What are the benefits to marking a field as `readonly` in C#?

Question

What are the benefits of having a member variable declared as read only? Is it just protecting against someone changing its value during the lifecycle of the class or does using this keyword result in any speed or efficiency improvements?

Answer 1

I don't believe there are any performance gains from using a readonly field. It's simply a check to ensure that once the object is fully constructed, that field cannot be pointed to a new value.

However "readonly" is very different from other types of read-only semantics because it's enforced at runtime by the CLR. The readonly keyword compiles down to .initonly which is verifiable by the CLR.

The real advantage of this keyword is to generate immutable data structures. Immutable data structures by definition cannot be changed once constructed. This makes it very easy to reason about the behavior of a structure at runtime. For instance, there is no danger of passing an immutable structure to another random portion of code. They can't changed it ever so you can program reliably against that structure.

Here is a good entry about one of the benefits of immutability: Threading

Answer 2

The readonly keyword is used to declare a member variable a constant, but allows the value to be calculated at runtime. This differs from a constant declared with the const modifier, which must have its value set at compile time. Using readonly you can set the value of the field either in the declaration, or in the constructor of the object that the field is a member of.

Also use it if you don't want to have to recompile external DLLs that reference the constant (since it gets replaced at compile time).

Answer 3

There are no apparent performance benefits to using readonly, at least none that I've ever seen mentioned anywhere. It's just for doing exactly as you suggest, for preventing modification once it has been initialised.

So it's beneficial in that it helps you write more robust, more readable code. The real benefit of things like this come when you're working in a team or for maintenance. Declaring something as readonly is akin to putting a contract for that variable's usage in the code. Think of it as adding documentation in the same way as other keywords like internal or private, you're saying "this variable should not be modified after initialisation", and moreover you're enforcing it.

So if you create a class and mark some member variables readonly by design, then you prevent yourself or another team member making a mistake later on when they're expanding upon or modifying your class. In my opinion, that's a benefit worth having (at the small expense of extra language complexity as doofledorfer mentions in the comments).

Answer 4

To put it in very practical terms:

If you use a const in dll A and dll B references that const, the value of that const will be compiled into dll B. If you redeploy dll A with a new value for that const, dll B will still be using the original value.

If you use a readonly in dll A and dll B references that readonly, that readonly will always be looked up at runtime. This means if you redeploy dll A with a new value for that readonly, dll B will use that new value.

Answer 5

There is a potential case where the compiler can make a performance optimization based on the presence of the readonly keyword.

This only applies if the read-only field is also marked as static. In that case, the JIT compiler can assume that this static field will never change. The JIT compiler can take this into account when compiling the methods of the class.

A typical example: your class could have a static read-only IsDebugLoggingEnabled field that is initialized in the constructor (e.g. based on a configuration file). Once the actual methods are JIT-compiled, the compiler may omit whole parts of the code when debug logging is not enabled.

I have not checked if this optimization is actually implemented in the current version of the JIT compiler, so this is just speculation.

Answer 6

Keep in mind that readonly only applies to the value itself, so if you're using a reference type readonly only protects the reference from being change. The state of the instance is not protected by readonly.

Answer 7

Don't forget there is a workaround to get the readonly fields set outside of any constructors using out params.

A little messy but:

private readonly int _someNumber;
private readonly string _someText;

public MyClass(int someNumber) : this(data, null)
{ }

public MyClass(int someNumber, string someText)
{
    Initialise(out _someNumber, someNumber, out _someText, someText);
}

private void Initialise(out int _someNumber, int someNumber, out string _someText, string someText)
{
    //some logic
}

Further discussion here: http://www.adamjamesnaylor.com/2013/01/23/Setting-Readonly-Fields-From-Chained-Constructors.aspx

Answer 8

Surprisingly, readonly can actually result in slower code, as Jon Skeet found when testing his Noda Time library. In this case, a test that ran in 20 seconds took only 4 seconds after removing readonly.

https://codeblog.jonskeet.uk/2014/07/16/micro-optimization-the-surprising-inefficiency-of-readonly-fields/

Answer 9

Be careful with private readonly arrays. If these are exposed a client as an object (you might do this for COM interop as I did) the client can manipulate array values. Use the Clone() method when returning an array as an object.

Answer 10

Another interesting part of usage of readonly marking can be protecting field from initialization in singleton.

for example in code from csharpindepth:

public sealed class Singleton
{
    private static readonly Lazy<Singleton> lazy =
        new Lazy<Singleton>(() => new Singleton());

    public static Singleton Instance { get { return lazy.Value; } }

    private Singleton()
    {
    }
}

readonly plays small role of protecting field Singleton from being initialized twice. Another detail is that for mentioned scenario you can't use const because const forces creation during compile time, but singleton makes creation at run time.

Answer 11

If you have a pre defined or pre calculated value that needs to remain same through out the program then you should use constant but if you have a value that needs to be provided at the runtime but once assigned should remain same throughout the program u should use readonly. for example if you have to assign the program start time or you have to store a user provided value at the object initialization and you have to restrict it from further changes you should use readonly.

Answer 12

readonly can be initialized at declaration or get its value from the constructor only. Unlike const it has to be initialized and declare at the same time. readonly has everything const has, plus constructor initialization

code https://repl.it/HvRU/1

using System;

class MainClass {
    public static void Main (string[] args) {

        Console.WriteLine(new Test().c);
        Console.WriteLine(new Test("Constructor").c);
        Console.WriteLine(new Test().ChangeC()); //Error A readonly field 
        // `MainClass.Test.c' cannot be assigned to (except in a constructor or a 
        // variable initializer)
    }


    public class Test {
        public readonly string c = "Hello World";
        public Test() {

        }

        public Test(string val) {
          c = val;
        }

        public string ChangeC() {
            c = "Method";
            return c ;
        }
    }
}

Answer 13

Adding a basic aspect to answer this question:

Properties can be expressed as readonly by leaving out the set operator. So in most cases you will not need to add the readonly keyword to properties:

public int Foo { get; }  // a readonly property

In contrast to that: Fields need the readonly keyword to achieve a similar effect:

public readonly int Foo; // a readonly field

So, one benefit of marking a field as readonly can be to achieve a similar write protection level as a property without set operator - without having to change the field to a property, if for any reason, that is desired.

Answer 14

There can be a performance benefit in WPF, as it removes the need for expensive DependencyProperties. This can be especially useful with collections