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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.

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.

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.