Value types vs Reference types

A question came up recently in the newsgroup, asking why adding seemingly different items to an ArrayList resulted in the ArrayList being full of the same item:

PS >$table = new-object Collections.ArrayList
PS >$row = 1 | select Col1,Col2,Col3
PS >$row.Col1 = "Column 1"
PS >$row.Col2 = "Column 2"
PS >[void] $table.Add($row)
PS >$row.Col1 = "Column 1, again"
PS >$row.Col2 = "Column 2, again"
PS >[void] $table.Add($row)
PS >$table

Col1                       Col2                       Col3
----                       ----                       ----
Column 1, again            Column 2, again
Column 1, again            Column 2, again

The source of the difference comes from the type of data that you assign to a variable.  Data falls into two categories: value types, and reference types.  Some data falls in the "value type" category, where its value is set in stone.  Changing the value actually creates a new one, as does passing it around:

PS > $a = "Foo"
PS > $b = $a
PS > $a += " Test"
PS > $a
Foo Test
PS > $b
Foo

Think of this like a promise you give to somebody.  If that person gives the same promise to somebody else, it's a new promise.  If you change your promise, it's a new promise yet again. 
 
Value types are the .Net primitives: string, int, bool, double, float, etc.
 
The other type category is called a "reference type."  Think of this like a bag you give to somebody.  If that person gives the bag to somebody else, it's still the same bag.  If you put stuff in the bag, take stuff out of the bag, or modify the bag -- it's still the same bag.
 
Most types are reference types.  Examples include ArrayList, StringBuilder, and some of the PowerShell primitives (ie: the Hashtable) -- since they are not .Net primitives.
 
The expression, "$row = 1 | select Col1, Col2, Col3" gives $row a PsObject -- which is a reference type.  When you change values in $row, you change it for everything that is talking about $row – including the copies inside the ArrayList.

In this case, the solution is to create new rows each time:

PS >$table = new-object Collections.ArrayList
PS >$row = 1 | select Col1,Col2,Col3
PS >$row.Col1 = "Column 1"
PS >$row.Col2 = "Column 2"
PS >[void] $table.Add($row)
PS >$row = 1 | select Col1,Col2,Col3
PS >$row.Col1 = "Column 1, again"
PS >$row.Col2 = "Column 2, again"
PS >[void] $table.Add($row)
PS >$table

Col1                       Col2                       Col3
----                       ----                       ----
Column 1                   Column 2
Column 1, again            Column 2, again

 

2 Responses to “Value types vs Reference types”

  1. jachymko writes:

    actually, string is a reference type. it is however immutable, so every modification results in a new instance created.

  2. Lee writes:

    Absolutely — I attempted to gloss over that fact to not confuse the larger issue 🙂

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