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Home Forums What is the difference between readonly, const, and static? – MindStick
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1 Answers

Ponu Maurya
Ponu Maurya 24 Jun 2025
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const

  • Compile-time constant.
  • Must be assigned at the time of declaration.
  • Implicitly static.
  • Cannot be changed after compilation.
  • Value is inlined wherever it's used — changing it requires recompilation of all dependent assemblies.
public class Example
{
    public const double Pi = 3.14159;  // Compile-time constant
}

Use when: Value is truly constant and known at compile time (like Pi, conversion factors, etc.)

readonly

  • Run-time constant.
  • Can be assigned in the declaration or constructor.
  • Value can differ between instances.
  • Ideal for values that should not change after the object is created.
public class Example
{
    public readonly int Id;

    public Example(int id)
    {
        Id = id;  // Allowed in constructor
    }
}

Use when: Value should be set once at runtime (e.g., ID, creation timestamp).

static

  • Belongs to the type itself, not to any instance.
  • Shared across all instances.
  • Can be used with fields, methods, constructors, classes, etc.
public class Example
{
    public static int Counter = 0;

    public Example()
    {
        Counter++;  // Shared by all instances
    }
}

Use when: You want a member that is shared across all instances (like configuration, counters, etc.)

Combined Usage

You can combine static with readonly:

public static readonly string AppName = "MyApp";

Means: shared across all instances, value set at runtime, but not modifiable afterward.

Summary Table

Keyword Assigned When Modifiable? Scope Use For
const Compile-time No Static only Hardcoded values
readonly Runtime (in ctor) After assigned Per-instance or static Init-once values
static Any time Yes (if not readonly) Type-wide Shared data/methods across class
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