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2014-05-30

About PowerShell's PSBoundParameters

PSBoundParameters are useful, simple and could trip you!
Here is our test script t.ps1

param(
    $Name="foo",
    $LastName="bar"
)

"Name: $Name LastName: $LastName"
"PSBoundParameters : " 
$PSBoundParameters
    
"Removing 'Name'"
$PSBoundParameters.Remove('Name')

"After Removing 'Name'"
"Name: $Name LastName: $LastName"
"PSBoundParameters : " 
$PSBoundParameters

    
"Removing 'LastName'"
$PSBoundParameters.Remove('LastName')
    
"After Removing 'LastName'"
"Name: $Name LastName: $LastName"
"PSBoundParameters : " 
$PSBoundParameters

## Let's start testing
D:\> C:\temp\t.ps1
Name: foo LastName: bar
PSBoundParameters :      ## <--- It's empty

Removing 'Name'
False                    ## There was no 'Name' parameter to remove

After Removing 'Name'
Name: foo LastName: bar
PSBoundParameters :

Removing 'LastName'
False

After Removing 'LastName'
Name: foo LastName: bar
PSBoundParameters :

D:\>

So, what happened? Well, $PSBoundParameters did not have anything, because nothing was passed to the script.

Now let's try passing it parameters

D:\> C:\temp\t.ps1 -name Adil -LastName Hindistan
Name: Adil LastName: Hindistan
PSBoundParameters :       ## This returns 2 key/value pairs      

Key                                Value
---                                -----
Name                               Adil
LastName                           Hindistan

Removing 'Name'           ## <-- returns True, b/c it could remove the binding
True

After Removing 'Name'
Name: Adil LastName: Hindistan
PSBoundParameters :
LastName                           Hindistan  ## <- now, $PSBoundParameters only has one key/value

Removing 'LastName'
True

After Removing 'LastName'
Name: Adil LastName: Hindistan
PSBoundParameters :

Note that $Name and $LastName got their values assigned and are not affected from $PSBoundParameters.Remove() operation at all. All we are doing is removing keys from a dictionary.

We can also add key/value pairs to it using this:
$PSBoundParameters.Add('Key','Value')

E.g. 
$LastName='Doe'
$PSBoundParameters.Add('NewLastName',$LastName)
$PSBoundParameters are quite useful to determine whether a parameter is using its default value or was it passed a value. It is also useful for splatting:
Some-CommandLet @PSBoundParameters
This supplies the key value pairs a CMDLET or function would require
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