Module 1 · Values, Parameters and Byrefs · Lesson 1 of 34

A value is not a variable, and assignment vs ref assignment

3 min readTechInterviewNotes Editorial TeamUpdated 16 Sept 2026Save

1.1 A value is not the same thing as a variable

The value 42 is an abstract number. A variable is a typed storage location that can currently contain the bit pattern representing 42. Two variables may contain equal values while occupying different storage. A reference variable is different again: it stores a reference to another variable, called its referent, rather than storing the referent’s value directly.[S1]

Four terms that must stay separate

Value
The information being represented, such as the integer 42 or a Point with coordinates.
Variable
A typed storage location whose current contents form a value.
Object reference
A GC-tracked reference identifying a managed object. Copying it does not copy the object.
Managed byref
A GC-aware reference to a variable or interior storage location. In IL signatures it is written as T&.

Equal values, different storage

stack frame	stack frame
`a: int = 42`	`x: int = 42`
`b: int = 42`	`r: ref int`
`a = b` copies `42` into `a`	`r = 99` writes into `x`
no alias exists	`x` and `r` observe the same bytes

1.2 Assignment and ref assignment are different operations

Normal assignment changes the value stored in the destination. Ref assignment changes which storage location a reference variable aliases.

int[] numbers = [10, 20, 30];
ref int slot = ref numbers[0]; // slot aliases element 0
slot = 111;
// ordinary assignment: numbers[0] = 111
slot = ref numbers[2];
// ref assignment: alias now targets element 2
slot = 333;
// ordinary assignment: numbers[2] = 333

After the code runs, the array is [111, 20, 333]. The expression slot = 333 does not replace the alias; it writes through the alias. The expression slot = ref numbers[2] changes the alias itself.

Under The Hood

CLR / IL. The CLI type system has managed pointer types, written T&. Address-producing instructions include ldloca for a local, ldarga for an argument, ldflda for a field, and ldelema for an array element. Indirection instructions such as ldind.i4 and stind.i4 read and write through the managed pointer.[S8]
JIT / CPU. A live byref is normally represented by a machine-sized address-like value in a register or stack slot. It introduces aliasing: the JIT may need to assume that a store through the byref changes another value whose address could overlap.
Memory layout. A byref can target stack storage, an argument, an array element, or a field inside a managed object. It is not an object header and does not carry a MethodTable pointer of its own.
GC impact. A managed byref into the managed heap is reported as an interior GC pointer. The GC can keep the containing object alive and account for relocation; the byref does not automatically pin the object.[S9][S13]
Performance cost. Creating an alias normally allocates no managed object. Costs are indirect access, possible register pressure, and optimization restrictions caused by uncertain aliasing.

Naive mental model
“ref means C-style pointer, so the object is pinned and I can do arbitrary address arithmetic.”
Accurate mental model
A C# byref is a typed, GC-aware alias whose uses and lifetime are constrained by the compiler and verifier. It can point into movable managed memory without granting arbitrary pointer arithmetic.
Explain it in one sentence
What is the difference between slot = value; and slot = ref other;?