Readonly structs in C Sharp (C#) - Step-by-Step Execution

Concept Flow - Readonly structs

Define readonly struct

↓

Create instance with values

↓

Try to modify field

No→Compile error

Yes↓

Read field values

↓

Use instance safely without changes

This flow shows how a readonly struct is defined, instantiated, and used without allowing field changes.

Execution Sample

C Sharp (C#)

readonly struct Point
{
    public readonly int X;
    public readonly int Y;

    public Point(int x, int y) => (X, Y) = (x, y);
}

Defines a readonly struct Point with two readonly fields X and Y set only at creation.

Execution Table

Step	Action	Field Values	Result
1	Define readonly struct Point	X: -, Y: -	Struct ready, fields immutable
2	Create Point p = new Point(3, 4)	X: 3, Y: 4	Instance created with values
3	Read p.X and p.Y	X: 3, Y: 4	Values read successfully
4	Try p.X = 5 (modify)	X: 3, Y: 4	Compile error: cannot assign to field of readonly struct
5	Use p without modification	X: 3, Y: 4	Safe usage, no changes allowed

💡 Modification attempt fails because fields are readonly, stopping changes.

Variable Tracker

Variable	Start	After Creation	After Read	After Failed Modify	Final
p.X	-	3	3	3	3
p.Y	-	4	4	4	4

Key Moments - 2 Insights

Why can't I change the value of p.X after creating the Point?

Because Point is a readonly struct, its fields are immutable after creation. See execution_table step 4 where modification causes a compile error.

Can I create a readonly struct without a constructor?

You can, but you won't be able to set fields after creation. The constructor sets the values once, as shown in step 2.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of p.Y after creation?

A 4

B 0

C Undefined

D Cannot read

💡 Hint

Check execution_table row 2 under Field Values.

At which step does the program stop allowing changes to the struct fields?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Look at execution_table step 4 where modification causes a compile error.

If Point was not readonly, what would happen at step 4?

A Compile error still occurs

B Field p.X would change to 5

C Program crashes

D Nothing happens

💡 Hint

Readonly keyword prevents modification; without it, assignment works (compare step 4).

Concept Snapshot

readonly struct StructName
{
  public readonly Type Field;
  public StructName(Type val) => Field = val;
}
- Fields set only at creation
- No changes allowed after
- Helps make immutable value types

Full Transcript

A readonly struct in C# is a special kind of struct where all fields or properties are immutable after creation. You define it with the readonly keyword before struct. When you create an instance, you set the values using a constructor. After that, any attempt to change the fields causes a compile error. This makes the struct safe to use without accidental changes. The execution table shows defining the struct, creating an instance with values, reading those values, and then trying to modify a field which fails. The variable tracker confirms the values stay the same throughout. Remember, readonly structs help keep your data safe and predictable.