Announcing poshrunner.exe so MyScript.ps1 can use MyScript.ps1.config instead of powershell.exe.config

I have a tendency to do odd things with technology so that things don’t just work. When I point out the obscure edge cases I find, most people tell me, “well don’t do that.” I usually ignore them and dream of tilting windmills. Well today a windmill has been tilted, and this is the epic tale.

I’m a developer that fell in love with PowerShell. As such I often call .NET API functions from powershell scripts. This usually just works. However, it kind of falls apart when you have to use settings in the app.config file. This means its basically impossible call functions from a DLL that use NHibernate, Entity Framework or WCF Service references. (However, WCF Services can be called direcctly from PowerShell quite easily)

The solution is to run the PowerShell script in a new PowerShell Runspace in a second AppDomain that uses its own app.config. However, things quickly fall apart because you need to write three classes that inherit from PSHostRawUserInterface, PSHostUserInterface and PSHost respectively or else Write-Host will throw an exception.

Now all this is a lot scarier than it sounds. However, it stops two important groups of people from ever using PowerShell to call DLLs that absolutely require you to manipulate your app.config:

  • People scared off by the word AppDomain
  • People that realize they have better things to do than everything I described above

Lucky for these two groups of people, I wasted my time so they didn’t have to! The project is currently called AppDomainPoshRunner, and I ILMerge it (via IL-Repack) into poshrunner.exe. Right now poshrunner takes one command line argument, the path to a script. If the script exists it will run it in an AppDomain whose config file is scriptname.config. Log4net configuration is read from a file called ADPR.log4net.config in the same directory as poshrunner.config.

The full background is to long and convoluted for this post. This was all born out of a problem with calling New-WebServiceProxy twice in the same PowerShell console. I use log4net to write the console messages so this has the potential to be quite extendable. Then Stan needed to run PowerShell scripts from msbuild and was complaining to me about it over twitter. He didn’t like the hacky solution I had then. Eventually I realized this was the way to simplify my previous solution.

So download the zip file. Try it out. Complain to me when you find bugs!

TLDR; Unlike powershell.exe -file foo.ps1, which uses the shared powershell.exe.config, poshrunner.exe foo.ps1 uses foo.ps1.config, for great justice. Download it now!