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In my previous two posts about hashtables I provided examples for creating a hashtable, returning a specific value and a description around performance. This post will focus on the errors you should expect if you start using a hashtable.

The most common exception I see happens when the data I’m storing in the hashtable is not unique. For example, in the first post in the series we created a simple hashtable from a list of HTML colors. If my input list had the color ‘red’ listed on two different lines, I would have seen an error similar to this:

Add : Exception calling “Add” with “2” argument(s): “Item has already been added. Key in dictionary: ‘Red’ Key being
added: ‘Red'”
At line:1 char:17
+ $colorCodeHT.Add <<<< ('Red','123456') + CategoryInfo : NotSpecified: (:) [], MethodInvocationException + FullyQualifiedErrorId : DotNetMethodException

This is an easy error message to generate if you want to see the actual error. Follow the examples in the first post and then try and re-add the color red using the following line:

$colorCodeHT.Add('Red','123456')

The hashtable object includes a .Contains method, which can be used in an if statement to prevent this exception from happening:

if ($colorCodeHT.Contains('Red')) { "Color 'red' has already been added" } else { $colorCodeHT.Add('Red','123456') }

This statement checks for the presence of a ‘Red’ item and only attempts to add if a value does not already exist.

To help describe these concepts, I have listed a few examples based on a CSV file containing HTML color codes.

For purposes of sample data, create an array object of colors and HTML color codes.

$colorCodes = Import-Csv ColorCodes.csv

Define a hashtable object:

$colorCodeHT = @{}

Examples for adding to a hashtable:

# For the old vbscripter (this is the method I use)
# $colorCodeHT.Add('key','value')
$colorCodes | %{$colorCodeHT.Add($_.ColorName, $_.Code)}

# For the person who likes equal signs
# $colorCodeHT['key'] = 'Value'
$colorCodes | %{$colorCodeHT[$_.ColorName] = $_.Code}

Examples for updating existing entries

# Using square brackets, just like the above 'add' example
$colorCodeHT['Red'] = '#123456'

Examples for removing from a hashtable:

# Remove a single item by key
$colorCodeHT.Remove('Red')

# Remove all of the entries from the hashtable:
$colorCodeHT.Clear()

In the previous post I provided a quick introduction to creating and using a hashtable object. If you are paying attention, you might have asked “Why would I go to extra work to create a hashtable when I can make the original object work?” The answer is performance…but lets expand on this answer.

We can use Measure-Command to show how long each of these statements takes to execute. Here is a quick refresher on using Measure-Command to show how long it takes to execute a command…just replace “PUT EACH COMMAND HERE” with your actual command:

(Measure-Command { "PUT EACH COMMAND HERE" }).TotalMilliseconds

Using the examples from the previous post, we return Total Millisecond execution times of:

In this sample, returning a value from the hashtable (example 4) is 600 times faster than using the array object (example 2)! Just because this step was faster, it doesn’t mean hashtables are always best. If we take into account the overhead from creating the hashtable (add the execution times of example 3 and 4 together), then using the array object is actually more efficient. The performance gain is only noticeable if you plan to re-use the hashtable over several iterations.

# Example 5: Selecting multiple times from array object
(Measure-Command {
($colorCodes | Where-Object {$_.ColorName -eq 'Red'}).Code
($colorCodes | Where-Object {$_.ColorName -eq 'Blue'}).Code
($colorCodes | Where-Object {$_.ColorName -eq 'Green'}).Code
($colorCodes | Where-Object {$_.ColorName -eq 'Black'}).Code
}).TotalMilliseconds

# Example 6: Creating hashtable and selecting from it multiple times
(Measure-Command {
$colorCodeHT = @{}
$colorCodes | %{ $colorCodeHT.Add($_.ColorName, $_.Code) }
$colorCodeHT['Red']
$colorCodeHT['Blue']
$colorCodeHT['Green']
$colorCodeHT['Black']
}).TotalMilliseconds

Even though a hashtable requires a bit of setup, it becomes more efficient with repetitive use. The example with HTML color codes only has around 290 items in the hashtable. This performance improvement is more noticeable with larger hashtables, especially when they are used many times within the script.

I was recently working on a script for a co-worker who is trying to learn more powershell. In the script I had used a couple of hashtables to store information…which caused a couple of questions as he was reviewing the code. This post is an introduction to powershell hashtables.

A hashtable stores indexed key-value pairs in memory and is similar to the vbscript dictionary object. To help describe these concepts, I have listed a few examples based on a CSV file containing HTML color codes.

The examples below will:
1.) Import the CSV into a powershell array object,
2.) Show how to use the array object to return a specific code given a color name
3.) Convert the object from step 1 into a hashtable
4.) Show how to use the hashtable to return a specific code given a color name

# Example 1:
$colorCodes = Import-Csv ColorCodes.csv

# Example 2:
# Using just the array object created from Import-CSV
# we can select the color code for Red with just one line
($colorCodes | Where-Object {$_.ColorName -eq 'Red'}).Code

# Example 3:
# We could also create a hash table and store each item
# from the array using a couple lines of code
$colorCodeHT = @{}
$colorCodes | %{ $colorCodeHT.Add($_.ColorName, $_.Code) }

# Example 4
# And then we can retreive the code for red using either of the
# examples below. Note: you only need one as they return the same
# value.  The first sample is my preferred method, but either works.
$colorCodeHT['Red']
$colorCodeHT.Item('Red')