How do I conditionally add a class with Add-Type -TypeDefinition if it isn't added already?

Question

Consider the following PowerShell snippet:

$csharpString = @" using System;  public sealed class MyClass {     public MyClass() { }     public override string ToString() {         return "This is my class. There are many others " +             "like it, but this one is mine.";     } } "@ Add-Type -TypeDefinition $csharpString; $myObject = New-Object MyClass Write-Host $myObject.ToString(); 

If I run it more than once in the same AppDomain (e.g. run the script twice in powershell.exe or powershell_ise.exe) I get the following error:

Add-Type : Cannot add type. The type name 'MyClass' already exists. At line:13 char:1 + Add-Type -TypeDefinition $csharpString; + ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~     + CategoryInfo          : InvalidOperation: (MyClass:String) [Add-Type],  Exception     + FullyQualifiedErrorId :  TYPE_ALREADY_EXISTS,Microsoft.PowerShell.Commands.AddTypeCommand 

How do I wrap the call to Add-Type -TypeDefinition so that its only called once?

Answer 1

This technique works well for me:

if (-not ([System.Management.Automation.PSTypeName]'MyClass').Type) {     Add-Type -TypeDefinition 'public class MyClass { }' } 

• The type name can be enclosed in quotes 'MyClass', square brackets [MyClass], or both '[MyClass]' (v3+ only).
• The type name lookup is not case-sensitive.
• You must use the full name of the type, unless it is part of the System namespace (e.g. [System.DateTime] can be looked up via 'DateTime', but [System.Reflection.Assembly] cannot be looked up via 'Assembly').
• I've only tested this in Win8.1; PowerShell v2, v3, v4.

Internally, the PSTypeName class calls the LanguagePrimitives.ConvertStringToType() method which handles the heavy lifting. It caches the lookup string when successful, so additional lookups are faster.

I have not confirmed whether or not any exceptions are thrown internally as mentioned by x0n and Justin D.