跨AppDomain边界代理匿名对象

Question

One of the most requested features I'm trying to iron out for RazorEngine vNext is support for loading template assemblies in a separate AppDomain so we can unload the assemblies when we need to. Great feature request, but it has the knock-on effect of introducing constraints on the possible model types that can be injected into templates.

One of the nice features of the current v2.1 release is the ability to use anonymous types as models. We do the work of determining that the model type in the template is an anonymous type, and we set the base template as TemplateBase<dynamic> . The runtime binder takes care late-bound calls for the model members for us. All very nice.

When we introduce support for running templates in a separate AppDomain we now have a constraint that models can only be of types that are [Serializable] (which is implied through inheritance of MarshalByRefObject ). Anonymous types are not serialisable, and are also private .

My thinking would be to somehow make a proxy model (declared as dynamic ) in the template base which despatches calls to the model (which would be in the calling domain, not the domain the template is running in). In essence:

Template :

<h1>@Model.Name</h1>

The call to @Model.Name would do something like:

Template.Model (ModelProxy) -> GetMember(Name) -> |BOUNDARY| -> Model.Name

Does anyone know or have experience with the best method of attempting to proxy a call to an anonymous (or dynamic object) in another AppDomain ?

Importantly, I'm not trying to push the anonymous object through AppDomain boundary, that can't be done.

Answer 1

Ok. Assuming you know about reflection and creating a new AppDomain . Which I know you know how to do... :)

I've created two helper classes that will allow you to pass anonymous objects. ProxyAnonymousObject and ProxyDynamicObject . You create the ProxyAnonymousObject in the first AppDomain and use the ProxyDynamicObject in the other AppDomain . (both of these objects exist in the primary AppDomain library)

[Serializable]
public class ProxyAnonymousObject : ISerializable {

    static Dictionary<string, Type> cached = new Dictionary<string, Type>();

    object model;

    public Dictionary<string, object> ModelProperties = new Dictionary<string, object>();

    public ProxyAnonymousObject(object model) { this.model = model; }
    public ProxyAnonymousObject(SerializationInfo info, StreamingContext ctx) {
        try {

            string fieldName = string.Empty;
            object fieldValue = null;

            foreach (var field in info) {
                fieldName = field.Name;
                fieldValue = field.Value;

                if (string.IsNullOrWhiteSpace(fieldName))
                    continue;

                if (fieldValue == null)
                    continue;

                ModelProperties.Add(fieldName, fieldValue);

            }

        } catch (Exception e) {
            var x = e;
        }
    }

    public void GetObjectData(SerializationInfo info, StreamingContext context) {

        foreach (var pi in model.GetType().GetProperties()) {
            info.AddValue(pi.Name, pi.GetValue(model, null), pi.PropertyType);
        }

    }
}

public class ProxyDynamicObject : DynamicObject{
    internal ProxyAnonymousObject Proxy { get; set; }

    public ProxyDynamicObject(ProxyAnonymousObject model) {
        this.Proxy = model;
    }

    public override bool TryGetMember(GetMemberBinder binder, out object result) {
        result = Proxy.ModelProperties[binder.Name];
        return true;
    }
}

To get this to work in your MarshalByRefObject inherited class you just set the target dynamic object equal to a new ProxyDynamicObject(model) . In the example I've written up I make a call like this.

instance = Activator.CreateInstance(type);
var setModel = type.GetMethod("SetModel", BindingFlags.Public | BindingFlags.Instance);
var render = type.GetMethod("Render", BindingFlags.Public | BindingFlags.Instance);

setModel.Invoke(instance, new object[] { new ProxyDynamicObject(model) });
render.Invoke(instance, null);

I've written a blog post about it http://buildstarted.com/2011/06/28/getting-anonymous-types-to-cross-the-appdomain-boundary/ to explain it in a bit more detail. (though it's something I'm not very good at)

There are definitely problems with this implementation. It doesn't support nested Anonymous Types and I'm fairly certain it will break in general. But it's definitely something to get you on the right track.