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How to dynamically load and unload (reload) a .dll assembly

I'm developing a module for an external application, which is a dll that is loaded.
However in order to develop, you have to restart the application to see the results of your code.
We have built a piece of code that loads a dll dynamically from a startassembly:
startassembly
var dllfile = findHighestAssembly(); // this works but omitted for clarity
Assembly asm = Assembly.LoadFrom(dllFile);
Type type = asm.GetType("Test.Program");
MethodInfo methodInfo = type.GetMethod("Run");
object[] parametersArray = new object[] { };
var result = methodInfo.Invoke(methodInfo, parametersArray);
Effectively we have a solution with a startassembly which will be static and a test assembly which will be invoked dynamically, which allows us to swap the assembly during runtime.
The problem
This piece of code will load a new dll every time and search for the highest version at the end of the assembly name. e.g. test02.dll will be loaded instead of test01.dll, because the application locks both startassemly.dll as well as test01.dll. Now we have to edit the properties > assembly name all the time.
I want to build a new dll while the main application still runs. However for now I get the message
The process cannot access the file test.dll because it is being used
by another process
I have read that you can unload a .dll using AppDomains however the problem is that I don't know how to properly unload an AppDomain and where to do this.
The goal is to have to reload the new test.dll everytime the window is re-opened (by a button click from the main application).

You cannot unload a single assembly, but you can unload an Appdomain. This means you need to create an app domain and load the assembly in the App domain.
Exmaple:
var appDomain = AppDomain.CreateDomain("MyAppDomain", null, new AppDomainSetup
{
ApplicationName = "MyAppDomain",
ShadowCopyFiles = "true",
PrivateBinPath = "MyAppDomainBin",
});
ShadowCopyFiles property will cause the .NET runtime to copy dlls in "MyAppDomainBin" folder to a cache location so as not to lock the files in that path. Instead the cached files are locked. For more information refer to article about Shadow Copying Assemblies
Now let's say you have an class you want to use in the assembly you want to unload. In your main app domain you call CreateInstanceAndUnwrap to get an instance of the object
_appDomain.CreateInstanceAndUnwrap("MyAssemblyName", "MyNameSpace.MyClass");
However, and this is very important, "Unwrap" part of CreateInstanceAndUnwrap will cause the assembly to be loaded in your main app domain if your class does not inherit from MarshalByRefObject. So basically you achieved nothing by creating an app domain.
To solve this problem, create a 3rd Assembly containing an Interface that is implemented by your class.
For example:
public interface IMyInterface
{
void DoSomething();
}
Then add reference to the assembly containing the interface in both your main application and your dynamically loaded assembly project. And have your class implement the interface, and inherit from MarshalByRefObject. Example:
public class MyClass : MarshalByRefObject, IMyInterface
{
public void DoSomething()
{
Console.WriteLine("Doing something.");
}
}
And to get a reference to your object:
var myObj = (IMyInterface)_appDomain.CreateInstanceAndUnwrap("MyAssemblyName", "MyNameSpace.MyClass");
Now you can call methods on your object, and .NET Runtime will use Remoting to forward the call to the other domain. It will use Serialization to serialize the parameters and return values to and from both domains. So make sure your classes used in parameters and return values are marked with [Serializable] Attribute. Or they can inherit from MarshalByRefObject in which case the you are passing a reference cross domains.
To have your application monitor changes to the folder, you can setup a FileSystemWatcher to monitor changes to the folder "MyAppDomainBin"
var watcher = new FileSystemWatcher(Path.GetFullPath(Path.Combine(".", "MyAppDomainBin")))
{
NotifyFilter = NotifyFilters.LastWrite,
};
watcher.EnableRaisingEvents = true;
watcher.Changed += Folder_Changed;
And in the Folder_Changed handler unload the appdomain and reload it again
private static async void Watcher_Changed(object sender, FileSystemEventArgs e)
{
Console.WriteLine("Folder changed");
AppDomain.Unload(_appDomain);
_appDomain = AppDomain.CreateDomain("MyAppDomain", null, new AppDomainSetup
{
ApplicationName = "MyAppDomain",
ShadowCopyFiles = "true",
PrivateBinPath = "MyAppDomainBin",
});
}
Then when you replace your DLL, in "MyAppDomainBin" folder, your application domain will be unloaded, and a new one will be created. Your old object references will be invalid (since they reference objects in an unloaded app domain), and you will need to create new ones.
A final note: AppDomains and .NET Remoting are not supported in .NET Core or future versions of .NET (.NET 5+). In those version, separation is achieved by creating separate processes instead of app domains. And using some sort of messaging library to communicate between processes.

Not the way forward in .NET Core 3 and .NET 5+
Some of the answers here assume working with .NET Framework. In .NET Core 3 and .NET 5+, the correct way to load assemblies (with ability to unload them) in the same process is with AssemblyLoadContext. Using AppDomain as a way to isolate assemblies is strictly for .NET Framework.
.NET Core 3 and 5+, give you two possible ways to load dynamic assemblies (and potentially unload):
Load another process and load your dynamic assemblies there. Then use an IPC messaging system of your choosing to send messages between the processes.
Use AssemblyLoadContext to load them in the same process. Note that the scope does NOT provide any kind of security isolation or boundaries within the process. In other words, code loaded in a separate context is still able to invoke other code in other contexts within the same process. If you want to isolate the code because you expect to be loading assemblies that you can't fully trust, then you need to load it in a completely separate process and rely on IPC.
An article explaining AssemblyLoadContext is here.
Plugin unloadability discussed here.
Many people who want to dynamically load DLLs are interested in the Plugin pattern. The MSDN actually covers this particular implementation here:
https://learn.microsoft.com/en-us/dotnet/core/tutorials/creating-app-with-plugin-support
2021-9-12 UPDATE
Off-the-Shelf Library for Plugins
I use the following library for plugin loading. It has worked extremely well for me:
https://github.com/natemcmaster/DotNetCorePlugins

what you're trying to do in the code you posted is unload the default app domain which your program will run in if another isn't specified. What you're probably wanting is to load a new app domain, load the assembly into that new app domain, and then unloaded the new app domain when the user destroys the page.
https://learn.microsoft.com/en-us/dotnet/api/system.appdomain?view=netframework-4.7
the reference page above should give you a working example of all of this.

Here is an example for loading and unloading an AppDomain.
In my example I have 2 Dll's: DynDll.dll and DynDll1.dll.
Both Dll's have the same class DynDll.Class and a method Run (MarshalByRefObject is required):
public class Class : MarshalByRefObject
{
public int Run()
{
return 1; //DynDll1 return 2
}
}
Now you can create a dynamic AppDomain and load a Assembly:
AppDomain loDynamicDomain = null;
try
{
//FullPath to the Assembly
string lsAssemblyPath = string.Empty;
if (this.mbLoad1)
lsAssemblyPath = Path.Combine(Application.StartupPath, "DynDll1.dll");
else
lsAssemblyPath = Path.Combine(Application.StartupPath, "DynDll.dll");
this.mbLoad1 = !this.mbLoad1;
//Create a new Domain
loDynamicDomain = AppDomain.CreateDomain("DynamicDomain");
//Load an Assembly and create an instance DynDll.Class
//CreateInstanceFromAndUnwrap needs the FullPath to your Assembly
object loDynClass = loDynamicDomain.CreateInstanceFromAndUnwrap(lsAssemblyPath, "DynDll.Class");
//Methode Info Run
MethodInfo loMethodInfo = loDynClass.GetType().GetMethod("Run");
//Call Run from the instance
int lnNumber = (int)loMethodInfo.Invoke(loDynClass, new object[] { });
Console.WriteLine(lnNumber.ToString());
}
finally
{
if (loDynamicDomain != null)
AppDomain.Unload(loDynamicDomain);
}

Here is an idea, instead of loading the DDL directly (as is), let the application rename it, then load the renamed ddl (e.g. test01_active.dll). Then, just check for the original file (test01.dll) before loading the assembly and if exists, just delete the current one(test01_active.dll) and then rename the updated version then reload it, and so on.
Here is a code shows the idea :
const string assemblyDirectoryPath = "C:\\bin";
const string assemblyFileNameSuffix = "_active";
var assemblyCurrentFileName = "test01_active.dll";
var assemblyOriginalFileName = "test01.dll";
var originalFilePath = Path.Combine(assemblyDirectoryPath, assemblyOriginalFileName);
var currentFilePath = Path.Combine(assemblyDirectoryPath, assemblyCurrentFileName);
if(File.Exists(originalFilePath))
{
File.Delete(currentFilePath);
File.Move(originalFilePath, currentFilePath);
}
Assembly asm = Assembly.LoadFrom(currentFilePath);
Type type = asm.GetType("Test.Program");
MethodInfo methodInfo = type.GetMethod("Run");
object[] parametersArray = new object[] { };
var result = methodInfo.Invoke(methodInfo, parametersArray);

Facing error during catalog refresh, the new dll is not used

I am trying to create a POC with mef where i have the requirement to load dll dynamically in an all ready running project , for this i have created one console application project and a class Library
project .
the code for console application project is as follows-
namespace MefProjectExtension
{
class Program
{
DirectoryCatalog catalog = new DirectoryCatalog(#"D:\MefDll", "*.dll");
[Import("Method1", AllowDefault = true, AllowRecomposition = true)]
public Func<string> method1;
static void Main(string[] args)
{
AppDomainSetup asp = new AppDomainSetup();
asp.ShadowCopyFiles = "true";
AppDomain sp = AppDomain.CreateDomain("sp",null,asp);
string exeassembly = Assembly.GetEntryAssembly().ToString();
BaseClass p = (BaseClass)sp.CreateInstanceAndUnwrap(exeassembly, "MefProjectExtension.BaseClass");
p.run();
}
}
public class BaseClass : MarshalByRefObject
{
[Import("Method1",AllowDefault=true,AllowRecomposition=true)]
public Func<string> method1;
DirectoryCatalog catalog = new DirectoryCatalog(#"D:\MefDll", "*.dll");
public void run()
{
FileSystemWatcher sw = new FileSystemWatcher(#"D:\MefDll", "*.dll");
sw.NotifyFilter = NotifyFilters.CreationTime | NotifyFilters.Size;
sw.Changed += onchanged;
CompositionContainer container = new CompositionContainer(catalog);
container.ComposeParts(this);
Console.WriteLine(this.method1());
sw.EnableRaisingEvents = true;
Console.Read();
}
void onchanged(object sender, FileSystemEventArgs e)
{
catalog.Refresh();
Console.WriteLine(this.method1());
}
}
}
the library project which satisfy import looks as follow-
namespace MefLibrary
{
public interface IMethods
{
string Method1();
}
public class CallMethods : IMethods
{
[Export("Method1")]
public string Method1()
{
return "Third6Hello";
}
}
}
once i compile the library project(MefLibrary) and put the dll in D:\MefDll location and run the console application for first time i will see the output as
Third6hello on screen
but now if i change the implementation of method1 and make it return "third7hello" build MEF Library project and replace at D:\MefDll while my console app is running the onchanged handler even after calling catalog refresh prints
Third6hello on screen rather than third7hello
Whether anyone knows what is the reason for this , if yes please help.

DirectoryCatalog.Refresh will only add new or remove existing assemblies. It will not update an assembly. A crude workaround is:
Move the updated assembly to a temp folder.
Call DirectoryCatalog.Refresh. This will remove the part(s) contained in the assembly.
Move the assembly back to the watched folder
Call DirectoryCatalog.Refresh. This will add the updated part(s) contained in the assembly.
Note:
For this to work your "plugin" assemblies have to be strong named and with different version numbers (AssemblyVersionAttribute). This is needed because when parts are removed using the DirectoryCatalog.Refresh the actual assembly will not be unloaded. Assemblies can only be unloaded when the whole application domain is unloaded. So if DirectoryCatalog.Refresh finds a new assembly it will create an AssemblyCatalog using the assembly filepath. AssemblyCatalog will then call Assembly.Load to load the assembly. But this method will not load an assembly that has the same AssemblyName.FullName with an already loaded assembly.
Make sure that the steps I mention will not trigger another FileSystemWatcher.Changed event. For example you could use this approach.
Your program will need to have write access on the watched folder. This can be a problem if you deploy in the %ProgramFiles% folder.
If you need thread-safety you can consider creating your CompositionContainer with the CompositionOption.IsThreadSafe flag.
As I mentioned this is a workaround. Another approach would be to download MEF's source code and use DirectoryCatalog.cs as a guideline for your own directory catalog implementation.

Once a dll is loaded in an app domain it can't be unloaded from that domain. Only the whole domain can be unloaded. As such it is not easy to implement what you are after. It is possible to constantly scan for the changes and load new copies and repoint the calls (you will be accumulating more and more useless assemblies in your domain this way), but I don't believe this is something that MEF implements out of the box. In other words the behaviour you are observing is by design.
The implementation of this can be also tricky and bug prone because of state. Imagine you set a filed in a class instance of the old DLL and assign it to a variable. Then the new dll comes through. What happens to the old instance and its fields? Apparently they will stay the same and now you have different version of your plug-in in use in memory. What a mess!
And in case you are interested here is the reason why there isn't an Assembly.Unload method. And possible (conceptual) workaround.

Get web application assembly name, regardless of current executing assembly

Is is possible to get the assembly name of an ASP.NET web application, from a referenced assembly??
Assembly.GetEntryAssembly worked fine in desktop and console apps but it seems to be always null in web apps, and GetExecuting\GetCallingAssebly return my referenced assembly, not the one from the web app.
Long explanation:
I wrote a custom Settings Provider, that instead of reading configuration from the app config file, it gets the settings from a centralized configuration service.
The custom provider is in a separate assembly so it can be used by the different applications.
The ApplicationName property needs to be overriden with the app assembly name.
The way to use the provider is though a .net custom attribute, so I can't send any params to it.
Since non of the Assembly.Get*Assembly methods seem to work, the only thing a I can think of is requiring an appSetting with the app name for web apps, but I'm not really happy with that. Any help with this is appreciated, thanks!

Try
BuildManager.GetGlobalAsaxType().BaseType.Assembly

You can use
HttpContext.Current.ApplicationInstance.GetType().Assembly

I know this is an old question but this was my approach to a somewhat similar situation. In my case a was using another assembly for formatting a string with the version to show for multiple programs that have the same core.
Version v = null;
var a = Assembly.GetEntryAssembly() ?? GetWebEntryAssembly() ?? Assembly.GetExecutingAssembly();
SnapshotVersion = FileVersionInfo.GetVersionInfo(a.Location).ProductVersion;
if (ApplicationDeployment.IsNetworkDeployed)
{
var d = ApplicationDeployment.CurrentDeployment;
v = d.CurrentVersion;
v = new Version(v.Major, v.Minor, v.Revision);
}
else
v = a.GetName().Version;
if (v != null)
version = string.Format("{0}.{1}.{2}", v.Major, v.Minor, v.Build);
Because this is in a static constructor all I needed to do was to call any property of this static class from the Web Application and then find the last calling assembly that is different from the assembly that the static class is on. This was achieve with the method GetWebEntryAssembly.
private static Assembly GetWebEntryAssembly()
{
var frames = new StackTrace().GetFrames();
var i = frames.FirstOrDefault(c => Assembly.GetAssembly(c.GetMethod().DeclaringType).FullName != Assembly.GetExecutingAssembly().FullName).GetMethod().DeclaringType;
return Assembly.GetAssembly(i);
}

Class design/best approach for initializing a user configuration file

I want to initialize a user configuration through a user configuration file. The path for the file can be read from the registry.
The file is located in a folder with the user's name.
So I need the following functionality:
Reading a string from registry
building the path to the configuration file
Reading the file into a configuration object
Now there are several approaches to handle this:
First, I need
one "helper"-class for getting the file path (let's call it Shared)
one "container"-class for the configuration information (let's call it Configuration)
So, Shared has a function/property like UserConfigurationFile which returns the path to the configuration file.
To get the path to the file I have a function InitializeUserConfigurationFile() which is called in the constructor of Shared:
class Shared {
public Shared()
{
InitializeUserConfigurationFile();
}
void InitializeUserConfigurationFile()
{
//
// Reads username
//
//
// Reads path from Registry
//
//
// etc.
//
}
//
// etc.
//
}
Any better suggestions?
When I want to Initialize my Container I have different options:
Is it best to initialize the user configuration within the constructor?
Sth. like:
class Container
{
Shared shared = new Shared();
public Container()
{
InitializeUserConfiguration();
}
void InitializeUserConfiguration()
{
LoadConfiguration(shared.UserConfigurationFile);
}
void LoadConfiguration(string filename)
{
//
// Initializes all parameters frome filename
//
}
}
Or through two steps (through an own method LoadConfiguration())?
Sth. like:
Shared shared = new Shared();
Container container = new Container();
container.LoadConfiguration(shared.UserConfigurationFile);
Or inside the constructor of Container by delivering a filename?
Sth. like:
Shared shared = new Shared();
Container container = new Container(shared.UserConfigurationFile);
or everything in Container..?
There are so many ways...
I hope somebody knows a best-approch...
Regards,
Inno

It is better to use standard configuration classes exist in .net. Such as ApplicationSettingsBase and Configuration.
Here you can find good article series:
Unraveling the Mysteries of .NET 2.0 Configuration
Unraveling the Mysteries of .NET 2.0 Configuration
Cracking the Mysteries of .NET 2.0 Configuration

For best practices, don't use the registry, and don't reinvent the wheel.
Since you didn't mention it, have you looked at the System.Configuration namespace?
The .NET Framework constains a perfectly good configuration system that is well tested. It is also the domain of Sys Admins, who also know about config files and the accompanying tools.
So it is unclear why you are reinventing the wheel, possibly making it a little less round.
There are practical reasons to shun the Registry (distribution, backup) but also, as arbiter points out, it is not going to move to other (future) platforms. Did you notice that those namespaces are not starting with System ?

How do I specify the name of my application's App.config file in WPF?

This is very frustrating... I can set the Configuration File for a Windows Forms Application just fine. Consider this:
public static void Main(){
AppDomain.CurrentDomain.SetData("APP_CONFIG_FILE", #"SharedAppConfig.config");
//do other things
}
However, in a WPF application, this doesn't seem to work! If I set this value, the value of the AppDomain.CurrentDomain.SetupInformation.ConfigurationFile property is correct, but any calls to that configuration file while debugging yield no results. There are WCF configuration settings in an App.config that I need to share between application, so this is my proposed solution. Is it possible to dynamically set the location of my config file in WPF?
Help! Thanks!

You should be able to do something along the lines of:
using System.Configuration;
public class TryThis
{
Configuration config = ConfigurationManager.OpenExeConfiguration("C:\PathTo\app.exe");
public static void Main()
{
// Get something from the config to test.
string test = config.AppSettings.Settings["TestSetting"].Value;
// Set a value in the config file.
config.AppSettings.Settings["TestSetting"].Value = test;
// Save the changes to disk.
config.Save(ConfigurationSaveMode.Modified);
}
}
NOTE: This will attempt to open a file named app.exe.config at C:\PathTo. This also REQUIRES that a file exists at the same path with the name "app.exe". The "app.exe" file can just be an empty file though. For your case I'd almost make a shared "Config.dll" library that would handle the config file.
~md5sum~

Is this on the service side or the client side? If on the service side, it is often the case that the service is running in its own AppDomain, so that if you set AppDomain.CurrentDomain.SetData(...) it won't apply to the service configuration.
I'm not entirely sure how to get around this, but you should be able to control the service's configuration by implementing your own ServiceHost.