How do I create a custom type in PowerShell for my scripts to use?

Prior to PowerShell 3

PowerShell's Extensible Type System didn't originally let you create concrete types you can test against the way you did in your parameter. If you don't need that test, you're fine with any of the other methods mentioned above.

If you want an actual type that you can cast to or type-check with, as in your example script ... it cannot be done without writing it in C# or VB.net and compiling. In PowerShell 2, you can use the "Add-Type" command to do it quite simmple:

add-type @"
public struct contact {
   public string First;
   public string Last;
   public string Phone;
}
"@

Historical Note: In PowerShell 1 it was even harder. You had to manually use CodeDom, there is a very old function new-struct script on PoshCode.org which will help. Your example becomes:

New-Struct Contact @{
    First=[string];
    Last=[string];
    Phone=[string];
}

Using Add-Type or New-Struct will let you actually test the class in your param([Contact]$contact) and make new ones using $contact = new-object Contact and so on...

In PowerShell 3

If you don't need a "real" class that you can cast to, you don't have to use the Add-Member way that Steven and others have demonstrated above.

Since PowerShell 2 you could use the -Property parameter for New-Object:

$Contact = New-Object PSObject -Property @{ First=""; Last=""; Phone="" }

And in PowerShell 3, we got the ability to use the PSCustomObject accelerator to add a TypeName:

[PSCustomObject]@{
    PSTypeName = "Contact"
    First = $First
    Last = $Last
    Phone = $Phone
}

You're still only getting a single object, so you should make a New-Contact function to make sure that every object comes out the same, but you can now easily verify a parameter "is" one of those type by decorating a parameter with the PSTypeName attribute:

function PrintContact
{
    param( [PSTypeName("Contact")]$contact )
    "Customer Name is " + $contact.First + " " + $contact.Last
    "Customer Phone is " + $contact.Phone 
}

In PowerShell 5

In PowerShell 5 everything changes, and we finally got class and enum as language keywords for defining types (there's no struct but that's ok):

class Contact
{
    # Optionally, add attributes to prevent invalid values
    [ValidateNotNullOrEmpty()][string]$First
    [ValidateNotNullOrEmpty()][string]$Last
    [ValidateNotNullOrEmpty()][string]$Phone

    # optionally, have a constructor to 
    # force properties to be set:
    Contact($First, $Last, $Phone) {
       $this.First = $First
       $this.Last = $Last
       $this.Phone = $Phone
    }
}

We also got a new way to create objects without using New-Object: [Contact]::new() -- in fact, if you kept your class simple and don't define a constructor, you can create objects by casting a hashtable (although without a constructor, there would be no way to enforce that all properties must be set):

class Contact
{
    # Optionally, add attributes to prevent invalid values
    [ValidateNotNullOrEmpty()][string]$First
    [ValidateNotNullOrEmpty()][string]$Last
    [ValidateNotNullOrEmpty()][string]$Phone
}

$C = [Contact]@{
   First = "Joel"
   Last = "Bennett"
}

Creating custom types can be done in PowerShell.
Kirk Munro actually has two great posts that detail the process thoroughly.

• Naming Custom Objects
• Defining Default Properties for Custom Objects

The book Windows PowerShell In Action by Manning also has a code sample for creating a domain specific language to create custom types. The book is excellent all around, so I really recommend it.

If you are just looking for a quick way to do the above, you could create a function to create the custom object like

function New-Person()
{
  param ($FirstName, $LastName, $Phone)

  $person = new-object PSObject

  $person | add-member -type NoteProperty -Name First -Value $FirstName
  $person | add-member -type NoteProperty -Name Last -Value $LastName
  $person | add-member -type NoteProperty -Name Phone -Value $Phone

  return $person
}