Constants and readonly fields in C Sharp (C#) - Step-by-Step Execution

Concept Flow - Constants and readonly fields

Declare constant with 'const'

↓

Value fixed at compile time

↓

Use constant anywhere

↓

Declare readonly field with 'readonly'

↓

Value set at declaration or in constructor

↓

Value fixed after construction

↓

Use readonly field anywhere

Constants are fixed values set at compile time. Readonly fields can be set once during object creation and then stay fixed.

Execution Sample

C Sharp (C#)

class Example {
  public const int DaysInWeek = 7;
  public readonly int Year;
  public Example(int year) {
    Year = year;
  }
}

Defines a constant for days in a week and a readonly field for the year set during object creation.

Execution Table

Step	Action	Value Assigned	When Assigned	Can Change Later?
1	Declare const DaysInWeek	7	Compile time	No
2	Declare readonly Year field	unassigned	N/A	Yes, once in constructor
3	Create Example object with year=2024	Year = 2024	Constructor	No
4	Try to change DaysInWeek	Error	N/A	No
5	Try to change Year after construction	Error	N/A	No

💡 Constants fixed at compile time; readonly fields fixed after constructor; attempts to change cause errors.

Variable Tracker

Variable	Start	After Declaration	After Constructor	Final
DaysInWeek	N/A	7	7	7
Year	N/A	unassigned	2024	2024

Key Moments - 3 Insights

Why can't I assign a value to a const variable inside a constructor?

Constants are fixed at compile time and must be assigned where declared, not in constructors. See execution_table step 1 vs step 3.

Can I change a readonly field after the object is created?

No, readonly fields can only be assigned once during declaration or in the constructor. After that, they cannot be changed. See execution_table step 5.

Why does trying to change a const or readonly variable cause an error?

Because const values are fixed at compile time and readonly fields are fixed after construction, the compiler prevents changes to keep data safe. See execution_table steps 4 and 5.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of DaysInWeek after declaration?

A 0

B 7

C unassigned

D Cannot be assigned

💡 Hint

Check execution_table row 1 and variable_tracker for DaysInWeek after declaration.

At which step does the readonly field Year get its value?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at execution_table step 3 where Year is assigned in the constructor.

If you try to change Year after construction, what happens according to the execution_table?

A Error occurs

B Value changes successfully

C Value resets to default

D Value becomes null

💡 Hint

See execution_table step 5 about changing Year after construction.

Concept Snapshot

const: fixed value at compile time, assigned where declared
readonly: assigned once at declaration or in constructor
const cannot be changed anywhere
readonly can be set only during construction
both provide safety for fixed data

Full Transcript

This lesson shows how constants and readonly fields work in C#. Constants are values fixed at compile time and must be assigned where declared. Readonly fields can be assigned once either at declaration or inside the constructor. After that, readonly fields cannot be changed. The execution table traces declaring a const DaysInWeek with value 7, declaring a readonly Year field, assigning Year in the constructor, and shows errors if you try to change these values later. The variable tracker shows DaysInWeek is always 7 and Year is assigned 2024 during construction. Key moments clarify why const cannot be assigned in constructors and why readonly fields cannot be changed after construction. The quiz tests understanding of when values are assigned and what happens if you try to change them. Remember, const is for fixed compile-time values, readonly is for values fixed after object creation.