Constructor Injection

noun

say it: “kun-STRUK-ter in-JEK-shun” /kənˈstrʌktər ɪnˈdʒɛkʃən/

A class taking the objects it needs as constructor parameters instead of creating them itself.

What is Constructor Injection?

Constructor Injection — A class lists the objects it needs as constructor parameters and keeps them, instead of creating them with new. Whoever creates the class decides which ones to pass: the real email sender in the app, a fake one in a test.

class OrderService
{
    private readonly IEmailSender email;
    public OrderService(IEmailSender email) => this.email = email;
    public void Place(Order order) => email.Send(order.CustomerEmail, "Thanks!");
}

In a sentence

• “With constructor injection, OrderService gets its email sender from whoever creates it.”

Level: Journeyman · More OOP terms at this level

Updated October 28, 2026