Constructor

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In the lending API

The same file before and after the fix, plus its test. Press Next line to walk through them.

What they ask next

Interviewers check that you can explain every line. Tap a question to open it.

• Explain this lineWhy throw, and not fix the value or return false?

  A constructor has no return value, so throwing is the only way to say no. Quietly turning 0 into a default hides the typo, and the customer gets a loan nobody asked for.

• Why not…Why not check in the controller instead?

  Do both. The API checks input first, so the user gets a clear message. The constructor is the last guard: every path that builds a Loan goes through it, including an import job, a test, or a screen added next year.

  • What does the user see if the constructor throws?

    If nothing catches it, a 500 error. Teams catch these errors in one place in the request pipeline and turn them into a 400 with the message.

• Why not…Why not the .NET 8 one-line check helpers?

  They are a good choice. ArgumentOutOfRangeException has static helpers, like ThrowIfNegativeOrZero(rate): one line per value, and the message names the parameter and the bad value. Many teams prefer it. Either way, the check runs before the object can be used. If you switch, the test must expect ArgumentOutOfRangeException, or use Assert.ThrowsAny<ArgumentException>.

• What is constructor chaining?

  One constructor calling another with : this(...), so the checks live in one place. public Loan(decimal amount, int months) : this(amount, 9.5m, months) { } runs the main constructor first. : base(...) calls the parent class's constructor instead.

• What is a static constructor?

  It runs once per class, before the class is first used, to set up static data. No parameters, no public, and you never call it yourself. If it throws, the class stays broken until the app restarts (TypeInitializationException).

• What is a private constructor for?

  It stops outside code from calling new. Used for a factory method (a static method that builds the object for you) like Loan.Create(...), which can return an error instead of throwing; for a class with only static members; and for a singleton, a class with exactly one shared object (see static).

• What happens if you write no constructor?

  The compiler adds a public one with no parameters, so new Loan() works. Write any constructor yourself and that free one is gone.

• In what order do the parts run?

  Static parts first, once per class: static field initializers, then the static constructor. Then for each new: this class’s field initializers, then the base class constructor (its initializers, then its body), then this constructor’s body.

• Does EF Core call this constructor when it loads a loan?

  It can. EF Core fills constructor parameters whose names match properties (amount, rate, months) and sets the rest, like Id, itself. Some teams add a private constructor with no parameters for EF, so loading old rows never runs today's checks.

• What is a primary constructor?

  C# 12: parameters written on the class line, public class LoanService(ILoanRepository repo). They can be used anywhere in the class. Common for services that get their parts from the DI container. There is no body, so checks must go into property initializers, which gets clumsy; classes like Loan keep a normal constructor. Note: the parameters are not readonly fields, so any method can reassign them.

• Can a constructor be async?

  No. If setup needs a database or web call, use a static CreateAsync method that awaits the call and then calls new. Constructors should be quick and never wait on I/O.

• What about new Loan { Rate = 0 }?

  That is an object initializer. It sets properties after the constructor has finished, so the constructor can't check those values. That is one reason Loan uses private set and takes every value through the constructor. C# 11 required makes the compiler insist a property is set, but it still can’t check the value.

Traps

• Don't sayThe constructor creates the object.Saynew makes the object, with every value at 0. The constructor then sets it up.
• Don't sayIf I don't write a constructor, the class has none.SayThe compiler adds a public one with no parameters. Write any constructor yourself and that one is gone.
• Don't sayLoad the data from the database in the constructor.SayConstructors can't be async and should be quick. Load data in an async method instead.