Common Type System—Interface Method Dispatch

Visual C# Tutorials

.NET Framework Tutorials

Common Type System Type Systems Intro § Types and Objects § Type Unification § Reference, Value Types § Accessibility, Visibility § Type Members § Fields § Methods § Locals Overloading Argument Passing Style Pass-by-Value and Pass-by-Reference Output Parameters Managed Pointers Variable Argument Methods Methods, Subclassing Virtuals, Overriding New Slots Exception Handlers Constructors, Instantiation § Default Constructors Value Construction Constructor Chaining Field Initialization Type Initializers Unhandled Exceptions Properties § Indexers Mixed Mode Accessibility Events Operator Overloading Coercion Subclassing, Polymorphism Styles of Inheritance § Implementation Inheritance Interface Inheritance Abstract Classes Interfaces § Method Dispatch Multiple Inheritance Private Interface Inheritance Private Inheritance Accessibility Interface Reimplementation Abstract and Interface? Sealing Types and Methods Runtime Type Checking Namespaces: Organizing Types § Define a Namespace Namespace Resolution Special Types § Delegates § CTS Support Co- and Contravariance Anonymous Delegates Custom Attributes Enumerations § Flags-Style Limitations Helper Methods Generics § Further Reading §

This Book Reviewed!

© 2006 Wiley Publishing Inc.

Interface Method Dispatch

An interface call is roughly equivalent to making a virtual method call. While the IL looks the same— that is, it gets emitted as a callvirt instruction—there is actually an extra layer of indirection in the implementation necessary to perform the method dispatch. If you look at the machine code that the JIT produces, it includes a lookup into the interface map (which hangs off the method table) in order to correlate the implementation of an interface to the correct slot in the method table. In most circumstances, this performance overhead shouldn’t be a concern.

As noted earlier—and it applies here too—to make a virtual method invocation on a value type requires the value to be boxed first. Constrained calls, a new feature in 2.0, enable the runtime to optimize this away in some circumstances.