Is there a strong reason why Microsoft chose not to support AppDomains in .NET Core?
AppDomains are particularly useful when building long running server apps, where we may want to update the assemblies loaded by the server is a graceful manner, without shutting down the server.
Without AppDomains, how are we going to replace our assemblies in a long running server process?
AppDomains also provide us a way to isolate different parts of server code. Like, a custom websocket server can have socket code in primary appdomain, while our services run in secondary appdomain.
Without AppDomains, the above scenario is not possible.
I can see an argument that may talk about using VMs concept of Cloud for handling assembly changes and not having to incur the overhead of AppDomains. But is this what Microsoft thinks or says? or they have a specific reason and alternatives for the above scenarios?
The point of the .NETCore subset was to keep a .NET install small. And easy to port. Which is why you can, say, run a Silverlight app on both Windows and OSX and not wait very long when you visit the web page. Downloading and installing the complete runtime and framework takes a handful of seconds, give or take.
Keeping it small inevitably requires features to be cut. Remoting was very high on that list, it is quite expensive. Otherwise well hidden, but you can for example see that delegates no longer have a functional BeginInvoke() method. Which put AppDomain on the cut list as well, you can't run code in an app domain without remoting support. So this is entirely by design.
Update for .NET Standard 2 and .NET Core 2
In .NET Standard 2 the AppDomain class is in there. However, many parts of that API will throw a PlatformNotSupportedException for .NET Core.
The main reason it's still in there is for basic stuff like registering an unhandled exception handler which will work.
The .NET Standard FAQ has this explanation:
Is AppDomain part of .NET Standard?
The AppDomain type is part of .NET Standard. Not all platforms will support the creation of new app domains, for example, .NET Core will not, so the method AppDomain.CreateDomain while being available in .NET Standard might throw PlatformNotSupportedException.
The primary reason we expose this type in .NET Standard is because the usage is fairly high and typically not associated with creating new app domains but for interacting with the current app domain, such as registering an unhandled exception handler or asking for the application's base directory.
Apart from that, the top answer and other answers also nicely explain why the bulk of AppDomain was still cut (e.g. throws a not supported exception).
App Domains
Why was it discontinued? AppDomains require runtime support and are generally quite expensive. While still implemented by CoreCLR, it’s not available in .NET Native and we don’t plan on adding this capability there.
What should I use instead? AppDomains were used for different purposes. For code isolation, we recommend processes and/or containers. For dynamic loading of assemblies, we recommend the new AssemblyLoadContext class.
At one point, I heard that unloading assemblies would be enabled without using domains. I think that the System.Runtime.Loader.AssemblyLoadContext type in System.Runtime.Loader.dll is related to this work, but I don't see anything there that enables unloading yet.
I have heard in a community standup or some talk of Microsoft that the isolation feature of AppDomains are better handled by processes (and actually the common pattern in other platforms) and the unloading is indeed planned as a normal feature unrelated to AppDomains.
来源：https://stackoverflow.com/questions/27266907/no-appdomains-in-net-core-why