Utilizing pass-by-ref Parameters

I recently helped a friend of mine code an application and at some point I created a method using a pass-by-reference parameter. Apparently this is new to him. I asked several other programmer friends that I know and most of them either know about it but never use it or don't know about it. So - here is a post about it.

To illustrate the usage of pass-by-reference, I will create to a simplistic example - where we want to calculate a loan amortization: with initial loan, interest rate, and loan period - and resulting in: monthly payment and total interest over loan term

So in code, without the pass-by-reference, we have to do this somewhat in 2 steps/methods:

    decimal monthlyPayment = CalculateMonthlyPayment(initialLoan, rate, period);
    decimal totalInterest = CalculateTotalInterest(initialLoan, rate, period);

With the definitions:

public decimal CalculateMonthlyPayment(decimal initialLoan, double rate, int period) { 
        decimal result;
        // calculate monthly payment
        return result;
    }

    public decimal CalculateTotalInterest(decimal initialLoan, double rate, int period) { 
        decimal result;
        // calculate interest
        return result;
    }

While the 2 steps thing is OK, but we know that we can optimize that better since both methods use the same parameter and both methods have a lot of similarity in their calculation. So it would be better if we can combine both methods - how can can we get 2 results with 1 methods? Maybe we should return the results in an array or make a "MortgageAmortization" class? Both ideas are OK and no doubt are also easily doable. But, I think this is also the right place to use pass-by-reference parameter in simplifying and optimizing our methods.

There are 2 ways to do pass-by-reference in C#: out or ref. The difference between them is small but important: argument passed to a ref parameter must be initialized - where out param does not have to be. You can read about ref and out further here.

So, using pass-by-reference, our code becomes something like this:

decimal monthlyPayment = 0;
    decimal totalInterest = 0;
    monthlyPayment = CalculateMortgage(initialLoan, rate, period, out totalInterest);
    // or you can use ref instead of out as long as the method signatures match
    // monthlyPayment = CalculateMortgage(initialLoan, rate, period, ref totalInterest);

public decimal CalculateMortgage(decimal initialLoan, double rate, int period, out totalInterest) { 
        decimal result;

        // calculate monthly payment

        // set total interest
        // totalInterest = 12500;

        return result;
    }