powershell hashtable manipulation

Question

Can someone explain please why we have different behaviors when we are working with hash tables?

In fact, when the value is a simple type (integer, string…) or an object type, the behavior is different. When we are working with a simple type and we affect the value to a variable and update it; this will not update the hash table. But when we are working with an object type and we affect the value to a variable and update it; this will update the hash table.

It will be easier to understand with an example ^^.

Simple type:

$hash=@{
    a=1
    b=2
}
$variable = $hash['a']
$variable = 3

Result: $hash
Name                           Value
----                           -----
a                              1
b                              2

Object type:

$hash=@{
    a=New-Object PSObject -Property @{ obj=1 }
    b=New-Object PSObject -Property @{ obj=2 }
}
$variable = $hash['a']
$variable.obj = 3

Result: $hash
Name                           Value
----                           -----
a                              @{obj=3}
b                              @{obj=2}

Community · Accepted Answer · 2017-05-23 12:14:02Z

7

It is because you use reference types and value types.

Object type:

$variable = $hash['a']
$variable.obj = 3

Here you retrieve the object reference and then access properties inside the object.

Simple type:

$variable = $hash['a']
$variable = 3

Here you copy the entire int to a new variable. So when the variable is updated, it is updated independently without influencing the hashtable. To update the value of the hashtable you have to use $hash['a'] = 3

Check out difference between reference types and values types.

Edit: string is special because it is saved in the heap with a reference but it behaves like a value type. So it is copied when assigning to a new string and behaves like the int example.

edited May 23, 2017 at 12:14

CommunityBot

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answered Jul 24, 2015 at 10:07

Fabian

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1 Comment

Mathias R. Jessen Over a year ago

Very nice explanation. Pretty sure though that the value type-like behavior of strings stem from the fact that they're immutable in the runtime

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