Unloading a dll file in mef

Question

I have some plugins as dll files. My application loads the dll and it runs fine. but when I try to delete the old plugin and replace it with a new plugin it doesn't allow to do me. as it has been loaded by application. I have found that by using appdomain we can do that. but I am not able to find a solution in which mef is used.

I need a code which can run on mef. Below is my code which is used to load plugins.

//Creating an instance of aggregate catalog. It aggregates other catalogs
var aggregateCatalog = new AggregateCatalog();

//Build the directory path where the parts will be available
var directoryPath = "Path to plugins folder";

//Load parts from the available dlls in the specified path using the directory catalog
var directoryCatalog = new DirectoryCatalog(directoryPath, "*.dll");

//Add to the aggregate catalog
aggregateCatalog.Catalogs.Add(directoryCatalog);

//Crete the composition container
var container = new CompositionContainer(aggregateCatalog);


// Composable parts are created here i.e. the Import and Export components assembles here
container.ComposeParts(this);

Smythe

Answer 1

i have found that by using appdomain we can do that. but i am not able to find a solution in which mef is used.

Unfortunately, this is not supported by MEF. MEF was designed specifically for application extensibility, not as a general purpose plugin system which supports unloading and replacement of code at runtime.

The only way to make this work would be to use MEF within a separate AppDomain, and unload the AppDomain as a whole. The CLR itself has no support for unloading a loaded assembly other than unloading the entire AppDomain in which the assembly is opened.

Anisaanise

there should be a solution to this. i heard that this is going to be in mef next release. dont know it is true or not. – Smythe

@Smythe Nope - at least not in anything publicly shown so far. This is really outside of the intended use case for MEF, so I doubt it will happen there. THe only supported way to remove an assembly in the CLR is to unload the entire AppDomain. MEF, being a library, can't "work around" that. – Anisaanise

well then what is the meaning of "AllowRecomposition = true" in ImportMany. If i write it or not behaviour is same. – Smythe

@Smythe It allows MEF to bring in more assemblies - but it never removes them. Basically, the IEnumerable<T> is recreated with extra, new types, but the old assemblies stay loaded in memory. – Anisaanise

Answer 2

If you try to insert on catalog one Assembly Object like this:

Assembly assembly = Assembly.Load(System.IO.File.ReadAllBytes(Path.Combine(directoryPath, ItemPlugin)));
aggregateCatalog.Catalogs.Add(new AssemblyCatalog(assembly));

You can Delete/Change the file later...

Upturn

You can delete it from aggregateCatalog but you wont be able to unload the assembly from the appdomain without unloading the hole appdomain. – Esmerolda