Load and unload assemblies

Pedro Rainho — Sun, 15 Mar 2009 17:59:14 GMT

Last week, I had a problem that probably some .Net developer had faced. The problem consists in load and unload an assembly?.

Can I do that??? Well, I can’t do it in a simple way. One of the things that Assembly doesn’t support in unload, check this post to see why.

So, like I wrote, last week I was developing a new feature for a software factory and that feature consisted in get all references and do something else.

My recipe does something like this:

Get project assembly
Copy the assembly DLL to a location
Get all assembly references
Generate a file

Do number 1 is simple.

Do number 2 is also simple, but has a problem.

Do number 3 is also simple, but has a problem.

Do number 4 is simple.

I’m going to explain first problem number 3 then problem 2

The problem with number 3 it’s because I load a assembly and they I get referenced assemblies. To get referenced assemblies I just do:

Assembly assembly = Assembly.LoadFrom(@"c:\myAssembly.dll");
AssemblyName[] references = assembly.GetReferencedAssemblies();

This piece of code has a problem, since I’m loading the assembly and they get referenced assemblies the file myAssembly.dll is now locked this means that the next time I try to run number 2 I will get an exception and the file will not ne copied. And this is why I had problem number 2.

Now, how can I load the assembly and then unload that assembly so the file won’t be locked???

I came across some solutions.

One of the possible solutions is just copy the assembly to something unique like {GUID}.DLL and then load that {GUID}.DLL, and do everything I need to do, this way I won’t lock my file.
1. Problem: I will have the assembly {GUID}.DLL loaded (in memory) every time, and can’t delete file {GUID}.DLL, because it’s locked. And finally I can’t unload unless I close Visual Studio.
Why don’t I just load the assembly into an array of bytes and then load the assembly, like this:
```
Assembly assembly = Assembly.Load(File.ReadAllBytes(@"c:\myAssembly.dll"));
```
1. Problem: I can do this and my file will not be locked but that array of bytes will be in memory and my visual studio memory will grow every time I run that recipe. That memory will be released only when I close the Visual Studio.
I can load the assembly in a new AppDomain and then unload the AppDomain.
1. Problem: tThis was a strange problem and I don’t know why but I load the assembly in a new AppDomain and that assembly become part of the new AppDomain and the AppDomain.Current. This was strange. Don’t know if I was doing something wrong. Now since the Assembly is loaded in AppDomain.Current I wasn’t able to copy the file, because it was locked.
The last solution I came across to load the assembly and unload it. This solution Consists in create a new AppDomain and then use method DoCallBack:

Here it is the source code:

AppDomainSetup setup = new AppDomainSetup();
setup.ApplicationBase = ApplicationBasePath;

AppDomain newDomainReferences = AppDomain.CreateDomain("AppDomain", null, setup);

CallBackAssemblyReferences call = new CallBackAssemblyReferences(newDomainReferences, filepath);
newDomainReferences.DoCallBack(new CrossAppDomainDelegate(call.LoadAssemblyReferences));
AssemblyName[] assName = (AssemblyName[])newDomainReferences.GetData("AssemblyReferences");

AppDomain.Unload(newDomainReferences);

This solution enables me to do a callback in a different domain. My class CallBackAssemblyReferences just does this:

[Serializable]
internal class CallBackAssemblyReferences
{
    #region Members Variables
    private string assemblyFile;
    private AppDomain domain;
    #endregion

    #region Public Implementation
    public CallBackAssemblyReferences()
    {

    }
    public CallBackAssemblyReferences(AppDomain domain, string assemblyFile)
    {
        this.assemblyFile = assemblyFile;
        this.domain = domain;
    }

    public void LoadAssemblyReferences()
    {
        Assembly assembly = this.domain.Load(AssemblyName.GetAssemblyName(this.assemblyFile));

        domain.SetData("AssemblyReferences", assembly.GetReferencedAssemblies());
    }
    #endregion
}

This class has a constructor that receives the new domain I’m working on and the path to the assembly I want to load.

public CallBackAssemblyReferences(AppDomain domain, string assemblyFile)

Then I just have my callback method WITH NO PARAMETERS.

public void LoadAssemblyReferences()

This is the method that will be called when I do

newDomainReferences.DoCallBack(new CrossAppDomainDelegate(call.LoadAssemblyReferences));

The callback method only loads the assembly in the new domain and then sets data in the new domain. These data are the assembly references.

When the DoCallBack returns I just had to do

AssemblyName[] assName = (AssemblyName[])newDomainReferences.GetData("AssemblyReferences");

To get the references I had saved.

Finally I just unload the AppDomain.

Now I can run my recipe every time I need without lock the file and without decrease visual studio performance.

Gat & Gax

Load and unload assemblies