What is difference between const and readonly?

Q: What is const in C#?

const is a compile-time constant.

Q: What is readonly in C#?

readonly is a runtime constant assigned once.

💡 Concept: const vs readonly in C#

const and readonly are keywords used to define immutable values, but differ in when and how their values are assigned and used.

📘 Quick Intro

const is a compile-time constant; readonly is a runtime constant assigned once, typically in constructor.

🧠 Analogy

Think of const as a tattoo—permanent and unchangeable; readonly as a signed contract—set once and fixed afterward.

🔧 Technical Explanation

const values are embedded at compile time; readonly values assigned at runtime.
const must be assigned where declared; readonly can be assigned in constructor.
const is static by default; readonly can be instance-level or static.
const only supports primitive types and strings.
readonly can hold reference types and complex objects.

🎯 Use Cases

✅ Use const for fixed values known at compile time.
✅ Use readonly for values assigned once during object construction.
✅ Use readonly for complex types needing runtime initialization.
✅ Use const for performance benefits in embedded values.

💻 Code Example


public class Example {
    public const double Pi = 3.1415;
    public readonly int MaxValue;

    public Example(int max) {
        MaxValue = max;
    }
}

❓ Interview Q&A

Q1: What is the difference between const and readonly?
A: const is compile-time, readonly is runtime.

Q2: Can readonly be assigned multiple times?
A: No, only once in constructor or declaration.

Q3: Are const values static?
A: Yes, implicitly static.

Q4: Can readonly hold complex objects?
A: Yes.

Q5: Can const be changed at runtime?
A: No.

Q6: Is readonly slower than const?
A: Slightly, due to runtime assignment.

Q7: Can const be instance level?
A: No, always static.

Q8: Can readonly be static?
A: Yes.

Q9: When to prefer readonly?
A: When value known only at runtime.

Q10: Are const and readonly thread-safe?
A: Yes, inherently immutable.

📝 MCQs

Q1. What is const in C#?

Runtime constant
Compile-time constant
Variable
Property

Q2. What is readonly in C#?

Runtime constant
Compile-time constant
Variable
Method

Q3. Can readonly be assigned multiple times?

Yes
No
Sometimes
Always

Q4. Are const values static?

No
Yes
Maybe
Sometimes

Q5. Can readonly hold complex objects?

No
Yes
Maybe
Sometimes

Q6. Can const be changed at runtime?

Yes
No
Sometimes
Rarely

Q7. Is readonly slower than const?

Faster
Slightly
Same
No

Q8. Can const be instance level?

Yes
No
Maybe
Sometimes

Q9. Can readonly be static?

No
Yes
Sometimes
Never

Q10. Are const and readonly thread-safe?

No
Yes
Sometimes
Never

💡 Bonus Insight

Understanding const vs readonly helps write safer and more predictable C# code.

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