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


consider following powershell snippet:

$csharpstring = @" using system;  public sealed class myclass {     public myclass() { }     public override string tostring() {         return "this class. there many others " +             "like it, 1 mine.";     } } "@ add-type -typedefinition $csharpstring; $myobject = new-object myclass write-host $myobject.tostring();

if run more once in same appdomain (e.g. run script twice in powershell.exe or powershell_ise.exe) following error:

add-type : cannot add type. type name 'myclass' exists. @ line:13 char:1 + add-type -typedefinition $csharpstring; + ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~     + categoryinfo          : invalidoperation: (myclass:string) [add-type],  exception     + fullyqualifiederrorid :  type_already_exists,microsoft.powershell.commands.addtypecommand

how wrap call add-type -typedefinition called once?

this technique works me:

if (-not ([system.management.automation.pstypename]'myclass').type) {     add-type -typedefinition 'public class myclass { }' }
  • the type name can enclosed in quotes 'myclass', square brackets [myclass], or both '[myclass]' (v3+ only).
  • the type name lookup not case-sensitive.
  • you must use full name of type, unless part of system namespace (e.g. [system.datetime] can looked via 'datetime', [system.reflection.assembly] cannot looked via 'assembly').
  • i've tested in win8.1; powershell v2, v3, v4.

internally, pstypename class calls languageprimitives.convertstringtotype() method handles heavy lifting. caches lookup string when successful, additional lookups faster.

i have not confirmed whether or not exceptions thrown internally mentioned x0n , justin d.


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