What is a Read-Only Property in C#?
💡 Concept: Read-Only Property
A read-only property in C# allows data to be exposed via a get accessor but restricts modification by omitting the set accessor. The value is set only once—typically in a constructor or initializer.
📘 Quick Intro
Use a read-only property when a value should be publicly accessible but not changeable after initialization. It is useful for exposing immutable state while protecting the integrity of your object.
🧠 Analogy
Think of a read-only property like your date of birth on an ID card. It can be read by anyone, but no one can change it. It’s fixed once and visible forever, ensuring the record stays accurate and trusted.
🔧 Technical Explanation
- 🔹 Declared using only the
getaccessor. - 🔹 Commonly initialized via constructors or inline property initializers.
- 🔹 Protects data from accidental external modification.
- 🔹 Useful in immutable types and domain models.
- 🔹 Can be combined with the
initkeyword (C# 9+) for initialization-only setting.
🎯 Use Cases
- ✅ Use in DTOs and ViewModels for non-editable properties.
- ✅ Use in configuration or environment objects that should not be altered.
- ✅ Use in immutable classes where state is set once at creation.
- ✅ Use when designing APIs where you want consumers to read but not write.
💻 Code Example
public class Student
{
public string Name { get; }
public DateTime EnrollmentDate { get; } = DateTime.Now;
public Student(string name)
{
Name = name;
}
}
❓ Interview Q&A
Q1: What is a read-only property?
A: A property that has only a getter and no setter, meaning it cannot be modified externally.
Q2: How is a read-only property set?
A: Usually through the constructor or inline initialization.
Q3: Is a read-only property immutable?
A: Yes, once initialized, its value cannot be changed.
Q4: Can a read-only property be set outside the constructor?
A: Only if using init or inline initialization, but not from external code directly.
Q5: What happens if you try to set a read-only property?
A: It results in a compile-time error.
Q6: Can you use private setters to simulate read-only access?
A: Yes, a private setter allows internal modification but not external.
Q7: Are read-only properties thread-safe?
A: They can be considered thread-safe if set only once at construction.
Q8: What C# version introduced init-only properties?
A: C# 9.0.
Q9: Can records use read-only properties?
A: Yes, records favor immutability and often use them.
Q10: Can you override a read-only property?
A: Yes, if the base property is declared as virtual.
📝 MCQs
Q1. What does a read-only property lack?
- Get accessor
- Constructor
- Set accessor
- Backing field
Q2. How is a read-only property commonly initialized?
- In Main()
- Via constructor
- Via event
- Via delegate
Q3. Can you write to a read-only property externally?
- Yes
- No
- Sometimes
- Only via lambda
Q4. What C# version introduced 'init' keyword?
- C# 6
- C# 7
- C# 8
- C# 9
Q5. What is a benefit of read-only properties?
- Performance
- Data integrity
- Fewer references
- Serialization speed
Q6. Can a read-only property be virtual?
- No
- Yes
- Only in sealed class
- Only with abstract class
Q7. Is a backing field auto-generated?
- No
- Yes
- Only in static
- Only in structs
Q8. Can a read-only property use 'init'?
- No
- Yes
- Only in properties
- Only in delegates
Q9. Where is a read-only property most useful?
- Static classes
- Partial classes
- Immutable types
- Private members
Q10. Can read-only properties be initialized inline?
- No
- Yes
- Only in methods
- Only in constructors
💡 Bonus Insight
Read-only properties are powerful tools for enforcing immutability and protecting data integrity. Combined with init, they provide a flexible yet safe way to initialize objects in a predictable, secure manner.
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