Dependency Injections & Inversion of Control

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Dependencies

What is a dependency? The best way to show this is through an code example:

public void CalculateTaxFromIncome(int income) {
    TaxRules taxRules = new TaxRules();
    int taxDeduction = taxRules.calculateTax(income);
}

In the above code, taxRules is a dependency, that is, an object that is required in order for the function CalculateTaxFromIncome to work. Without taxRules, the function will not be able to operate. Thus we say that taxRules is a dependency.

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Inversion of Control (IoC)

So, how is inversion of control related to dependency injections. Lets take a look at the below code example.

public void CalculateTaxFromIncome(int income, TaxRules taxRules) {
    int taxDeduction = taxRules.calculateTax(income);
}

Instead of giving the function CalculateTaxFromIncome control over instantiating taxRules, we instead leave that responsibility to the class that calls CalculateTaxFromIncome. Instead we are thereby injecting the dependency into the function. From this, we can show that we are inverting the creation/control of the dependency.

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Why IoC & Dependency Injections?

There’s several reasons why we attempt to use IoC and Dependency Injections.

1. Agglomerate the creation of all necessary objects into a single point (file). By doing this, it becomes easier to handle changes that may occur further down the track.
2. Typically, when we don’t use dependency injections and IoC, unit testing becomes a lot more difficult, as our classes are likely to be more closely coupled.

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