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

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.