Getting the type argument of an ICollection<T> implementing class

Question

I am writing a little serialization library in .net. The goal is to have a replacement for XmlSerialize but easier to configure and without messing up the model with attributes.

The problem that I face is that I need the type of each ICollection<T> I find during traversing the model. The naive approach is this:

 var theType=myModel.GetType().GetGenericArguments()[0];

But it does not help for classes that derive from ICollection<T> with a specific T.

public class MyClass:A,ICollection<B>{}

I tried getting the interface with reflection

  var iCollectionInterface =
  o.GetType().GetInterfaces()
     .Where(i => i.IsGenericType)
     .Select(i => i.GetGenericTypeDefinition())
     .FirstOrDefault(i => i == typeof(ICollection<>));

but iCollectionInterface.GetGenericArguments()[0] is just T, not B because it describes only the definition of the interface and not the usage of it.

Any ideas? I also need it for IDictionary<TKey, TValue> but that is basically the same problem and will have the same solution.

Thank you!

Edit

Thank you all, here is what I ended up with:

public static Type GetTypeParameter(object o, Type typeToSearch, int argumentNumber)
{
    return o.GetType()
            .GetInterfaces()
            .Where(i => i.IsGenericType)
            .Where(i => i.GetGenericTypeDefinition() == typeToSearch)
            .Select(t => t.GetGenericArguments()[argumentNumber])
            .FirstOrDefault();
}

Answer 1

From the comments it seems you need to figure out how to filter the collection to only include ICollection. You can reach that goal by:

var iCollectionInterfaces =
      from i in o.GetType().GetInterfaces()
      where i.IsGenericType 
            && i.GetGenericTypeDefinition() == typeof(IColection<>)
      select i;

You can then iterate over the collection and do whatever you need to with each type argument

//same type might implement ICollection<T> more than once
foreach(var collection in iCollectionInterfaces) {
     //More than one is invalid for ICollection<T>
    var T = collection.GetGenericArguments().Single();
    //do what ever you need
}

of course if you want to make it more generic. Ie to support interfaces/types with more than one type argument you need to remove the call to Single and replace with iteration of the type arguments