How do the keywords const and readonly differ in programming, and in which cases is their use preferable?

In C#, the keyword const is used to declare constants whose values are known at compile time and cannot be changed during execution. Such values should be primitive or literal and initialized immediately.

readonly is used for fields whose values can only be set at declaration or in the class constructor. After initialization, the value cannot be changed, but it can be computed at runtime.

When to use:

• const — for immutable values known at compile time (e.g., numbers, strings).
• readonly — when the value is only known at runtime, for example, depending on constructor parameters.

Example:

public class Example {
    public const double Pi = 3.1415; // known at compile time
    public readonly DateTime CreatedAt;

    public Example() {
        CreatedAt = DateTime.Now; // initialized at runtime
    }
}