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.

Practice

1.

What is the main purpose of declaring a struct as readonly in C#?

easy

A. To allow inheritance from the struct

B. To make the struct mutable

C. To enable dynamic resizing of the struct

D. To prevent modification of its fields after creation

Solution

1. Step 1: Understand what readonly means for structs

   A readonly struct means its fields cannot be changed after the struct is created.

2. Step 2: Compare options with this meaning

   Only To prevent modification of its fields after creation matches this behavior by preventing modification after creation.

3. Final Answer:

   To prevent modification of its fields after creation -> Option D

4. Quick Check:

   Readonly struct = Immutable fields [OK]

Hint: Readonly structs make data unchangeable after creation [OK]

Common Mistakes:

• Thinking readonly allows inheritance
• Confusing readonly with mutable structs
• Assuming readonly structs can resize dynamically

2.

Which of the following is the correct way to declare a readonly struct in C#?

?

easy

A. readonly struct Point { public int X; public int Y; }

B. struct readonly Point { public int X; public int Y; }

C. struct Point readonly { public int X; public int Y; }

D. readonly Point struct { public int X; public int Y; }

Solution

1. Step 1: Recall correct syntax for readonly structs

   The keyword readonly comes before struct in the declaration.

2. Step 2: Check each option's keyword order

   Only readonly struct Point { public int X; public int Y; } places readonly before struct correctly.

3. Final Answer:

   readonly struct Point { public int X; public int Y; } -> Option A

4. Quick Check:

   Keyword order: readonly struct [OK]

Hint: Put 'readonly' before 'struct' keyword [OK]

Common Mistakes:

• Placing readonly after struct
• Mixing order of keywords
• Using readonly as a suffix

3.

What will be the output of the following C# code?

readonly struct Point
{
    public int X { get; }
    public int Y { get; }
    public Point(int x, int y) { X = x; Y = y; }
}

var p = new Point(3, 4);
Console.WriteLine($"X: {p.X}, Y: {p.Y}");

medium

A. X: 3, Y: 4

B. X: 0, Y: 0

C. Compilation error due to readonly struct

D. Runtime error when accessing properties

Solution

1. Step 1: Understand readonly struct with get-only properties

   The struct has get-only properties set in the constructor, which is allowed.

2. Step 2: Check the constructor and output

   The constructor sets X=3 and Y=4, so printing them outputs "X: 3, Y: 4".

3. Final Answer:

   X: 3, Y: 4 -> Option A

4. Quick Check:

   Constructor sets values correctly [OK]

Hint: Readonly structs allow get-only properties set in constructor [OK]

Common Mistakes:

• Assuming readonly structs cannot have properties
• Thinking constructor cannot set properties
• Expecting runtime errors accessing properties

4.

Identify the error in this code snippet using a readonly struct:

readonly struct Rectangle
{
    public int Width;
    public int Height;

    public void Resize(int w, int h)
    {
        Width = w;
        Height = h;
    }
}

medium

A. Fields must be properties in readonly structs

B. Missing constructor for readonly struct

C. Cannot assign to fields inside a method of a readonly struct

D. Readonly structs cannot have methods

Solution

1. Step 1: Understand mutability rules in readonly structs

   Readonly structs cannot modify their fields inside instance methods.

2. Step 2: Analyze the Resize method

   Resize tries to assign new values to Width and Height fields, which is not allowed.

3. Final Answer:

   Cannot assign to fields inside a method of a readonly struct -> Option C

4. Quick Check:

   Readonly struct fields are immutable in methods [OK]

Hint: Readonly struct methods cannot change fields [OK]

Common Mistakes:

• Thinking methods can modify fields in readonly structs
• Assuming missing constructor causes error
• Believing readonly structs cannot have any methods

5.

You want to create a readonly struct Circle that stores a radius and calculates area. Which of these implementations is correct and efficient?

?

hard

A. readonly struct Circle { public double Radius; public void SetRadius(double r) { Radius = r; } }

B. readonly struct Circle { public double Radius { get; } public Circle(double r) => Radius = r; public double Area => Math.PI * Radius * Radius; }

C. struct Circle { public double Radius; public double Area => Math.PI * Radius * Radius; }

D. readonly struct Circle { public double Radius { get; set; } public double Area() => Math.PI * Radius * Radius; }

Solution

1. Step 1: Check readonly struct with immutable radius

   readonly struct Circle { public double Radius { get; } public Circle(double r) => Radius = r; public double Area => Math.PI * Radius * Radius; } uses a readonly struct with a get-only property set in constructor, which is correct.

2. Step 2: Verify area calculation and mutability

   Area is a property calculated from Radius, no setters modify Radius, ensuring immutability and efficiency.

3. Final Answer:

   readonly struct Circle { public double Radius { get; } public Circle(double r) => Radius = r; public double Area => Math.PI * Radius * Radius; } -> Option B

4. Quick Check:

   Immutable radius with calculated area [OK]

Hint: Use get-only property and constructor for readonly struct fields [OK]

Common Mistakes:

• Trying to set fields in readonly struct after creation
• Using mutable fields in readonly structs
• Adding setters that break immutability