What is the difference between readonly, const, and static?

Question

Ponu Maurya · Answer

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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What is the difference between readonly, const, and static?

Anubhav Kumar

Can you answer this question?

1 Answers

`const`

`readonly`

`static`

Combined Usage

Summary Table

Liked By

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