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References and new

“What is the difference between a value type and a reference type?”

Some variables hold a value. Others hold only an address, and copying one never copies the object.

See it

1 / 9int months = 60; The variable holds the value 60 itself. int is a value type.

In plain words

Some variables hold the value itself, like int months = 60. These are value types. Other variables hold only the address of an object that lives somewhere else in memory, like var car = new Loan(...). These are reference types. Copy a value and you get two separate values. Copy a reference and you get two names for the same object. The stack is a method’s short-lived scratch space for its local variables. The heap is where objects made by live until the garbage collector, .NET’s automatic memory cleaner, frees them.

Say it in the interview

Value types (int, decimal, bool, , any struct or enum) hold their data directly, so assignment copies the data.
A struct is a type you define whose variables hold the data itself.
Reference types (class, string, arrays, record class) hold a reference to an object on the managed heap, so assignment copies the reference and both variables see the same object.
A is a class or struct that compares by values.
For a class, allocates the object and returns its reference.
When nothing references it any more, the reclaims it later, at a time it chooses.

In the lending API

One real test file from the lending API. The last test is a production bug and its fix. Press Next line to walk through it.

Lending.sln · where this file sits

tests/Domain.UnitTests/Servicing/

Diagram · step 1int months = 60; The variable holds the value 60 itself. int is a value type.

ReferenceTests.cs · line 10 · 1 of 15int is a value type. The variable months holds the number 60 itself, not an address.

What they ask next

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

Why not…So is a class passed to a method "by reference"?
No. C# passes every argument by value unless you write ref. For a class, the value copied is the address. So the method can change the object, but if it points its parameter at a new object, the caller never sees that.
- What does ref do, then?
  ref passes the caller's variable itself. Now the method can point the caller's car at a different Loan, or change the caller's int. out is the same, but the method must set it.
Explain this lineWhy and not ?
checks one thing: same object, same address. calls , which a class may change to compare values. Same says exactly what the test means.
Explain this lineWhat is the ! in car!.Emi()?
The null-forgiving operator. It only silences the compiler's "this may be null" warning. It does not stop the crash. Here the crash is the point of the test.
Is string a value type? It behaves like one.
No, string is a reference type. It feels like a value because strings never change: every "change" builds a new string, so a shared one can't change under you. == on strings compares the text, not the address.
What is boxing?
Putting a value type into an object variable. object o = 60; copies the 60 into a new object on the heap. (int)o copies it back out, which is unboxing. Each box is an allocation, so busy loops avoid it.
When would you write a struct instead of a class?
For a small value with no identity that rarely changes, like a coordinate. Microsoft's guideline: under about 16 bytes, immutable, and not boxed often. Anything with an identity and a life, like a Loan, is a class.
Two loans with the same values: are they equal?
Not with == on a class: it compares addresses, and two calls give two addresses. A compares values instead, so two records with the same values are equal.

Traps

Don't say~~Value types live on the stack, reference types on the heap.~~SayObjects live on the heap. A value lives where it is declared: a local int usually on the stack, but the Months field of a Loan sits on the heap, inside the Loan.
Don't say~~A class is passed to a method by reference.~~SayEverything is passed by value unless you write ref. For a class, the value that gets copied is the address.
Don't say~~var preview = loan; makes a copy of the loan.~~SayIt copies the address. Both names reach one loan. A second loan needs .

With AI tools

Canva's interview guide says it grades whether you find and fix the issues in AI-written code. var preview = loan; is a typical find-the-bug line: it compiles, it looks fine, and it changes the real loan. Spot it by asking of every assignment: does this copy a value or an address?