Researching the optimal; implementing the practical

PowerShell is built on top of .NET. This allows you to tap into the power of the .NET framework as well as any DLLs/Assemblies you build. In version 2 of PowerShell the Add-type cmdlet was added which is the preferred way to load .NET assemblies.

In PowerShell Version 1 that cmdlet is not available so you need to use the one of the Load* static methods found in System.Reflection.Assembly.

Why?

For example, in PowerShell, if you want to encode a Url you can use the UrlEncode method found in the .NET HttpUtility Class.

If you fire up PowerShell and try to call the static method UrlEncode found in System.Web.HttpUtility you’ll get this error.

[System.Web.HttpUtility]::UrlEncode("this is a test")

Unable to find type [System.Web.HttpUtility]: make sure that the assembly containing this type is loaded.

You can solve this by loading the System.Web .NET assembly. This is equivalent to being in the Visual Studio IDE, adding a reference and then adding a using statement.

Using Reflection.Assembly in PowerShell Version 1

[Reflection.Assembly]::LoadFile( `
  'C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\System.Web.dll')`
  | out-null            

[System.Web.HttpUtility]::UrlEncode("this is a test")

Using Add-Type in PowerShell Version 2

Add-Type -AssemblyName System.Web
[System.Web.HttpUtility]::UrlEncode("this is a test")

Additional Reflection.Assembly usage

[void] [Reflection.Assembly]::LoadWithPartialName("System.Windows.Forms")
[reflection.assembly]::Load("mscorlib.dll") | Out-Null
[void][system.reflection.assembly]::loadfrom("c:\windows\system32\inetsrv\microsoft.web.administration.dll")

Additional Add-Type usage

Add-Type -AssemblyName "System.Windows.Forms"
Add-Type -AssemblyName "mscorlib.dll"
Add-Type -Path "c:\windows\system32\inetsrv\microsoft.web.administration.dll"

Compiling C# on the fly in Version 2

Add-Type -TypeDefinition @"
public class Test
{
    public static int Add(int n1, int n2)
    {
        return n1 + n2;
    }
}
"@

If you change the signature of the Add method you will get this error:

When compiling code on the fly or loading assemblies they cannot be unloaded.

.NET assemblies can’t be unloaded from a session (it’s a .NET thing, not a PowerShell thing) therefore DLLs can’t be unloaded from a session. This means that you can’t update a DLLs once it’s been loaded. We can’t even update the assembly on disk because the file is locked when the assembly is loaded.

- Bruce Payette Windows PowerShell in Action, Second Edition

Another way to use the –Path parameter on Add-Type

You can point to a C# source code file directly.

Add-Type –Path c:\test\Hello.cs

Add-Type is not limited to compiling only C#

You can inline these languages using the –Language parameter CSharp, CSharpVersion3, VisualBasic, JScript.

Notice, F# is not supported. Here is a post How to add an F# type to a PowerShell session that inlines F#.

NOTE: This uses the F# DLLs shipped with .NET 4.0

Add-TypeFSharp -TypeDefinition @"
module MyModule

let Add a b = a + b;;
"@                        

1..10 | % { [MyModule]::Add( $_, $_*2 ) }

Finally

There are additional parameters that the Add-Type cmdlet supports. You can generate a DLL file for the assembly with the specified name in the location, specify compiler parameters, specify assemblies which your code depends on and use wild cards when loading assemblies.

Check out the online help for Add-Type.

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