How would I return an object or multiple values from PowerShell to executing C# code

In your powershell script you can build an Hashtable based on your necessity:

[hashtable]$Return = @{} 
$Return.ReturnCode = [int]1 
$Return.ReturnString = [string]"All Done!" 
Return $Return 

In C# code handle the Psobject in this way

 ReturnInfo ri = new ReturnInfo();
 foreach (PSObject p in psObjects)
 {
   Hashtable ht = p.ImmediateBaseObject as Hashtable;
   ri.ReturnCode = (int)ht["ReturnCode"];
   ri.ReturnText = (string)ht["ReturnString"];
 } 

//Do what you want with ri object.

If you want to use a PsCustomobject as in Keith Hill comment in powershell v2.0:

powershell script:

$return = new-object psobject -property @{ReturnCode=1;ReturnString="all done"}
$return

c# code:

ReturnInfo ri = new ReturnInfo();
foreach (PSObject p in psObjects)
   {
     ri.ReturnCode = (int)p.Properties["ReturnCode"].Value;
     ri.ReturnText = (string)p.Properties["ReturnString"].Value;
   }