Understanding Double Dispatch in C#

When working with object-oriented programming languages like C#, you might come across various design patterns and techniques that aim to solve specific programming problems. One such technique is double dispatch, which can initially seem puzzling. In this blog post, we’ll unravel the concept of double dispatch, discuss when to use it, and provide a concrete code sample to illustrate its application.

What is Double Dispatch?

To put it simply, double dispatch is a programming technique that allows a method to be selected based not only on the type of the object it belongs to but also on the type of another argument passed to it. This means that the decision about which method to invoke occurs at runtime, rather than at compile time.

Single vs. Double Dispatch

• Single Dispatch: This occurs when you call a virtual method on an object. The method that gets invoked is determined solely by the type of that object. In other words, the dispatching depends on one “dispatcher”: the base object.

• Double Dispatch: This takes it a step further. Both the type of the object and the type of the method’s argument are evaluated to determine which method to execute. This is an important distinction because it opens up more flexibility in method execution based on runtime types.

Multiple Dispatch Explained

Double dispatch is actually a specific case of multiple dispatch. In multiple dispatch, a method can take multiple arguments, and the choice of which implementation to execute depends on the type of each argument. Different programming languages handle multiple dispatch differently, often utilizing generic functions that can adapt to the types of their arguments.

When to Use Double Dispatch?

You might ask: “When should I consider using double dispatch?” Here are a few scenarios:

• Complex Object Interactions: When your program requires objects to interact in various ways based on their specific types, double dispatch enables that flexibility.
• Visitor Pattern Applicability: The visitor pattern, a popular use case for double dispatch, allows you to define new operations on a structure without changing that structure’s classes.
• Dynamic Method Resolution: If you need to resolve methods based on runtime types rather than compile-time types, double dispatch can provide the necessary capabilities.

Implementing Double Dispatch in C#

Now that we understand the concept, let’s take a closer look at how to implement double dispatch in C#. Below is a code example to demonstrate this technique.

Example Code

using System.Linq;  

class DoubleDispatch
{ 
    public T Foo<T>(object arg)
    { 
        var method = from m in GetType().GetMethods()
                     where m.Name == "Foo" 
                     && m.GetParameters().Length==1
                     && arg.GetType().IsAssignableFrom(m.GetParameters()[0].GetType())
                     && m.ReturnType == typeof(T)
                     select m;

        return (T) method.Single().Invoke(this, new object[]{arg});          
    }

    public int Foo(int arg) { /* Implementation here */ }

    static void Test() 
    { 
        object x = 5;
        Foo<int>(x); // Calls Foo(int) via Foo<T>(object).
    }
}  

Code Breakdown

1. Generic Method Definition: The Foo<T> method is declared generically to allow for type inference.
2. Method Selection: The method selects the appropriate Foo implementation by checking the argument type at runtime.
3. Method Invocation: Finally, the selected method is invoked with the argument.

Conclusion

Double dispatch is a powerful technique that allows for more dynamic method executions based on runtime types in C#. Whether you’re implementing the visitor pattern or simply need flexible method resolution, double dispatch is an invaluable tool in a C# developer’s arsenal. By understanding how it works and how to implement it, you can create more sophisticated and adaptable applications.

By integrating double dispatch into your programming practices, you can enhance the design and functionality of your C# applications. Happy coding!