The Blog of Ian Mercer.

Class-free persistence and multiple inheritance in C# with MongoDB

Much as I appreciate Object Relational Mappers and the C# type system there's a lot of work to do if you just want create and persist a few objects. MongoDB alleviates a lot of that work with its Bson serialization code that converts almost any object into a binary serialized object notation and provides easy round tripping with JSON.

But there's no getting around the limitations of C# when it comes to multiple inheritance. You can use interfaces to get most of the benefits of multiple inheritance but implementing a tangled set of classes with multiple interfaces on them can lead to a lot of duplicate code.

What if there was a way to do multiple inheritance without every having to write a class? What if we could simply declare a few interfaces and then ask for an object that implements all of them and a way to persist it to disk and get it back? What if we could later take one of those objects and add another interface to it? "Crazy talk" I hear you say!

Well, maybe not so crazy ... take a look at the open source project impromptu-interface and you'll see some of what you'll need to make this reality. It can take a .NET dynamic object and turn it into an object that implements a specific interface.

Combine that with a simple MongoDB document store and some cunning logic to link the two together and voila, we have persistent objects that can implement any interface dynamically and there's absolutely no classes in sight anywhere!

Let's take a look at it in use and then I'll explain how it works. First, let's define a few interfaces:

public interface ILegs { int Legs { get; set; } }

public interface IMammal { double BodyTemperatureCelcius { get; set; } }

// Interfaces can use multiple inheritance:

public interface IHuman: IMammal, ILegs { string Name { get; set; } }

// We can have interfaces that apply to specific instances of a class: not all humans are carnivores

public interface ICarnivore { string Prey { get; set; } }

Now let's take a look at some code to create a few of these new dynamic documents and treat them as implementors of those interfaces. First we need a MongoDB connection:

MongoServer MongoServer = MongoServer.Create(ConnectionString);
MongoDatabase mongoDatabase = MongoServer.GetDatabase("Remember",
credentials);

Next we grab a collection where we will persist our objects.

var sampleCollection = mongoDatabase.GetCollection<SimpleDocument>("Sample");

Now we can create some objects adding interfaces to them dynamically and we get to use those strongly typed interfaces to set properties on them.

var person1 = new SimpleDocument();
person1.AddLike<IHuman>().Name = "John";
person1.AddLike<ILegs>().Legs = 2;
person1.AddLike<ICarniovore>().Prey = "Cattle";
sampleCollection.Save(person1);

var monkey1 = new SimpleDocument(); monkey1.AddLike<IMammal>(); 
// mark as a mammal 
monkey1.AddLike<ILegs>().Legs = 2;
monkey1.AddLike<ICarniovore>().Prey = "Bugs";
sampleCollection.Save(monkey1);

Yes, that's it! That's all we needed to do to create persisted objects that implement any collection of interfaces. Note how the IHuman is also an IMammal because our code will also support inheritance amongst interfaces. We can load them back in from MongoDB and get the strongly typed versions of them by using .AsLike() which returns a value of type T or null if that object doesn't implement the interface T. But that's not all, we can even add new interfaces to them later allowing an object to change type over time! Now, of course, you could do a lot of this just with dynamic types but then you lose Intellisense and compile time checking.

So next, let's take a look at how we can query for objects that support a given interface and how we can get strongly typed objects back from MongoDB:

var query = Query.EQ("int", typeof(IHuman).Name); 
var humans = sampleCollection.Find(query);

Console.WriteLine("Examine the raw documents");

foreach (var doc in humans) { Console.WriteLine(doc.ToJson()); }

Console.WriteLine("Use query results strongly typed");

foreach (IHuman human in humans.Select(m => m.AsLike<IHuman>())) 
{
    Console.WriteLine(human.Name); 
}
Console.ReadKey();

So how does this 'magic' work? First we need a simple Document class. It can be any old object class, no special requirements. At the moment it does wrap these interface properties up in a document inside it called 'prop' making it just a little bit harder to query and index but still fairly easy.

/// <summary>
/// A very simple document object 
/// </summary>
public class SimpleDocument : DynamicObject
{
    public ObjectId Id { get; set; }

// All other properties are added dynamically and stored wrapped in another Document
[BsonElement("prop")] 
protected BsonDocument properties = new BsonDocument();

/// <summary> 
/// Interfaces that have been added to this object 
///</summary>
[BsonElement("int")] 
protected HashSet<string> interfaces = new HashSet<string>();

/// <summary>
/// Add support for an interface to this document if it doesn't already have it 
/// </summary>
public T AddLike<T>() where T:class 
{
    interfaces.Add(typeof(T).Name);
    foreach (var @interface in typeof(T).GetInterfaces()) 
      interfaces.Add(@interface.Name); 
    return Impromptu.ActLike<T>(new Proxy(this.properties)); 
}

/// <summary>
/// Cast this object to an interface only if it has previously been created as one of that kind
/// </summary>
public T AsLike<T>() where T : class 
{
    if (!this.interfaces.Contains(typeof(T).Name)) return null; 
    else return Impromptu.ActLike<T>(new Proxy(this.properties)); }
}

Then we need a simple proxy object to wrap up the properties as a dynamic object that we can feed to Impromptu:

public class Proxy : DynamicObject 
{
    public BsonDocument document { get; set; }

    public Proxy(BsonDocument document) 
    {
        this.document = document;
    }
    
    public override bool TryGetMember(GetMemberBinder binder, out object result)
    {
        BsonValue res = null; 
        this.document.TryGetValue(binder.Name, out res);
        result = res.RawValue; 
        return true; // We always support a member even if we don't have it in the dictionary 
    }

    /// <summary> 
    /// Set a property (e.g. person1.Name = "Smith") 
    /// </summary>
    public override bool TrySetMember(SetMemberBinder binder, object value)
    {
        this.document.Add(binder.Name, BsonValue.Create(value));
        return true; 
    } 
}

And that's it! There is no other code required. Multiple-inheritance and code-free persistent objects are now a reality! All you need to do is design some interfaces and objects spring magically to life and get persisted easily.

[NOTE: This is experimental code: it's a prototype of an idea that's been bugging me for some time as I look at how to meld Semantic Web classes which have multiple inheritance relationships with C# classes (that don't) and with MongoDB's document-centric storage format. Does everything really have to be stored in a triple-store or is there some hybrid where objects can be stored with their properties and triple-store statements can be reserved for more complex relationships? Can we get semantic web objects back as meaningful C# objects with strongly typed properties on them? It's an interesting challenge and this approach appears to have some merit as a way to solve it.]

Related Stories

My love/hate relationship with Stackoverflow

Stackoverflow is a terrific source of information but can also be infuriating.

Ian Mercer
Ian Mercer

Xamarin Forms Application For Home Automation

Building a Xamarin Forms application to control my home automation system

Ian Mercer
Ian Mercer

JSON Patch - a C# implementation

Ian Mercer
Ian Mercer

Websites should stop using passwords for login!

A slightly radical idea to eliminate passwords from many of the websites you use just occasionally

Ian Mercer
Ian Mercer

Dynamically building 'Or' Expressions in LINQ

How to create a LINQ expression that logically ORs together a set of predicates

Ian Mercer
Ian Mercer

VariableWithHistory - making persistence invisible, making history visible

A novel approach to adding history to variables in a programming language

Ian Mercer
Ian Mercer

Neo4j Meetup in Seattle - some observations

Some observations from a meetup in Seattle on graph databases and Neo4j

Ian Mercer
Ian Mercer

Updated Release of the Abodit State Machine

A hierarchical state machine for .NET

Ian Mercer
Ian Mercer

My first programme [sic]

At the risk of looking seriously old, here's something found on a paper tape

Ian Mercer
Ian Mercer

Building a better .NET State Machine

A state machine for .NET that I've released on Nuget

Ian Mercer
Ian Mercer

The Internet of Dogs

Connecting our dog into the home automation

Ian Mercer
Ian Mercer

A simple state machine in C#

State machines are useful in many contexts but especially for home automation

Ian Mercer
Ian Mercer

MongoDB substring search with a difference

Ian Mercer
Ian Mercer

Convert a property getter to a setter

Ian Mercer
Ian Mercer

MongoDB - Map-Reduce coming from C#

Ian Mercer
Ian Mercer

MongoDB Map-Reduce - Hints and Tips

Ian Mercer
Ian Mercer

Weather Forecasting for Home Automation

Ian Mercer
Ian Mercer

Lengthening short Urls in C#

Ian Mercer
Ian Mercer

Why don't you trust your build system?

Ian Mercer
Ian Mercer

ASP.NET MVC SEO - Solution Part 1

Ian Mercer
Ian Mercer

Elliott 803 - An Early Computer

Ian Mercer
Ian Mercer

Building sitemap.xml for SEO ASP.NET MVC

Ian Mercer
Ian Mercer

Continuous Integration -> Continuous Deployment

What is "quality" in terms of a released software product or website?

Ian Mercer
Ian Mercer

Making a bootable Windows 7 USB Memory Stick

Here's how I made a bootable USB memory stick for Windows 7

Ian Mercer
Ian Mercer

Tip: getting the index in a foreeach statement

A tip on using LINQ's Select expression with an index

Ian Mercer
Ian Mercer

SQL Server - error: 18456, severity: 14, state: 38 - Incorrect Login

A rant about developers using the same message for different errors

Ian Mercer
Ian Mercer

WCF and the SYSTEM account

Namespace reservations and http.sys, my, oh my!

Ian Mercer
Ian Mercer

404 errors on IIS6 with ASP.NET 4 Beta 2

Ian Mercer
Ian Mercer

Mixed mode assembly errors after upgrade to .NET 4 Beta 2

Fixing this error was fairly simple

Ian Mercer
Ian Mercer

The EntityContainer name could not be determined

How to fix the exception "the entitycontainer" name could not be determined

Ian Mercer
Ian Mercer

Shortened URLs should be treated like a Codec ...

Expanding URLs would help users decide whether or not to click a link

Ian Mercer
Ian Mercer

Tagging File Systems

Isn't it time we stopped knowing which drive our file is on?

Ian Mercer
Ian Mercer

A great site for developing and testing regular expressions

Just a link to a site I found useful

Ian Mercer
Ian Mercer

Introducing Jigsaw menus

A novel UI for menus that combines a breadcrumb and a menu in one visual metaphor

Ian Mercer
Ian Mercer

Entity Framework in .NET 4

Ian Mercer
Ian Mercer

Fix for IE's overflow:hidden problem

Ian Mercer
Ian Mercer

A better Tail program for Windows

A comparison of tail programs for Windows

Ian Mercer
Ian Mercer

Measuring website browser performance

Found this great resource on website performance

Ian Mercer
Ian Mercer

Amazon Instance vs Dedicated Server comparison

Some benchmark performance for Amazon vs a dedicated server

Ian Mercer
Ian Mercer

System.Data.EntitySqlException

Hints for dealing with this exception

Ian Mercer
Ian Mercer

Agile Software Development is Like Sailing

You cannot tack too often when sailing or you get nowhere. Agile is a bit like that.

Ian Mercer
Ian Mercer

Exception Handling using Exception.Data

My latest article on CodeProject covers the lesser known Exception.Data property

Ian Mercer
Ian Mercer

Javascript error reporting

Sending client-side errors back to a server for analysis

Ian Mercer
Ian Mercer

AntiVirus Software is the Worst Software!

When your anti-virus software starts stealing your personal data, it's time to remove it!

Ian Mercer
Ian Mercer

ASP.NET Custom Validation

How to solve a problem encountered with custom validation in ASP.NET

Ian Mercer
Ian Mercer

Optimization Advice

Some advice on software optimization

Ian Mercer
Ian Mercer

Linq's missing link

LinqKit came in handy back in 2009

Ian Mercer
Ian Mercer

Google Chart API

Ian Mercer
Ian Mercer

Cache optimized scanning of pairwise combinations of values

Using space-filling curves to optimize caching

Ian Mercer
Ian Mercer

Threading and User Interfaces

A rant about how few software programs get threading right

Ian Mercer
Ian Mercer

Take out the trash!

Why Windows shutdown takes so long

Ian Mercer
Ian Mercer

Dell upgrades - a pricey way to go

Ian Mercer
Ian Mercer

Programming mostly C#

Ian's advice on programming

Ian Mercer
Ian Mercer