Adding_via_reflection.cs

private object ReadList(Type listType, object existingContainer)
{
    if (IsDictionary(listType))
    {
        return ReadDictionary(listType, existingContainer);
    }

    NewDocument(_reader.ReadInt32());
    var isReadonly = false;
    var itemType = ListHelper.GetListItemType(listType);
    var container = existingContainer == null ? ListHelper.CreateContainer(listType, itemType, out isReadonly) : (IList)existingContainer;

    try 
    {
        var fill = typeof(BsonDeserializer)
            .GetMethod("Fill").MakeGenericMethod(itemType)
            .Invoke(this, new object[] { existingContainer });
    } 
    catch (TargetInvocationException ex)
    {
        throw ex.InnerException;
    }
    
    if (listType.IsArray)
    {
        return ListHelper.ToArray((List<object>)container, itemType);
    }
    if (isReadonly)
    {
        return listType.GetConstructor(BindingFlags.Instance | BindingFlags.Public, null, new[] { container.GetType() }, null).Invoke(new object[] { container });
    }
    return container;
}

private void Fill<T>(ICollection<T> collection)
{
    var itemType = typeof(T);
    while (!IsDone())
    {
        var storageType = ReadType();

        ReadName();
        if (storageType == BSONTypes.Object)
        {
            NewDocument(_reader.ReadInt32());
        }
        var value = DeserializeValue(itemType, storageType);

        collection.Add((T)value);
    }
}

Converting_via_Expression.cs

// make something similar to Action<object> action = instance => collection.Add((T)instance)

var collectionType = typeof(ICollection<>).MakeGenericType(listType);
var collection = Expression.Constant(existingContainer, collectionType);
var instanceParameter = Expression.Parameter(typeof(object), "instance");
var filler = Expression.Lambda<Action<object>>(
    // collection.Add((T)instance)
    Expression.Call(
        collection,
        collectionType.GetMethod("Add"),
        Expression.Convert(instanceParameter, listType)
    ),
    instanceParameter
).Compile();

while (!IsDone())
{
    var storageType = ReadType();

    ReadName();
    if (storageType == BSONTypes.Object)
    {
        NewDocument(_reader.ReadInt32());
    }
    var value = DeserializeValue(itemType, storageType);
    filler(value);
    //container.Add(value);
}