Anyone whose ever had to deal with loading Assemblies using standard .NET reflection facilities understands that it's not straightforward. Which context should I load into? How do I know that my Assembly is imported into the correct application domain? What if I want to load two versions of the same assembly into an application domain? How can I tear down an application domain where I've loaded my own assemblies?

I've seen quite a few questions like these all over www.stackoverflow.com, and having dealt with these problems in my own research projects I decided to build a small library that provided some basic functionality:

1. Ability to set up transient app domains that easily tear themselves down.
2. Ability to execute remote delegates in an arbitrary app domain.

PM> Install-Package AppDomainToolkit

You may also search the package gallery for AppDomainToolkit and it should show up. Click install and you're all set.

There are a couple of things to be aware of when utilizing the library. Most of them are non-issues as long as you understand the load context concept of application domains, and some of them stem from the backwards idea of executing code remotely--that is it's not entirely intuitive. I've seen several answers on stackoverflow claiming to solve an application domain problem when the solution still executes the target code in the current domain. Thinking in a remoting context requires a deeper analysis of where you think call sites are executing.

The most important of issues to discuss first is how the currently executing assembly is discovered and loaded into the current and foreign app domains for class resolution. You can't execute any assembly loading into an AppDomainContext without first loading the AssemblyLoader class into that domain. There's an interface dedicated to handling domain assembly resolve events and an implementation called PathBasedAssemblyResolver. This class is responsible for loading missing assemblies into any the current application domain whenever an Assembly.Load* method is called or whenever the application domain deduces that it needs to load a specific Assembly. Events that can trigger this include all of the static Assembly methods, along with pretty much anything that reflects across that assembly. By default, the AppDomainContext class will load the currently executing assembly's path as a target for the contained IAssemblyResolver instance, but an overload of AppDomainContext.Create() allows you to pass your own AppDomainSetupInfo instance. When doing this, make sure that you set the ApplicationBase property to be the base directory where your application, and ideally the AppDomainToolkit assembly lives. Usually this can easily be done with a simple Assembly.GetExecutingAssembly.Location() call. Also note that you can access the remote and local IAssemblyResolver instances on an AppDomainContext through the context.AssemblyImporter and context.RemoteResolver properties.

The typical remoting scenario requires you to create an instance of a remote class using the CreateInstanceAndUnwrap method on an application domain and then utilize instance methods on that type--which is effectively instantiated in the foreign app domain. The .NET runtime also provides a way to execute a block of code in a foreign application domain via the CrossAppDomainDelegate class, but it's extremely limited. If you attempt to pass this class a lambda expression, for example, sometimes you'll get a runtime exception complaining about some mangled class name not being serializable. This will always happen if your lambda has a closure in it--that is some variable is hoisted in the class generated by the compiler to be passed back to your code. The root issue here is that the compiler does not mark the generated class as [Serializable], something MSFT says they may change in the future. But why wait? Building out our own Serializable delegates is easy enough. There are two classes equipped for handling this exact problem, that is the RemoteAction and the RemoteFunc. Please note that both of these classes require all inputs to be serializable, and all output values to be either serializable or extend MarshalObjectByRef. See the examples below for further clarification.

using(var context = AppDomainContext.Create())
{
    context.LoadAssembly(LoadMethod.LoadFile, "C:\path\to\my\test.dll");
}

The AppDomain will be set up with a temporary name, and the application base path and private bin paths will be set to the current executing assembly--which is likely the right thing to do since this assembly will be installed along side whatever program that is using it.

var rootDir = Path.GetDirectoryName(Assembly.GetExecutingAssembly().Location);
var setupInfo = new AppDomainSetup()
{
    ApplicationName = "My Application",
    ApplicationBase = rootDir,
    PrivateBinPath = rootDir
};

using(var context = AppDomainContext.Create(setupInfo))
{
    context.LoadAssembly(LoadMethod.LoadFrom, "C:\path\to\my\test.dll");
}

This overloaded factory method allows you to assign whatever attributes you wish to the domain setup info.

using(var context = AppDomainContext.Create())
{
    RemoteAction.Invoke(
        context.Domain,
        "Hello World",
        (message) =>
        {
            var msg = message + " from AppDomain " + AppDomain.CurrentDomain.SetupInformation.ApplicationName
            Console.WriteLine(msg);
        });
}

All arguments passed through the static Invoke method must be serializable or MarshalByRefObject derivatives.

using(var context = AppDomainContext.Create())
{
    var area = RemoteFunc.Invoke(
        context.Domain,
        3.146,
        5,
        (pi, r) =>
        {
            return pi * r * r;
        });
}

Return values and inputs may be serializable or MarshalByRefObject derivatives. Pay attention to the lifetime cycle of any objects marshaled by reference. If the time runs out, then the returned value will be GC'd before you're finished with it. Function arguments passed to Invoke must be serializable.

Reply reply = null;
using(var context = AppDomainContext.Create())
{
    reply = RemoteFunc.Invoke(
        context.Domain,
        new Request("Hello World"),
        (request) =>
        {
            return !string.IsNullOrEmpty(request.Message) ?
                new Reply("Hello yourself!") :
                new Reply("You said nothing!");

        });
}

// Since the reply is serializable, you can use it after the domain has
// been unloaded! This is super powerful!
Console.WriteLine(reply.Message);

[Serializable]
public class Request
{
    public Request(string message)
    {
        this.Message = message;
    }

    public string Message { get; private set; }
}

[Serializable]
public class Reply
{
    public Request(string message)
    {
        this.Message = message;
    }

    public string Message { get; private set; }
}

using(var context = AppDomainContext.Create())
{
    var result = RemoteFunc.Invoke(
        context.Domain,
        "Hello",
        (phrase) =>
        {
            return new Greeter(phrase);
        });

    // Executes in the remote domain.
    result.SayHello("jduv");
}

public class Greeter : MarshalByRefObject
{
    private readonly string helloPhrase;

    public void RemoteGreeter(string helloPhrase)
    {
        this.helloPhrase = helloPhrase;
    }

    public void SayHello(string name)
    {
        return this.helloPhrase + " " + name + "!";
    }
}

This code is quite powerful. It will create a remote object and return a handle to it as long as the remote class extends MarshalByRefObject. It's important to understand here, though, that the remote object won't live outside the using block. If you wish to persist a remote object longer, check out the next example. Also note that you can pass constructor arguments to the RemoteFunc.Invoke method. It's a simple params array after the AppDomain argument.

var context = AppDomainContext.Create()

// The second arg is ctor arguments.
var remoteGreeter = Remote<Greeter>.CreateProxy(context.Domain, "Hello");

remoteGreeter.SayHello("jduv");

// Eventually, it's a good idea to unload the app domain.
// *Always* use dispose for this. Don't call AppDomain.Unload(context.Domain)
context.Dispose();

var context = AppDomainContext.Create();

// Alternatively, you can place a proxy into a using block for automatic disposal.
using(var remoteGreeter = Remote.<Greeter>.CreateProxy(context.Domain, "Hello"))
{
    remoteGreeter.SayHello("jduv");
}

The Remote class was stolen off the internet and modified slightly to suite my needs. Credit should be placed where credit is due, so the original version of the Remote class lives here. Alot of the inspiration for this entire library was based on that implementation along with some stackoverflow questions I've been lurking.