Windows PowerShell Cookbook 2013 by Unknown

Author:Unknown
Language: eng
Format: epub

Discussion

You can access many simple Windows APIs using the script given in Recipe 17.5 . This approach is difficult for more complex APIs, however.

In PowerShell version one, it was possible to access these APIs in one of two ways: by generating a dynamic assembly on the fly (you wouldn’t really do this for one-off calls, but Recipe 17.5 uses this technique) or by looking up the P/Invoke definition for that API call and compiling the C# to access it.

These are both good approaches, but PowerShell version two introduces the Add-Type cmdlet to make this much easier.

Add-Type offers four basic modes of operation:

PS > Get-Command Add-Type | Select -Expand ParameterSets | Select Name

----

FromSource

FromMember

FromPath

FromAssemblyName

These modes of operation are:

FromSource

Compile some C# (or other language) code that completely defines a type. This is useful when you want to define an entire class, its methods, namespace, etc. You supply the actual code as the value to the -TypeDefinition parameter, usually through a variable. For more information about this technique, see Recipe 17.6 .

FromPath

Compile from a file on disk, or load the types from an assembly at that location. For more information about this technique, see Recipe 17.8 .

FromAssemblyName

Load an assembly from the .NET Global Assembly Cache (GAC) by its shorter name. This is not the same as the [Reflection.Assembly]::LoadWithPartialName method, since that method introduces your script to many subtle breaking changes. Instead, PowerShell maintains a large mapping table that converts the shorter name you type into a strongly named assembly reference. For more information about this technique, see Recipe 17.8 .

FromMember

Generates a type out of a member definition (or a set of them). For example, if you specify only a method definition, PowerShell automatically generates the wrapper class for you. This parameter set is explicitly designed to easily support P/Invoke calls.

Now, how do you use the FromMember parameter set to call a Windows API? The solution shows the end result of this process, but let’s take it step-by-step. First, imagine that you want to access sections of an INI file.

PowerShell doesn’t have a native way to manage INI files, and neither does the .NET Framework. However, the Windows API does, through a call to the function called GetPrivateProfileString . The .NET framework lets you access Windows functions through a technique called P/Invoke (Platform Invocation Services). Most calls boil down to a simple P/Invoke definition , which usually takes a lot of trial and error. How-ever, a great community has grown around these definitions, resulting in an enormous resource called P/Invoke .NET ( http://www.pinvoke.net/ ). The .NET Framework team also supports a tool called the P/Invoke Interop Assistant that generates these defini-tions as well, but we won’t consider that for now.

First, we’ll create a script called Get-PrivateProfileString.ps1 . It’s a template for now: ## Get-PrivateProfileString.ps1

param(

$Path,

$Category,

$Key)

$null

To start fleshing this out, we visit P/Invoke .NET and search for GetPrivateProfile String , as shown in Figure 17-1 .

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