How To Load A Class Into a New Appdomain, and Use It: Quick and Simple

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Introduction

This is a quick smash-and-grab type article. Download the code and you'll see how to load a class into a secondary appdomain and use it.

One of the things I miss most about VB6, is the ability to load an object into a separate process, detached from my current process, so if something goes wrong in that object, my main process does not hang (application isolation). In the VB6 world, we create an out of process activex EXE. The .NET way requires us to use Appdomains.

My first read on appdomains left me confused and asking myself "why is this so hard? All I want to do is create an out of process object and use it." After reading Wrox's book "Visual Basic.NET Reflection Handbook", the lights went on and I better understood how these types of things work.

I'm not an expert in this area, but this code will save you some time researching how to create an 'out of process' object the .NET way...

First, you create the app domain:

' Create the app domain
otherDomain = System.AppDomain.CreateDomain("DuffBeerIsGreatDomain")

Next, you create the object:

' Do it the easy way, but you have to pay attention to case in the object.classname

Dim herHello As echo.Hello = _
    CType(otherDomain.CreateInstanceAndUnwrap("echo", "echo.Hello"), echo.Hello)

Now, invoke a method on the object:

MessageBox.Show(herHello.echo("As createinstanceandunwrap"))

It is *that* simple. Now obviously, there are pitfalls, one of them being that in the above createinstanceandunwrap call, the echo.Hello parameter is case sensitive..

The sample code contains example calls which avoid this issue.

Enjoy!

History

• 1^st September, 2006: Initial post