Install-Package MetaObject

C# 4.0's new dynamic keyword brings a realm of new possibilities to C#.

If you inherit your class from DynamicObject, it becomes trivial to add support for dynamic methods, properties, indexes, and more.

As soon as you want to inherit your class from something else, however, it becomes a small nightmare ...

• for your class to support dynamic functionality, it needs to implement IDynamicMetaObjectProvider
• to implement IDynamicMetaObjectProvider, you need to have a GetMetaObject method that returns a DynamicMetaObject
• to return a DynamicMetaObject you need to be INTIMATELY familiar with pretty much everything in the System.Linq.Expressions namespace

As much as I'd love to learn how to create expression trees in C#, what I really want to do is add dynamic functionality to my classes!

I started digging around and discovered that DynamicObject is basically an empty class that has a rock solid DynamicMetaObject implementation that delegates dynamic calls to methods on the DynamicObject, eg. TryInvokeMember

Unfortunately, this DynamicMetaObject implementation is private and sealed!

Thankfully, the DLR (which is where this class comes from) is open-source, released under the Apache 2.0 license!

I took the source code, dealt with some dependency issues, removed the dependency on a DynamicObject and ended up with MetaObject

Assuming you have a reference to MetaObject.dll or you included MetaObject.cs in your project:

using System;
using System.Dynamic;

public class MyClass : Whatever, IDynamicMetaObjectProvider {

    // This 1 line is *ALL* you need to add support for all of the DynamicObject methods
    public DynamicMetaObject GetMetaObject(System.Linq.Expressions.Expression e){ return new MetaObject(e, this); }

    // Now, if you want to handle dynamic method calls, you can implement TryInvokeMember, just like you would in DynamicObject!
    public bool TryInvokeMember(InvokeMemberBinder binder, object[] args, out object result) {
        if (binder.Name.Contains("Cool")) {
            result = "You called a method with Cool in the name!";
            return true;
        } else {
            result = null;
            return false;
        }
    }
}

Now, if you want to invoke your dynamic methods:

// This will give you a compiler error because MyClass doesn't have a method called CoolMethod
var instance = new MyClass();
instance.CoolMethod();

// This will work and return "You called a method with Cool in the name!"
dynamic instance = new MyClass();
instance.CoolMethod();

All of the Try* methods that DynamicObject supports are supported by MetaObject:

• TryBinaryOperation
• TryConvert
• TryGetIndex
• TryGetMember
• TryInvoke
• TryInvokeMember
• TrySetIndex
• TrySetMember
• TryUnaryOperation