"You are creating a Function as Object using a Closure. How do you create instances of the object? [M]ake a function that returns your Function as Object. Give the function an Intention Revealing Name (Beck) such as make-object."

The constructor function must be explicitly typed, again, but we gain a tiny bit of brevity by changing the "delegate" literals into (slightly) shorter C# lambdas:

static void ConstructorFunction()
    Func<int,Func<int, int> makeAccount = 
        ((Func<int,Func<int,int>)( (bal) => {
            var balance = bal;
            return (int amount) =>
                if (balance >= amount)
                    balance -= amount;
                    return balance;
                    throw new Exception("Insufficient funds");

    Console.WriteLine("=============> Closure");
    var acctForEugene = makeAccount(100);
    Console.WriteLine("{0}", acctForEugene(20));

Were it not for the implicitly-typed local variable declaration syntax around "acctForEugene", it would be acutely obvious that "makeAccount" isn't creating any kind of object at all, but a function to be executed. Even so, the explicit typing requirement for the lambdas is kind of annoying, and will only get worse as we move through the pattern language.

Last updated: 16 March 2022

Tags: pattern implementation   creational   csharp