Conflicting dependencies when loading an assembly

Question

I am having some issues when trying to write a plugin handler.

I have a main application "AppA", that references "AssemblyX".

AppA also loads a number of plugin assemblies that implement "IPlugin" interface. However, these plugins might also reference "AssemblyX" and it may even be an older version.

So the initial problem was the conflict with AssemblyX that occurred when loading the plugins via Assembly.LoadFrom().

After a bit of research on here I tried loading the plugins in a new AppDomain. On its own this did not solve the issue. Next I created a ProxyDomain class that inherited from MarshalByRefObject. Still no joy.

Finally I made the plugins themselves inherit from MarshalByRefObject, which did yield more success, but when I tried to bind a List to a ListView it complained that "System.MarshalByRefObject' does not contain a property with the name 'SomeProperty'".

Please could somebody cast an eye over my code and see if any changes could be made:

public partial class WebForm1 : System.Web.UI.Page
{
    protected void Button1_Click(object sender, EventArgs e)
    {
        AssemblyX x = new AssemblyX();
        x.GetStuff("a", "b");

        lstConfig.DataSource = PluginLoader.Load(Path.Combine(PluginLoader.GetPluginFolderPath(), "ExamplePlugins.dll"));
        lstConfig.DataBind();
    }
}

public class PluginLoader
{
    public static List<IPlugin> Load(string file)
    {
        var plugins = new List<IPlugin>();

        ProxyDomain pd = new ProxyDomain();

        Assembly ass = pd.GetAssembly(file);

        try
        {
            AppDomainSetup adSetup = new AppDomainSetup();

            string fileName = Path.GetFileName(file);

            string path = file.Replace(fileName, "");

            adSetup.ApplicationBase = path;

            AppDomain crmAppDomain = AppDomain.CreateDomain("ProxyDomain", null, adSetup);

            foreach (Type t in ass.GetTypes())
            {
                Type hasInterface = t.GetInterface(typeof(IPlugin).FullName, true);

                if (hasInterface != null && !t.IsInterface)
                {

                    IPlugin plugin = (IPlugin)crmAppDomain.CreateInstanceAndUnwrap(ass.FullName, t.FullName);
                    plugins.Add(plugin);
                }
            }
        }
        catch (Exception ex)
        {

        }

        return plugins;
    }

public class ProxyDomain : MarshalByRefObject
{
    public Assembly GetAssembly(string assemblyPath)
    {
        try
        {
            return Assembly.LoadFrom(assemblyPath);
        }
        catch (Exception ex)
        {
            throw ex;
        }
    }
}

public interface IPlugin
{
    string SomeProperty { get; set; }
    void DoSomething();
}

[Serializable]
public class ExamplePlugin : MarshalByRefObject, IPlugin
{
    public string SomeValue
    {
        get
        {
            AssemblyX x = new AssemblyX(); // Referencing a previouus version of AssemblyX 
            return x.GetStuff("c");
        }
        set
        {

        }
    }

    public void DoSomething() { }
}

Note: PluginExamples.dll may contain multiple plugin classes.

Answer 1

If I understand you correct, you can specify the specific versions for AssemblyX like this in your app.config:

<configuration>
  <runtime>
    <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
      <dependentAssembly>
        <assemblyIdentity name="AssemblyX" publicKeyToken="3d67ed1f87d44c89" />
        <codeBase version="3.0" href=".\ver30\AssemblyX.dll"/>
        <codeBase version="5.0" href=".\ver50\AssemblyX.dll"/>
      </dependentAssembly>
    </assemblyBinding>
  </runtime>
</configuration>

If the public key token is different for the versions, you will have to specify 2 separate entries.