Computed properties in C Sharp (C#) - Step-by-Step Execution

Concept Flow - Computed properties

Define class with computed property

↓

Create object instance

↓

Access computed property

↓

Run get accessor code

↓

Return computed value

↓

Use value in program

The flow shows how a computed property is defined, accessed, and returns a value calculated on the fly.

Execution Sample

C Sharp (C#)

class Rectangle {
  public int Width { get; set; }
  public int Height { get; set; }
  public int Area => Width * Height;
}

var rect = new Rectangle { Width = 3, Height = 4 };
Console.WriteLine(rect.Area);

This code defines a Rectangle with Width and Height, and a computed property Area that multiplies them. It prints the area.

Execution Table

Step	Action	Variable/Property	Value	Explanation
1	Create Rectangle object	rect	Rectangle instance	Object with Width and Height properties created
2	Set Width	rect.Width	3	Width property set to 3
3	Set Height	rect.Height	4	Height property set to 4
4	Access computed property Area	rect.Area	12	Area getter runs: 3 * 4 = 12
5	Print Area	Output	12	Area value 12 printed to console

💡 All steps complete, program ends after printing computed Area

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
rect.Width	0 (default)	3	3	3	3
rect.Height	0 (default)	0	4	4	4
rect.Area	N/A	N/A	N/A	12	12

Key Moments - 2 Insights

Why does rect.Area not store a value but still returns 12?

Because Area is a computed property with only a get accessor that calculates Width * Height each time it is accessed, it does not store a value but computes it on demand (see execution_table step 4).

What happens if Width or Height changes after accessing Area once?

Area will reflect the new product because it computes the value every time it is accessed, not just once (not shown here but implied by the computed property behavior).

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of rect.Area at step 4?

A 0

B 7

C 12

D Undefined

💡 Hint

Check the 'Value' column for step 4 in execution_table where Area is accessed.

At which step is rect.Height set to 4?

A Step 3

B Step 4

C Step 2

D Step 5

💡 Hint

Look at the 'Action' and 'Variable/Property' columns in execution_table.

If rect.Width was changed to 5 after step 3, what would rect.Area be at step 4?

A 12

B 20

C 7

D 9

💡 Hint

Area computes Width * Height each time; changing Width to 5 and Height is 4 means 5*4=20.

Concept Snapshot

Computed properties in C# use get accessors to calculate values on demand.
Syntax: public int PropertyName => expression;
They do not store values but compute each time accessed.
Useful for values derived from other properties.
Example: Area => Width * Height.

Full Transcript

This example shows a Rectangle class with Width and Height properties and a computed property Area that multiplies them. When we create a Rectangle object and set Width to 3 and Height to 4, accessing Area runs the get accessor which calculates 3 times 4 and returns 12. This value is printed. The computed property does not store the area but calculates it fresh each time it is accessed. This means if Width or Height changes later, Area will reflect the new product automatically.

Practice

1. What is a computed property in C#?
public int Area => Width * Height;

easy

A. A property that calculates its value from other data automatically

B. A property that stores a fixed value in memory

C. A method that returns a value

D. A variable that can be changed anytime

Solution

1. Step 1: Understand the property syntax

   The property uses the lambda syntax with =>, which means it calculates the value when accessed.

2. Step 2: Recognize computed property behavior

   It does not store a value but returns Width * Height each time, so it is computed automatically.

3. Final Answer:

   A property that calculates its value from other data automatically -> Option A

4. Quick Check:

   Computed property = calculates value [OK]

Hint: Look for => syntax to spot computed properties [OK]

Common Mistakes:

• Thinking computed properties store values
• Confusing methods with properties
• Assuming computed properties can be set directly

2. Which of the following is the correct syntax for a read-only computed property in C#?

easy

A. public int Total() { return Price + Quantity; }

B. public int Total { get { return Price * Quantity; } }

C. public int Total => Price + Quantity { get; set; }

D. public int Total { set { Price = value; } }

Solution

1. Step 1: Identify read-only computed property syntax

   public int Total { get { return Price * Quantity; } } uses a property with only a get accessor returning a calculation, which is correct.

2. Step 2: Check other options for errors

   public int Total { set { Price = value; } } only has set, so not read-only. public int Total => Price + Quantity { get; set; } mixes expression body with get/set incorrectly. public int Total() { return Price + Quantity; } is a method, not a property.

3. Final Answer:

   public int Total { get { return Price * Quantity; } } -> Option B

4. Quick Check:

   Read-only computed property = get only [OK]

Hint: Read-only properties have only get accessor [OK]

Common Mistakes:

• Using set accessor in read-only properties
• Confusing methods with properties
• Incorrect expression body syntax

3. What is the output of this code?

class Box {
  public int Width { get; set; } = 3;
  public int Height { get; set; } = 4;
  public int Area => Width * Height;
}

var box = new Box();
Console.WriteLine(box.Area);

medium

A. 12

B. 7

C. 0

D. Error

Solution

1. Step 1: Identify property values

   Width is 3 and Height is 4 as set by default.

2. Step 2: Calculate computed property Area

   Area returns Width * Height = 3 * 4 = 12.

3. Final Answer:

   12 -> Option A

4. Quick Check:

   3 * 4 = 12 [OK]

Hint: Multiply Width and Height for Area [OK]

Common Mistakes:

• Adding instead of multiplying
• Assuming default values are zero
• Confusing property with method call

4. Find the error in this computed property code:

public class Circle {
  public double Radius { get; set; }
  public double Diameter => Radius * 2;
  public double Circumference { get { return 2 * Math.PI * Diameter; } set { Diameter = value; } }
}

medium

A. Circumference should not use Math.PI

B. Radius property is missing get accessor

C. Computed property Diameter cannot be assigned in set accessor

D. No error, code is correct

Solution

1. Step 1: Check computed property Diameter

   Diameter is read-only computed property with only get accessor (=>). It cannot be assigned a value.

2. Step 2: Analyze Circumference set accessor

   Circumference tries to set Diameter, which is not allowed because Diameter has no set accessor.

3. Final Answer:

   Computed property Diameter cannot be assigned in set accessor -> Option C

4. Quick Check:

   Read-only property cannot be set [OK]

Hint: Computed properties with => are read-only [OK]

Common Mistakes:

• Trying to set read-only computed properties
• Ignoring property accessors
• Confusing methods with properties

5. You want to create a computed property IsAdult in a Person class that returns true if Age is 18 or more, otherwise false. Which code correctly implements this?

hard

A. public bool IsAdult() { return Age >= 18; }

B. public bool IsAdult { get { return Age > 18; } }

C. public bool IsAdult { get; set; } = Age >= 18;

D. public bool IsAdult => Age >= 18;

Solution

1. Step 1: Understand requirement for computed property

   IsAdult should return true if Age is 18 or more, false otherwise, without storing a value.

2. Step 2: Check each option

   public bool IsAdult => Age >= 18; uses expression-bodied property correctly with >= 18. public bool IsAdult { get { return Age > 18; } } uses > 18 (wrong condition). public bool IsAdult { get; set; } = Age >= 18; tries to set property with Age comparison, which is invalid. public bool IsAdult() { return Age >= 18; } is a method, not a property.

3. Final Answer:

   public bool IsAdult => Age >= 18; -> Option D

4. Quick Check:

   Age >= 18 for IsAdult [OK]

Hint: Use => with condition for simple computed properties [OK]

Common Mistakes:

• Using > instead of >= for age check
• Trying to set computed property value
• Confusing methods with properties