Sometimes, we need to define classes in inline C#, like so:

Add-Type -TypeDefinition @'
using System.Collections;

public class MyClass 
    private Int32 myInt;

Typical reasons are that we might need to support a version of Powershell prior to the introduction of native classes, or we need to use P/Invoke to access native Win32 libraries.

When developing like this, the second time we run our code, we bang our heads on the type name already exists error:

Add-Type : Cannot add type. The type name 'MyClass' already exists.
At line:3 char:1
+ Add-Type -TypeDefinition @'
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidOperation: (MyClass:String) [Add-Type], Exception
    + FullyQualifiedErrorId : TYPE_ALREADY_EXISTS,Microsoft.PowerShell.Commands.AddTypeCommand

This is because of AppDomains. You absolutely, massively and totally cannot unload a class from an AppDomain. In general in .NET, you can create a new AppDomain and unload it when you are done, but this is impossible in Powershell because the engine is still in the first AppDomain or, more accurately, if it is possible, it’s beyond me!

I often like to sketch code snippets before formally introducing them into the project, so I increment the class name each time with a hack like this:

if (-not $i) {$Global:i = 1} else {$i++}
Add-Type -TypeDefinition (@'
using System;
using System.Management.Automation;

public class Foo
    //code here

'@ -replace 'Foo', "Foo$i")

which saves me reloading my session every time I update the inline C#. But of course, then my class becomes Foo1, Foo2, Foo3.. and I have to keep editing my test code to reflect the changing typenames. This change saves me that:

$Type = Add-Type -PassThru -TypeDefinition (@'
    // code here
$TypeName = $Type.FullName

so I can have my test object of the latest class:

$Obj = New-Object $TypeName

or, in Powershell 5 and above:

$Obj = $Type::new()

(You can get at all public static members with this syntax.)