Dynamic and the DLR

Dynamic Dispatch

On to the interop features in C# 4.0, starting with what is perhaps the biggest change.

C# now supports dynamic late-binding. The language has always been strongly typed, and it continues to be so in version 4.0. Microsoft believes this makes C# easy to use, fast and suitable for all the work .NET programmers are putting it to. But there are times when you need to communicate with systems not based on .NET.

Traditionally, there were at least two approaches to this. The first was simply to import the foreign model directly into .NET as a proxy. COM Interop provides one example. Since the original release of the .NET Framework, it has used this strategy with a tool called TLBIMP, which creates new .NET proxy types you can use directly from C#.

LINQ-to-SQL, shipped with C# 3.0, contains a tool called SQLMETAL, which imports an existing database into C# proxy classes for use with queries. You’ll also find a tool that imports Windows Management Instrumentation (WMI) classes to C#. Many technologies allow you to write C# (often with attributes) and then perform interop using your handwritten code as basis for external actions, such as LINQ-to-SQL, Windows Communication Foundation (WCF) and serialization.

The second approach abandons the C# type system entirely—you embed strings and data in your code. This is what you do whenever you write code that, say, invokes a method on a JScript object or when you embed a SQL query in your ADO.NET application. You’re even doing this when you defer binding to run time using reflection, even though the interop in that case is with .NET itself.

The dynamic keyword in C# is a response to dealing with the hassles of these other approaches. Let’s start with a simple example—reflection. Normally, using it requires a lot of boilerplate infrastructure code, such as:


With the dynamic keyword, instead of calling a method MyMethod on some object using reflection in this manner, you can now tell the compiler to please treat o as dynamic and delay all analysis until run time. Code that does that looks like this:


It works, and it accomplishes the same thing with code that’s much less convoluted.

The value of this shortened, simplified C# syntax is perhaps more clear if you look at the ScriptObject class that supports operations on a JScript object. The class has an InvokeMember method that has more and different parameters, except in Silverlight, which actually has an Invoke method (notice the difference in the name) with fewer parameters. Neither of these are the same as what you’d need to invoke a method on an IronPython or IronRuby object or on any number of non-C# objects you might come into contact with.

In addition to objects that come from dynamic languages, you’ll find a variety of data models that are inherently dynamic and have different APIs supporting them, such as HTML DOMs, the System.Xml DOM and the XLinq model for XML. COM objects are often dynamic and can benefit from the delay to run time of some compiler analysis.

Essentially, C# 4.0 offers a simplified, consistent view of dynamic operations. To take advantage of it, all you need to do is specify that a given value is dynamic, ensuring that analysis of all operations on the value will be delayed until run time.

In C# 4.0, dynamic is a built-in type, and a special pseudo-keyword signifies it. Note, however, that dynamic is different from var. Variables declared with var actually do have a strong type, but the programmer has left it up to the compiler to figure it out. When the programmer uses dynamic, the compiler doesn’t know what type is being used—the programmer leaves figuring it out up to the runtime.

Dynamic and the DLR

The infrastructure that supports these dynamic operations at run time is called the Dynamic Language Runtime (DLR). This new .NET Framework 4 library runs on the CLR, like any other managed library. It’s responsible for brokering each dynamic operation between the language that initiated it and the object it occurs on. If a dynamic operation isn’t handled by the object it occurs on, a runtime component of the C# compiler handles the bind. A simplified and incomplete architecture diagram looks something like Figure 1.

image: The DLR Runs on Top of the CLR

Figure 1 The DLR Runs on Top of the CLR

The interesting thing about a dynamic operation, such as a dynamic method call, is that the receiver object has an opportunity to inject itself into the binding at run time and can, as a result, completely determine the semantics of any given dynamic operation. For instance, take a look at the following code:


If MyDynamicObject was defined as shown here, then you can imagine what happens:


In fact, the code prints:


By declaring d to be of type dynamic, the code that consumes the MyDynamicObject instance effectively opts out of compile-time checking for the operations d participates in. Use of dynamic means “I don’t know what type this is going to be, so I don’t know what methods or properties there are right now. Compiler, please let them all through and then figure it out when you really have an object at run time.” So the call to Bar compiles even though the compiler doesn’t know what it means. Then at run time, the object itself is asked what to do with this call to Bar. That’s what TryInvokeMember knows how to handle.

Now, suppose that instead of a MyDynamicObject, you used a Python object:


If the object is the file listed here, then the code also works, and the output is much the same:


Under the covers, for each use of a dynamic value, the compiler generates a bunch of code that initializes and uses a DLR CallSite. That CallSite contains all the information needed to bind at run time, including such things as the method name, extra data, such as whether the operation takes place in a checked context, and information about the arguments and their types.

This code, if you had to maintain it, would be every bit as ugly as the reflection code shown earlier or the ScriptObject code or strings that contain XML queries. That’s the point of the dynamic feature in C#—you don’t have to write code like that!

When using the dynamic keyword, your code can look pretty much the way you want: like a simple method invocation, a call to an indexer, an operator, such as +, a cast or even compounds, like += or ++. You can even use dynamic values in statements—for example, if(d) and foreach(var x in d). Short-circuiting is also supported, with code such as d && ShortCircuited or d ?? ShortCircuited.

The value of having the DLR provide a common infrastructure for these sorts of operations is that you’re no longer having to deal with a different API for each dynamic model you’d like to code against—there’s just a single API. And you don’t even have to use it. The C# compiler can use it for you, and that should give you more time to actually write the code you want—the less infrastructure code you have to maintain means more productivity for you.

The C# language provides no shortcuts for defining dynamic objects. Dynamic in C# is all about consuming and using dynamic objects. Consider the following:


This code compiles, and it contains a lot of dynamic operations. First, there’s the dynamic pre-increment on index1, then the dynamic add with index2. Then a dynamic indexer get is called on list. The product of those operations calls the member Foo. Finally, the total result of the expression is converted to a string and stored in s. That’s five dynamic operations in one line, each dispatched at run time.

The compile-time type of each dynamic operation is itself dynamic, and so the “dynamicness” kind of flows from computation to computation. Even if you hadn’t included dynamic expressions multiple times, there still would be a number of dynamic operations. There are still five in this one line:


Because the results of the two indexing expressions are dynamic, the index itself is as well. And because the result of the index is dynamic, so is the call to Foo. Then you’re confronted with converting a dynamic value to a string. That happens dynamically, of course, because the object could be a dynamic one that wants to perform some special computation in the face of a conversion request.

Notice in the previous examples that C# allows implicit conversions from any dynamic expression to any type. The conversion to string at the end is implicit and did not require an explicit cast operation. Similarly, any type can be converted to dynamic implicitly.

In this respect, dynamic is a lot like object, and the similarities don’t stop there. When the compiler emits your assembly and needs to emit a dynamic variable, it does so by using the type object and then marking it specially. In some sense, dynamic is kind of an alias for object, but it adds the extra behavior of dynamically resolving operations when you use it.

You can see this if you try to convert between generic types that differ only in dynamic and object; such conversions will always work, because at runtime, an instance of List<dynamic> actually is an instance of List<object>:


You can also see the similarity between dynamic and object if you try to override a method that’s declared with an object parameter:


Although it resolves to a decorated object in your assembly, I do like to think of dynamic as a real type, because it serves as a reminder that you can do most things with it that you can do with any other type. You can use it as a type argument or, say, as a return value. For instance, this function definition will let you use the result of the function call dynamically without having to put its return value in a dynamic variable:


There are a lot more details about the way dynamic is treated and dispatched, but you don’t need to know them to use the feature. The essential idea is that you can write code that looks like C#, and if any part of the code you write is dynamic, the compiler will leave it alone until run time.

I want to cover one final topic concerning dynamic: failure. Because the compiler can’t check whether the dynamic thing you’re using really has the method called Foo, it can’t give you an error. Of course, that doesn’t mean that your call to Foo will work at run time. It may work, but there are a lot of objects that don’t have a method called Foo. When your expression fails to bind at run time, the binder makes its best attempt to give you an exception that’s more or less exactly what the compiler would’ve told you if you hadn’t used dynamic to begin with.

Consider the following code:


Here I have a string, and strings clearly do not have a method called Foo. When the line that calls Foo executes, the binding will fail and you’ll get a RuntimeBinderException. This is what the previous program prints:


Which is exactly the error message you, as a C# programmer, expect.

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Vidyasagar MSC

Polyglot & Pragmatic Programmer