C#: What is the difference between const and readonly?

const — compile-time. readonly — runtime (immutable after init).

const:

• Compile-time.
• Static.
• Literal only.

readonly:

• Runtime (constructor).
• Instance or static.
• Can be computed.

1// const
2public const int MaxSize = 100;
3
4// readonly
5public readonly DateTime CreatedAt;
6
7public MyClass()
8{
9    CreatedAt = DateTime.UtcNow;
10}

Key: const for compile-time, readonly for initialization.