Paulo Morgado

C# Proposal: Compile Time Static Checking Of Dynamic Objects

C# 4.0 introduces a new type: dynamic. dynamic is a static type that bypasses static type checking.

This new type comes in very handy to work with:

• The new languages from the dynamic language runtime.
• HTML Document Object Model (DOM).
• COM objects.
• Duck typing
• …

Because static type checking is bypassed, this:

dynamic dynamicValue = GetValue();
dynamicValue.Method();

is equivalent to this:

object objectValue = GetValue();
objectValue
    .GetType()
        .InvokeMember(
            "Method",
            BindingFlags.InvokeMethod,
            null,
            objectValue,
            null);

Apart from caching the call site behind the scenes and some dynamic resolution, dynamic only looks better. Any typing error will only be caught at run time.

In fact, if I’m writing the code, I know the contract of what I’m calling. Wouldn’t it be nice to have the compiler do some static type checking on the interactions with these dynamic objects?

Imagine that the dynamic object that I’m retrieving from the GetValue method, besides the parameterless method Method also has a string read-only Property property. This means that, from the point of view of the code I’m writing, the contract that the dynamic object returned by GetValue implements is:

string Property { get; }
void Method();

Since it’s a well defined contract, I could write an interface to represent it:

interface IValue
{
    string Property { get; }
    void Method();
}

If dynamic allowed to specify the contract in the form of dynamic(contract), I could write this:

dynamic(IValue) dynamicValue = GetValue();
dynamicValue.Method();

This doesn’t mean that the value returned by GetValue has to implement the IValue interface. It just enables the compiler to verify that dynamicValue.Method() is a valid use of dynamicValue and dynamicValue.OtherMethod() isn’t.

If the IValue interface already existed for any other reason, this would be fine. But having a type added to an assembly just for compile time usage doesn’t seem right. So, dynamic could be another type construct. Something like this:

dynamic DValue
{
    string Property { get; }
    void Method();
}

The code could now be written like this;

DValue dynamicValue = GetValue();
dynamicValue.Method();

The compiler would never generate any IL or metadata for this new type construct. It would only be used, at compile time, for static checking of dynamic objects. As a consequence, it makes no sense to have public accessibility, so it would not be allowed.

Once again, if the IValue interface (or any other type definition) already exists, it can be used in the dynamic type definition:

dynamic DValue : IValue, IEnumerable, SomeClass
{
    string Property { get; }
    void Method();
}

Another added benefit would be IntelliSense.

I’ve been getting mixed reactions to this proposal. What do you think? Would this be useful?