Computed properties in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a computed property in C#?

A computed property is a property that calculates its value dynamically using code in its getter, instead of storing a value directly.

beginner

How do you define a computed property in C#?

You define a computed property by providing only a getter with code that returns a calculated value, without a backing field.

beginner

Example: What does this computed property do?

public int DoubleAge => Age * 2;

This property returns twice the value of the Age property whenever accessed. It does not store a value but computes it on the fly.

intermediate

Can computed properties have setters in C#?

No, computed properties usually only have getters because their value is calculated. If you add a setter, it must store a value somewhere.

intermediate

Why use computed properties instead of fields?

Computed properties keep data consistent by calculating values on demand, avoiding errors from storing outdated values.

What keyword is used to define a computed property with a single expression in C#?

A var

B get

C set

D =>

The '=>' symbol defines an expression-bodied member, often used for computed properties.

Which of these is true about computed properties in C#?

A They store values in private fields.

B They calculate values when accessed.

C They must have both get and set.

D They cannot use other properties.

Computed properties calculate and return values dynamically when accessed.

What happens if you try to assign a value to a computed property with only a getter?

A It compiles and stores the value.

B It throws a runtime error.

C It causes a compile-time error.

D It ignores the assignment.

Computed properties with only getters cannot be assigned to; this causes a compile-time error.

Which is a benefit of using computed properties?

A They reduce memory by not storing redundant data.

B They make code slower.

C They require manual updates.

D They prevent any calculations.

Computed properties save memory by calculating values on demand instead of storing them.

How do you write a computed property that returns the length of a string property 'Name'?

A public int NameLength => Name.Length;

B public int NameLength { get; set; }

C public int NameLength = Name.Length;

D public int NameLength() { return Name.Length; }

Option A uses an expression-bodied computed property to return the length of 'Name'.

Explain what a computed property is and why it is useful in C#.

Think about properties that calculate values instead of storing them.

Write a simple computed property in C# that returns the full name by combining first and last names.

Use string interpolation or concatenation inside the getter.

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