C#•6d ago

✅ readonly keyword vs const keyword and use-case

Hello guys, I just read a bit about the readonly keywords. From what I've read, using the readonly keyword, we can only assign a particular value to a variable either at declaration level or when using the constructor. In constrast with the const keyword, I think we can only assign a value to it in the declaration part. I read that using const means the value is determined at compile-time itself while using readonly means value is determined at run-time. My first question concerning the readonly keyword: When we declare a property, consider the following:

C#
public struct Customer 
{ 
  
    public string Name { get; } 
    public int Price { get; } 
  
    // Readonly member 
    public readonly string Product { get; } 
  
    public Customer(string name, string product, int price) 
    { 
  
        this.Name = name; 
        this.Product = product; 
        this.Price = price; 
    } 
}

C#
public struct Customer 
{ 
  
    public string Name { get; } 
    public int Price { get; } 
  
    // Readonly member 
    public readonly string Product { get; } 
  
    public Customer(string name, string product, int price) 
    { 
  
        this.Name = name; 
        this.Product = product; 
        this.Price = price; 
    } 
}

My question is, if we omit the readonly keyword and just use { get; } without the set keyword, does the property behave as readonly? Second question is, what are the use cases of readonly vs const keyword please

20 Replies

𓀠𓀒𓀡𓀬•6d ago

Yes caps

ero•6d ago

readonly is not valid for properties

FakerOP•6d ago

yeah just read that we use readonly for fields while {get ; } for properties which are "read-only" ?

𓀠𓀒𓀡𓀬•6d ago

{ get; } is readonly

ero•6d ago

basically

FakerOP•6d ago

yep I see what about the use cases of readonly fields vs const fields please

𓀠𓀒𓀡𓀬•6d ago

readonly is runtime and not static const is compile time and static you have to use static readonly to make it static

Pobiega•6d ago

const behaves like static readonly, but as said, its a compile time constant if a value never changes and is known at compile time, use const if you can you can't have const arrays etc, so there are cases where static readonly is needed non-static readonly is almost only used for stuff passed in via the constructor

ero•6d ago

well, unless it's a null array x3

MODiX•6d ago

ero

REPL Result: Success

const int[] arr = null;

const int[] arr = null;

Compile: 194.036ms | Execution: 16.977ms | React with ❌ to remove this embed.

Pobiega•6d ago

I have never needed one of those :p

FakerOP•6d ago

yep I see Noted

ero•6d ago

(but this is useless and you shouldn't really use it...)

FakerOP•6d ago

So to summarise: If we know a value is going to be constant at compile itme, use const else use readonly. The difference is that const is for the entire class itself (static readonly) while readonly can differs for each instances. Now, const can only be use for value type (well I guess there are exceptions like the null array) so if we want to make a reference type "constant"; we need to use readonly keyword in this case?

Pobiega•6d ago

yeah, and the value of the reference type can still be mutated readonly only applies to the variable itself, not the thing it contains

FakerOP•6d ago

yep I see

ero•6d ago

for example you could make collections readonly a lot of the time

class C
{
  private readonly List<int> _someList = [];
}

class C
{
  private readonly List<int> _someList = [];
}

each instance of C would have a new _someList but the list will never need to be re-assigned

FakerOP•6d ago

yep, but we can add items to it though

ero•6d ago

you can just call .Clear() to empty the values of the list or add and remove, exactly

FakerOP•6d ago

yep 💯 It clearer now, thanks guys, really appreciate

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✅ readonly keyword vs const keyword and use-case

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