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Inheritance

“What is inheritance in C#, and in what order do the constructors run?”

One class is built on another and gets its data and methods. Write the shared part once.

See it

class Loanits own copy

Balance

ApplyPayment()

Rate

class Leaseits own copy

Balance

ApplyPayment()

EndDate

1/10Before. Loan and Lease each hold their own copy of Balance and .

In plain words

Inheritance lets one class (the child) be built on another (the parent, or base class). class Loan : Contract means every Loan has everything a Contract has, plus its own extras. Building a Loan makes one object; the parent's constructor runs first, then the child's. You then call the parent's methods on that same object, with no extra .

Say it in the interview

Inheritance is an "is-a" relationship (a Loan is a Contract): class Loan : Contract gives Loan every member of Contract that is not , so shared code lives in one place.
C# allows one base class only, but any number of .
Constructor order: the base constructor runs first, then the derived one. A derived constructor passes values up with .
Inherited methods are called on the same object: loan.ApplyPayment(200) runs Contract's code against this Loan's data. refers to the current object; reaches the parent's version of a member.

In the lending API

The parent, one child, and the code that builds and uses it. Press Next line to walk through them.

Lending.sln · where these 3 files sit

src/
- Domain/
- Billing/

Diagram · step 3The fix. Balance and move out of Loan and Lease into one parent, . Fixed once.

class Loanits own copy

Balance

ApplyPayment()

Rate

class Leaseits own copy

Balance

ApplyPayment()

EndDate

Contract.cs · line 3 · 1 of 3The parent. It holds only what every kind of contract shares.

What they ask next

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

What does mean, and when do you write it?
is the object the code is running on right now. Inside , and are the same. You write when a parameter has the same name as a member, or to call another constructor of the same class with .
When do you need outside the constructor line?
When a child replaces a parent method but still wants the parent's version too: calls the parent's code. That is covered on .
What if the parent has no constructor without parameters?
Then every child constructor must call with values, or it will not compile. Here Contract needs a balance, so Loan must pass one up.
Can a child use the parent's private members?
No. Private stays inside Contract. That is why Loan cannot write Balance directly; it calls . Mark a member to let children use it (see ).
Can a class inherit from two classes?
No, C# allows one base class. It can implement many interfaces. Deep chains (A : B : C : D) get hard to change, so teams keep them shallow, one or two levels.

Traps

Don't say~~Calling on a Loan creates a Contract object first.~~SayThere is one object. The Contract part is inside it from the moment ran.
Don't say~~The child constructor runs, then the parent's.~~SayParent first. The child's body runs once the parent part is ready.