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Picking Overloaded Methods at Runtime (Multiple Dispatch)

I don't spend a lot of time interacting with dynamic languages like Python, but I think that I've finally found a use for the dynamic keyword in C#. It's something that C++ developers call "multiple dispatch" (or so I'm told).

Normally, if you call a method that that has two overloaded versions, the choice between the two overloads is made at compile time, based on how the parameters passed to the method are declared. But, by using the dynamic keyword, you can defer the choice between the two methods until run time and have the choice based on the datatype of the data in the parameters, instead of on the way the parameters are declared.

For instance, assume two classes: one called BaseCustomer and another called PremiumCustomer that inherits from BaseCustomer. Also assume a method with two overloaded versions: one version that accepts a Customer object and one that accepts a PremiumCustomer object. Here are the two methods:

public void ProcessCustomer(Customer cust)
{
}

public void ProcessCustomer(PremiumCustomer cust)
{
}

Now, here's some code that calls the two versions of the ProcessCustomer method. Guess which version is called each time:

Customer cst = new Customer();
ProcessCustomer(cst);
PremiumCustomer pcst = new PremiumCustomer();
ProcessCustomer(pcst);

You probably guessed that the first two lines of code will call the first version of the ProcessCustomer method -- the one that accepts a Customer object parameter. After all, the cst variable passed to the method is declared as Customer and is referencing a Customer object. And, just as probably, you guessed that the second two lines of code will call the second version of the method that accepts a PremiumCustomer. You'd be right both times.

But how about the following example, which declares the variable passed to the ProcessCustomer method as Customer, but holds a reference to a PremiumCustomer? What's your guess for this code:

Customer cst = new PremiumCustomer();
ProcessCustomer(cst);

If you guessed that this example still calls the first version of the ProcessCustomer method, you'd be right. That's because the decision on which overloaded version to call is made at compile time, based on the datatype of the variable being passed (and the cst variable is declared as Customer).

You might prefer, however, to have the decision made at run time based on the datatype of the object actually being passed. In other words, in that last set of code you might prefer to have the second version of the ProcessCustomer method called because the cst variable is referencing a PremiumCustomer object. You can do that by casting the parameter using the dynamic keyword, as in this example:

Customer cst = new PremiumCustomer();
ProcessCustomer((dynamic) cst);

This code will call the second version of the ProcessCustomer method because, even though the cst parameter is still declared as Customer, the variable is pointing at a PremiumCustomer object. I won't always want that behavior, but there will be times when this technique is going to be very useful.

Posted by Peter Vogel on 01/10/2013 at 1:16 PM


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