How to unload an assembly from the primary AppDomain?

I would like to know how to unload an assembly that is loaded into the main AppDomain.

I have the following code:

var assembly = Assembly.LoadFrom( FilePathHere );

I need/want to be able to unload this assembly when I am done.

Thanks for your help.

Solution 1:

For .net versions core 3.0 and later:

You can now unload assemblies. Note that appdomains are no longer available in .net core. Instead, you can create one or more AssemblyLoadContext, load your assemblies via that context, then unload that context. See AssemblyLoadContext, or this tutorial that simulates loading a plugin then unloading it.

For .net versions before .net core 3, including netframework 4 and lower

You can not unload an assembly from an appdomain. You can destroy appdomains, but once an assembly is loaded into an appdomain, it's there for the life of the appdomain.

See Jason Zander's explanation of Why isn't there an Assembly.Unload method?

If you are using 3.5, you can use the AddIn Framework to make it easier to manage/call into different AppDomains (which you can unload, unloading all the assemblies). If you are using versions before that, you need to create a new appdomain yourself to unload it.

Solution 2:

I also know this is very old, but may help someone who is having this issue! Here is one way I have found to do it! instead of using:

var assembly = Assembly.LoadFrom( FilePathHere );

use this:

var assembly = Assembly.Load( File.ReadAllBytes(FilePathHere));

This actually loads the "Contents" of the assembly file, instead of the file itself. Which means there is NOT a file lock placed on the assembly file! So now it can be copied over, deleted or upgraded without closing your application or trying to use a separate AppDomain or Marshaling!

PROS: Very Simple to fix with a 1 Liner of code! CONS: Cannot use AppDomain, Assembly.Location or Assembly.CodeBase.

Now you just need to destroy any instances created on the assembly. For example:

assembly = null;

Solution 3:

You can't unload an assembly without unloading the whole AppDomain. Here's why:

1. You are running that code in the app domain. That means there are potentially call sites and call stacks with addresses in them that are expecting to keep working.

2. Say you did manage to track all handles and references to already running code by an assembly. Assuming you didn't ngen the code, once you successfully freed up the assembly, you have only freed up the metadata and IL. The JIT'd code is still allocated in the app domain loader heap (JIT'd methods are allocated sequentially in a buffer in the order in which they are called).

3. The final issue relates to code which has been loaded shared, otherwise more formally know as "domain neutral" (check out /shared on the ngen tool). In this mode, the code for an assembly is generated to be executed from any app domain (nothing hard wired).

It is recommended that you design your application around the application domain boundary naturally, where unload is fully supported.

Solution 4:

You should load your temporary assemblies in another AppDomain and when not in use then you can unload that AppDomain. It's safe and fast.