Nivot Ink - Create your own custom "Type Accelerators" in PowerShell

Thursday, March 27, 2008

Create your own custom "Type Accelerators" in PowerShell

PowerShell has several "type accelerators" which are used exactly like a casting operation. Examples of these special operators are [xml] and [wmi]. The former is used for quickly converting a string of xml into a fully-fledged System.Xml.XmlDocument object.

Often I find myself converting things to and from hexadecimal using the -f operator, but this always seemed like just a little too much typing for me. Enter the [hex] accelerator type:

As you can see from the source below, there's no magic here. This is just a straight cast, but I have no namespace. If I had a namespace, say, like "Nivot.PowerShell", we'd have to cast using [nivot.powershell.hex] instead of just [hex]. All of the trickery is done using operator overloads in C#. These tells .NET (and in turn, powershell) how to behave should someone try to add, subtract, remove or divide our instances.

using System;
using System.Collections.Generic;
using System.Text;
public class Hex
{
private readonly int _value = 0;
private Hex(int value)
{
_value = value;
}
private Hex(string value)
{
_value = Convert.ToInt32(value, 16);
}
public static implicit operator Hex(int value)
{
return new Hex(value);
}
public static implicit operator int(Hex value)
{
return value._value;
}
public static explicit operator Hex(string value)
{
return new Hex(value);
}
public static Hex operator +(Hex op1, Hex op2)
{
return new Hex(op1._value + op2._value);
}
public static Hex operator -(Hex op1, Hex op2)
{
return new Hex(op1._value - op2._value);
}
public static Hex operator *(Hex op1, Hex op2)
{
return new Hex(op1._value * op2._value);
}
public static Hex operator /(Hex op1, Hex op2)
{
return new Hex(op1._value / op2._value);
}
public override string ToString()
{
return "0x" + _value.ToString("X");
}
}

The next step is to tell PowerShell's formatter what to do with the new type. Here's a simple format definition that tells the formatter to call ToString() on the Hex instance. This is the method that does the conversion by calling ToString("X") on the integer field. "X" means format the integer as hexadecimal using upper case. A lower-case "x" would output the value using lower-case (if you couldn't guess ;-) ).

<Configuration>
<ViewDefinitions>
<View>
<Name>Hex</Name>
<ViewSelectedBy>
<TypeName>Hex</TypeName>
</ViewSelectedBy>
<CustomControl>
<CustomEntries>
<CustomEntry>
<CustomItem>
<ExpressionBinding>
<ScriptBlock>$_.ToString()</ScriptBlock>
</ExpressionBinding>
</CustomItem>
</CustomEntry>
</CustomEntries>
</CustomControl>
</View>
</ViewDefinitions>
</Configuration>

If we don't load this format file, PowerShell just emits a couple of blank lines when you try to use it. You'll notice from the screenshot above that the blank lines still appear. I'm not sure how to remove these - it looks pretty ugly compared to the output of the [int] object. If you want to play with this, you can download the zip file below and unzip the contents into a single folder and run "hex.ps1". I didn't bother with a full Snap-In, it just loads the DLL using reflection. It also loads the format ps1xml too. Have fun.

HexAccelerator1.zip (2.4 KB)

.NET | Monad | PowerShell | XML

posted on Thursday, March 27, 2008 8:19:15 AM (Eastern Standard Time, UTC-05:00)