps someone ending up here :)