> Mike Valenty

A Proper Closure in C#

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You’ve seen code like this:

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public class Order
{
    private ITaxCalculator taxCalculator;
    private decimal? tax;

    public Order(ITaxCalculator taxCalculator)
    {
        this.taxCalculator = taxCalculator;
    }

    public decimal CalculateTax()
    {
        if (!tax.HasValue)
        {
            tax = taxCalculator.Calculate(this);
        }

        return tax.Value;
    }

    ...
}

There are a few things to pick at. I’m looking at the member variable named tax that is null until you call CalculateTax for the first time. There isn’t anything to prevent the rest of the class from using the tax variable directly and possibly repeating the null check code in multiple places. I thought it would be fun to rewrite it using a closure. I don’t mean the kind of accidental closures we write with LINQ, but an honest to goodness proper closure.

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public class Order
{
    public Order(ITaxCalculator taxCalculator)
    {
        CalculateTax = new Func<Func<decimal>>(() =>
        {
            decimal? tax = null;
            return () =>
            {
                if (!tax.HasValue)
                {
                    tax = taxCalculator.Calculate(this);
                }
                return tax.Value;
            };
        })();
    }

    public Func<decimal> CalculateTax { get; set; }

    ...
}

A closure is created around the tax variable so only the CalculateTax function has access to it. That’s pretty awesome. I wouldn’t have thought of using this technique before learning JavaScript all over again. It’s fun code to write, but it’s not going to get checked-in to source control. It’s basically a landmine for the next guy that has to make changes. The mental energy it takes to wrap your head around it is like listening to a joke with a really long setup and a lousy punch line. The solution is more complicated than the problem.

I still thought it was worth the mental exercise. Each language has its wheelhouse which makes a certain class of problems easy to solve. Admittedly this was a bit forced here, but opening your mind to other solutions may lead to a breakthrough solving a legitimately tough problem.

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