C# 4.0 Pocket Reference, 3rd Edition

Dynamic Binding (C# 4.0)

Dynamic binding defers binding—the process of resolving types, members, and operations—from compile time to runtime. Dynamic binding is useful when at compile time you know that a certain function, member, or operation exists, but the compiler does not. This commonly occurs when you are interoperating with dynamic languages (such as IronPython) and COM, and in scenarios when you might otherwise use reflection.

A dynamic type is declared with the contextual keyword dynamic:

dynamic d = GetSomeObject();
d.Quack();

A dynamic type tells the compiler to relax. We expect the runtime type of d to have a Quack method. We just can’t prove it statically. Since d is dynamic, the compiler defers binding Quack to d until runtime. To understand what this means requires distinguishing between static binding and dynamic binding.

Static Binding Versus Dynamic Binding

The canonical binding example is mapping a name to a specific function when compiling an expression. To compile the following expression, the compiler needs to find the implementation of the method named Quack:

d.Quack();

Let’s suppose the static type of d is Duck:

Duck d = ...
d.Quack();

In the simplest case, the compiler does the binding by looking for a parameterless method named Quack on Duck. Failing that, the compiler extends its search to methods taking optional parameters, methods on base classes of Duck, and extension methods that take Duck as its first parameter. If no match is found, you’ll get a compilation ...