const vs readonly

const vs readonly

const

Definition: Declares a constant field that is initialized at compile time and cannot be changed after compilation.

Example:

public const double Pi = 3.14159;

Use Cases:

For values that never change throughout the application's lifetime (e.g., mathematical constants like Pi, e, or fixed version numbers).

Configuration values that are known at compile time and do not change during execution.

Key Characteristics:

Compile-time initialization.

Immutable (value can't be changed).

Typically replaced with the value directly by the compiler, which can improve performance.

Less flexible (must be initialized with a constant value).

 

readonly

Definition: Declares a field that can only be assigned a value during its declaration or within the constructor of the class.

Example:

public class MyClass

{

    public readonly string Name;

 

    public MyClass(string name)

    {

        Name = name;

    }

}

Use Cases:

For values that should be immutable after object creation but may be determined at runtime (e.g., initial object state, values read from a file or database).

Immutable objects: Ensures the field's value is not changed after initialization.

Suitable for both instance and static fields.

·       Key Characteristics:

o   Runtime initialization (via constructor or declaration).

o   Immutable after initialization.

o   More flexible as values can be computed at runtime (e.g., based on user input or external data).

Key Differences

Feature

const

readonly

Initialization

Compile time only

Compile time or constructor

Mutability

Immutable at compile time

Immutable after object creation

Memory

Compiler replaces with value directly

Stored in memory

Flexibility

Less flexible (must be compile-time)

More flexible (allows runtime values)

When to Use Each

·       Use const when:

o   The value is known at compile time (e.g., mathematical constants like Pi, static configuration values).

o   The value will never change during the app's lifetime.

o   Performance is critical (since constants can be directly replaced at compile-time, potentially improving performance).

·       Use readonly when:

o   The value is determined at runtime or needs to be set in a constructor.

o   You need to ensure that the value remains immutable after initialization, but it can vary across different objects or based on external factors like configuration files.

o   You need a runtime-computed value that can't be modified once the object is created.

Example Scenario: Physics Simulation

const: Use for values like the speed of light (c) or the gravitational constant (G), which are fixed, well-known, and determined at compile time.

readonly: Use for values like an object's initial position in a simulation, which could depend on user input or be calculated at runtime when the object is created.

 


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