C# Interview Question #19

What is an interface in C#?

OOP & Core C# Concepts Junior Beginner

Quick Interview Answer

An interface defines a contract that implementing types agree to provide.

Detailed Explanation

An interface defines a contract that implementing types agree to provide.

Interfaces are heavily used in ASP.NET Core for dependency injection, loose coupling, unit testing, mocking, Clean Architecture, and SOLID design.

Modern C# interfaces can also contain certain default implementations, but their primary role in application architecture remains defining capabilities and contracts.

Code Example

public interface IProductService
{
    Task<List<Product>> GetAllAsync();
    Task<Product?> GetByIdAsync(int id);
}

public class ProductService : IProductService
{
    public Task<List<Product>> GetAllAsync()
        => Task.FromResult(new List<Product>());

    public Task<Product?> GetByIdAsync(int id)
        => Task.FromResult<Product?>(null);
}

// ASP.NET Core DI registration:
// builder.Services.AddScoped<IProductService, ProductService>();