Computed properties in C Sharp (C#) - Time & Space Complexity

Time Complexity: Computed properties

O(n)

Understanding Time Complexity

Let's see how the time it takes to get a computed property changes as the data grows.

We want to know how the cost of calculating a property depends on the size of the input.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

public class Numbers
{
    public int[] Values { get; set; }

    public int Sum => Values.Sum();
}

This code defines a computed property Sum that calculates the total of all numbers in an array.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Summing all elements in the array inside the computed property.
• How many times: Once each time the Sum property is accessed, it loops through all elements.

How Execution Grows With Input

When the array has more numbers, the sum takes longer because it adds each number one by one.

Input Size (n)	Approx. Operations
10	10 additions
100	100 additions
1000	1000 additions

Pattern observation: The work grows directly with the number of items; double the items, double the work.

Final Time Complexity

Time Complexity: O(n)

This means the time to get the computed sum grows in a straight line with the number of elements.

Common Mistake

[X] Wrong: "Accessing the computed property is instant no matter the array size."

[OK] Correct: Each time you ask for the sum, the program adds up all numbers again, so bigger arrays take more time.

Interview Connect

Understanding how computed properties work helps you explain performance in real code and shows you think about efficiency clearly.

Self-Check

"What if we stored the sum once and updated it only when the array changes? How would the time complexity change when accessing the sum?"

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