Demonstration of PowerShell Tied / automatic variables

Tied_Variables.Ps1

<#    The template for our class looks like this  
// Constructor for our variable calls PSVariables' constructor with Name, value, options 
public class  XYZVariable            : System.Management.Automation.PSVariable {
       public XYZVariable ()         : base("XYZ",                          / Name
                                            0,                             // Value
                                            ScopedItemOptions.ReadOnly |   // Options
                                            ScopedItemOptions.AllScope){}
    // variable objects have a value property, override it to return our value 
        public override object Value {
            get {
               //  Whatever code you need to make an XYZ  ( Must have a return )    
                return  the_XYZ_thing ;     
            }
        }
}


Examples...  #>

Add-type  @"
using System.Management.Automation; 

public  class  WhoAmiVariable            : PSVariable {
        public WhoAmiVariable ()         : base("WhoAmI",  0, ScopedItemOptions.ReadOnly | ScopedItemOptions.AllScope){}
        public override object Value {get {

            return  System.Security.Principal.WindowsIdentity.GetCurrent().Name ;

}}}

public  class  DefaultGatewayVariable    : PSVariable {
        public DefaultGatewayVariable () : base("Gateway", 0, ScopedItemOptions.ReadOnly | ScopedItemOptions.AllScope) {}
        public override object Value {get{

            using (PowerShell ps = PowerShell.Create())
            {
                ps.AddScript("(Get-NetRoute -DestinationPrefix '0.0.0.0/0').Nexthop");
                return ps.Invoke(); 
            }

}}}

public  class  NowVariable                : PSVariable {
        public NowVariable ()             : base("Now",     0, ScopedItemOptions.ReadOnly | ScopedItemOptions.AllScope) {}
        public override object Value {get {

                return  System.DateTime.Now; 

}}}
"@

# Create a new object for our type(s) and add to out session

$executioncontext.SessionState.PSVariable.Set((New-Object -TypeName DefaultGatewayVariable))
$executioncontext.SessionState.PSVariable.Set((New-Object -TypeName NowVariable))
$executioncontext.SessionState.PSVariable.Set((New-Object -TypeName WhoAmiVariable))


#### After I showed the above at PSConf_EU @jborean93  showed me a trick for script properties in PowerShell classes.  

class  BatVariable            : System.Management.Automation.PSVariable {
       BatVariable ()         : base("Bat", 0, "ReadOnly, AllScope"){}
       [object] get_Value(){
      
       $b = (Get-CimInstance  -Namespace root\wmi -ClassName BatteryStatus ).RemainingCapacity /
            (Get-CimInstance  -Namespace root\wmi -ClassName BatteryFullChargedCapacity root\wmi).FullChargedCapacity
        return $b.tostring("p0")
       }
}
$executioncontext.SessionState.PSVariable.Set((New-Object -TypeName BatVariable))