By Readonly

2008-09-11 08:02:19 8 Comments

What is the difference between const and readonly in C#?

When would you use one over the other?


@Ryan Efendy 2019-06-04 20:09:07

  • when to use const or readonly

    • const

      • compile-time constant: absolute constant, value is set during declaration, is in the IL code itself
    • readonly

      • run-time constant: can be set in the constructor/init via config file i.e. App.config, but once it initializes it can't be changed

@Bigeyes 2019-04-29 20:26:40

Const: Absolute constant value during the application life time.

Readonly: It can be changed in running time.

@Ahmed 2019-12-26 05:48:07

Your definition of 'Readonly' that it can change is flawed. I guess by 'change' you meant 'set', like 'it can be set at runtime'.

@Salman Farsi 2019-01-17 16:27:57

In simple words Const is compile time and readonly is run time.

@Muhammad VP 2018-09-08 06:39:10


  1. const keyword can be applied to fields or local variables
  2. We must assign const field at time of declaration
  3. No Memory Allocated Because const value is embedded in IL code itself after compilation. It is like find all occurrences of const variable and replace by its value. So the IL code after compilation will have hard-coded values in place of const variables
  4. Const in C# are by default static.
  5. The value is constant for all objects
  6. There is dll versioning issue - This means that whenever we change a public const variable or property , (In fact, it is not supposed to be changed theoretically), any other dll or assembly which uses this variable has to be re-built
  7. Only C# built-in types can be declared as constant
  8. Const field can not be passed as ref or out parameter


  1. readonly keyword applies only to fields not local variables
  2. We can assign readonly field at the time of declaration or in constructor,not in any other methods.
  3. dynamic memory allocated for readonly fields and we can get the value at run time.
  4. Readonly belongs to the object created so accessed through only instance of class. To make it class member we need to add static keyword before readonly.
  5. The value may be different depending upon constructor used (as it belongs to object of the class)
  6. If you declare a non-primitive types (reference type) as readonly only reference is immutable not the object it contains.
  7. Since the value is obtained at run time, there is no dll versioning problem with readonly fields/ properties.
  8. We can pass readonly field as ref or out parameters in the constructor context.

@Sujit 2012-09-17 11:48:03

A constant member is defined at compile time and cannot be changed at runtime. Constants are declared as a field, using the const keyword and must be initialized as they are declared.

public class MyClass
    public const double PI1 = 3.14159;

A readonly member is like a constant in that it represents an unchanging value. The difference is that a readonly member can be initialized at runtime, in a constructor, as well being able to be initialized as they are declared.

public class MyClass1
     public readonly double PI2 = 3.14159;


     public readonly double PI3;

     public MyClass2()
         PI3 = 3.14159;


  • They can not be declared as static (they are implicitly static)
  • The value of constant is evaluated at compile time
  • constants are initialized at declaration only


  • They can be either instance-level or static
  • The value is evaluated at run time
  • readonly can be initialized in declaration or by code in the constructor

@nawfal 2013-12-13 20:21:44

They Can not be static, they are static. You should make it clear if you meant one can't declare static const int i = 0;

@Minh Tran 2017-10-29 17:38:16

Can you explain why const declarations can't be made inside methods?

@Yeasin Abedin Siam 2016-01-12 17:42:57

Read Only : Value can be changed through Ctor at runtime. But not through member Function

Constant : By defult static. Value cannot be changed from anywhere ( Ctor, Function, runtime etc no-where)

@Don Cheadle 2018-02-26 19:11:06

thanks for not making me read 4 paragraphs just for these two take-aways...

@Hallgrim 2008-09-17 13:02:06

They are both constant, but a const is available also at compile time. This means that one aspect of the difference is that you can use const variables as input to attribute constructors, but not readonly variables.


public static class Text {
  public const string ConstDescription = "This can be used.";
  public readonly static string ReadonlyDescription = "Cannot be used.";

public class Foo 
  public int BarThatBuilds {
    { get; set; }

  public int BarThatDoesNotBuild {
    { get; set; }

@Gishu 2008-09-11 08:24:41

Apart from the apparent difference of

  • having to declare the value at the time of a definition for a const VS readonly values can be computed dynamically but need to be assigned before the constructor exits.. after that it is frozen.
  • 'const's are implicitly static. You use a ClassName.ConstantName notation to access them.

There is a subtle difference. Consider a class defined in AssemblyA.

public class Const_V_Readonly
  public const int I_CONST_VALUE = 2;
  public readonly int I_RO_VALUE;
  public Const_V_Readonly()
     I_RO_VALUE = 3;

AssemblyB references AssemblyA and uses these values in code. When this is compiled,

  • in the case of the const value, it is like a find-replace, the value 2 is 'baked into' the AssemblyB's IL. This means that if tomorrow I'll update I_CONST_VALUE to 20 in the future. AssemblyB would still have 2 till I recompile it.
  • in the case of the readonly value, it is like a ref to a memory location. The value is not baked into AssemblyB's IL. This means that if the memory location is updated, AssemblyB gets the new value without recompilation. So if I_RO_VALUE is updated to 30, you only need to build AssemblyA. All clients do not need to be recompiled.

So if you are confident that the value of the constant won't change use a const.

public const int CM_IN_A_METER = 100;

But if you have a constant that may change (e.g. w.r.t. precision).. or when in doubt, use a readonly.

public readonly float PI = 3.14;

Update: Aku needs to get a mention coz he pointed this out first. Also I need to plug where I learned this.. Effective C# - Bill Wagner

@LCJ 2013-01-21 14:00:51

The static point seems to be the most important and useful point - consts are implicitly static

@CodingBarfield 2013-06-05 07:03:30

The part about reference values is the most important one. Const values can be optimized away.

@Bitterblue 2013-06-05 14:10:34

readonly variables can be changed outside the constructor (reflection). It's only the compiler that tries to hinder you from modifying the var outside the constructor.

@user743382 2014-04-14 11:55:40

@mini-me readonly variables are not allowed to be changed once the constructor has finished, even via reflection. The runtime happens to not enforce this. The runtime also happens not to enforce that you don't change string.Empty to "Hello, world!", but I still wouldn't claim that this makes string.Empty modifiable, or that code shouldn't assume that string.Empty will always be a zero-length string.

@user743382 2014-04-14 13:37:48

@mini-me That doesn't contradict what I said. They're not allowed to be changed, the CIL spec explicitly forbids it, but the runtime doesn't enforce it, and the CIL spec doesn't require (but does allow) the runtime to enforce it. Code modifying readonly fields is an active time bomb, just waiting to blow up when the runtime does start enforcing this.

@CAD bloke 2014-08-05 21:52:55… is an interesting read only the overhead cost of readonly

@dragan.stepanovic 2015-08-18 10:38:16

This is the same case as with method's default arguments. As with constants, their default value is embedded in the client's call, and if you change the value of the default argument in the method definition and recompile just that assembly, client assemblies will still have the old value.

@Nomi Ali 2017-03-23 09:11:34

what you mean by "BAKED", if you mean to say addition reference of AssemblyB into AssemblyA? if it is the case than const getting updated value.

@Gishu 2017-03-28 05:06:12

@NomiAli by Baked, i mean the value is embedded at each call site in the client code.. vs a reference to the value.

@Don Cheadle 2018-02-26 19:13:18

Read Only : Value can be changed through Ctor at runtime. But not through member Function Constant : By defult static. Value cannot be changed from anywhere ( Ctor, Function, runtime etc no-where)`

@Mikhail 2019-02-11 14:19:26

"having to declare the value at the time of a definition for a const VS readonly values can be computed dynamically but need to be assigned before the constructor exits.. after that it is frozen" - can you please simplify this sentence? I read it like 5 times, but I still don't get it :(

@Vikram 2015-08-04 05:34:27


We need to provide the value to the const field when it is defined. The compiler then saves the constant’s value in the assembly’s metadata. This means that a constant can be defined only for the primitive type like boolean, char, byte and so on. Constants are always considered static members, not instance members.


Readonly fields can only be resolved at runtime. That means we can define a value for a value using the constructor for the type in which the field is declared. The verification is done by the compiler that readonly fields are not written to by any method other than the constructor.

More about both explained here in this article

@Omar AMEZOUG 2015-04-09 21:09:25

Constant variables are declared and initialized at compile time. The value can’t be changed after wards. Read-only variables will be initialized only from the Static constructor of the class. Read only is used only when we want to assign the value at run time.

@Greg 2009-10-13 02:26:55

Yet another gotcha: readonly values can be changed by "devious" code via reflection.

var fi = this.GetType()
                       BindingFlags.Instance | BindingFlags.NonPublic);
fi.SetValue(this, 1);

Can I change a private readonly inherited field in C# using reflection?

@Chirag 2014-12-15 13:01:10

There is notable difference between const and readonly fields in C#.Net

const is by default static and needs to be initialized with constant value, which can not be modified later on. Change of value is not allowed in constructors, too. It can not be used with all datatypes. For ex- DateTime. It can not be used with DateTime datatype.

public const DateTime dt = DateTime.Today;  //throws compilation error
public const string Name = string.Empty;    //throws compilation error
public readonly string Name = string.Empty; //No error, legal

readonly can be declared as static, but not necessary. No need to initialize at the time of declaration. Its value can be assigned or changed using constructor. So, it gives advantage when used as instance class member. Two different instantiation may have different value of readonly field. For ex -

class A
    public readonly int Id;

    public A(int i)
        Id = i;

Then readonly field can be initialised with instant specific values, as follows:

A objOne = new A(5);
A objTwo = new A(10);

Here, instance objOne will have value of readonly field as 5 and objTwo has 10. Which is not possible using const.

@Chris S 2009-01-31 11:42:05

Here's another link demonstrating how const isn't version safe, or relevant for reference types.


  • The value of your const property is set at compile time and can't change at runtime
  • Const can't be marked as static - the keyword denotes they are static, unlike readonly fields which can.
  • Const can't be anything except value (primitive) types
  • The readonly keyword marks the field as unchangeable. However the property can be changed inside the constructor of the class
  • The readonly only keyword can also be combined with static to make it act in the same way as a const (atleast on the surface). There is a marked difference when you look at the IL between the two
  • const fields are marked as "literal" in IL while readonly is "initonly"

@aku 2008-09-11 08:15:15

There is a gotcha with consts! If you reference a constant from another assembly, its value will be compiled right into the calling assembly. That way when you update the constant in the referenced assembly it won't change in the calling assembly!

@springy76 2016-08-19 13:02:01

On decompilation (Reflector, ILSpy, ..) a constant NEVER EVER is referenced by any one, no matter of same assembly or another assembly, so you cannot analyze the usage of a constant in compiled code at all.

@Yonatan Nir 2014-04-05 14:39:18

The difference is that the value of a static readonly field is set at run time, so it can have a different value for different executions of the program. However, the value of a const field is set to a compile time constant.

Remember: For reference types, in both cases (static and instance), the readonly modifier only prevents you from assigning a new reference to the field. It specifically does not make immutable the object pointed to by the reference.

For details, please refer to C# Frequently Asked Questions on this topic:

@donstack 2014-01-24 07:34:17

ReadOnly :The value will be initialized only once from the constructor of the class.
const: can be initialized in any function but only once

@Deepthi 2011-05-21 13:21:27

const: Can't be changed anywhere.

readonly: This value can only be changed in the constructor. Can't be changed in normal functions.

@Ramesh Rajendran 2013-09-28 08:19:52

Const and readonly are similar, but they are not exactly the same. A const field is a compile-time constant, meaning that that value can be computed at compile-time. A readonly field enables additional scenarios in which some code must be run during construction of the type. After construction, a readonly field cannot be changed.

For instance, const members can be used to define members like:

struct Test
    public const double Pi = 3.14;
    public const int Zero = 0;

since values like 3.14 and 0 are compile-time constants. However, consider the case where you define a type and want to provide some pre-fab instances of it. E.g., you might want to define a Color class and provide "constants" for common colors like Black, White, etc. It isn't possible to do this with const members, as the right hand sides are not compile-time constants. One could do this with regular static members:

public class Color
    public static Color Black = new Color(0, 0, 0);
    public static Color White = new Color(255, 255, 255);
    public static Color Red = new Color(255, 0, 0);
    public static Color Green = new Color(0, 255, 0);
    public static Color Blue = new Color(0, 0, 255);
    private byte red, green, blue;

    public Color(byte r, byte g, byte b) {
        red = r;
        green = g;
        blue = b;

but then there is nothing to keep a client of Color from mucking with it, perhaps by swapping the Black and White values. Needless to say, this would cause consternation for other clients of the Color class. The "readonly" feature addresses this scenario. By simply introducing the readonly keyword in the declarations, we preserve the flexible initialization while preventing client code from mucking around.

public class Color
    public static readonly Color Black = new Color(0, 0, 0);
    public static readonly Color White = new Color(255, 255, 255);
    public static readonly Color Red = new Color(255, 0, 0);
    public static readonly Color Green = new Color(0, 255, 0);
    public static readonly Color Blue = new Color(0, 0, 255);
    private byte red, green, blue;

    public Color(byte r, byte g, byte b) {
        red = r;
        green = g;
        blue = b;

It is interesting to note that const members are always static, whereas a readonly member can be either static or not, just like a regular field.

It is possible to use a single keyword for these two purposes, but this leads to either versioning problems or performance problems. Assume for a moment that we used a single keyword for this (const) and a developer wrote:

public class A
    public static const C = 0;

and a different developer wrote code that relied on A:

public class B
    static void Main() {

Now, can the code that is generated rely on the fact that A.C is a compile-time constant? I.e., can the use of A.C simply be replaced by the value 0? If you say "yes" to this, then that means that the developer of A cannot change the way that A.C is initialized -- this ties the hands of the developer of A without permission. If you say "no" to this question then an important optimization is missed. Perhaps the author of A is positive that A.C will always be zero. The use of both const and readonly allows the developer of A to specify the intent. This makes for better versioning behavior and also better performance.

@splattne 2008-12-02 11:50:14


  • Constants are static by default
  • They must have a value at compilation-time (you can have e.g. 3.14 * 2, but cannot call methods)
  • Could be declared within functions
  • Are copied into every assembly that uses them (every assembly gets a local copy of values)
  • Can be used in attributes

Readonly instance fields

  • Must have set value, by the time constructor exits
  • Are evaluated when instance is created

Static readonly fields

  • Are evaluated when code execution hits class reference (when new instance is created or a static method is executed)
  • Must have an evaluated value by the time the static constructor is done
  • It's not recommended to put ThreadStaticAttribute on these (static constructors will be executed in one thread only and will set the value for its thread; all other threads will have this value uninitialized)

@Greg 2011-02-20 12:29:26

A const has to be hard-coded, where as readonly can be set in the constructor of the class.

@Brett Ryan 2009-09-08 12:20:05

A constant will be compiled into the consumer as a literal value while the static string will serve as a reference to the value defined.

As an exercise, try creating an external library and consume it in a console application, then alter the values in the library and recompile it (without recompiling the consumer program), drop the DLL into the directory and run the EXE manually, you should find that the constant string does not change.

@ljs 2009-09-08 12:23:53

I sincerely doubt that is true... I will go check.

@Russ Cam 2009-09-08 12:24:56

this is one of the 50 specific ways to improve your C# -‌​660/…

@Russ Cam 2009-09-08 12:27:32

@ljs 2009-09-08 12:29:18

@Andrew Hare - yes, I just checked. I am very surprised, that is a real gotcha, I'm really very surprised by that, amazed that is the case...!

@ljs 2009-09-08 12:31:23

I do object, however, to the use of the word pointer here. It's not a pointer, it's a reference, and there is a difference in C# as you can manipulate unmanaged pointers in unsafe mode so it's important to distinguish between the two.

@ljs 2009-09-08 12:32:33

Not to mention the value itself is immutable, so you can't change it (well, not without some funky evil underhand manipulation). Anyway :)

@ljs 2009-09-08 12:24:24

Principally; you can assign a value to a static readonly field to a non-constant value at runtime, whereas a const has to be assigned a constant value.

@Wedge 2008-09-11 08:41:51

Variables marked const are little more than strongly typed #define macros, at compile time const variable references are replaced with inline literal values. As a consequence only certain built-in primitive value types can be used in this way. Variables marked readonly can be set, in a constructor, at run-time and their read-only-ness is enforced during run-time as well. There is some minor performance cost associated with this but it means you can use readonly with any type (even reference types).

Also, const variables are inherently static, whereas readonly variables can be instance specific if desired.

@Jason Baker 2009-01-05 22:35:21

Added that consts are strongly typed #define macros. Otherwise, we may scare off all the C or C++ people. :-)

@Mike Two 2008-10-19 22:14:57

There is a small gotcha with readonly. A readonly field can be set multiple times within the constructor(s). Even if the value is set in two different chained constructors it is still allowed.

public class Sample {
    private readonly string ro;

    public Sample() {
        ro = "set";

    public Sample(string value) : this() {
        ro = value; // this works even though it was set in the no-arg ctor

@Scott Lawrence 2008-10-02 20:43:37

One of the team members in our office provided the following guidance on when to use const, static, and readonly:

  • Use const when you have a variable of a type you can know at runtime (string literal, int, double, enums,...) that you want all instances or consumers of a class to have access to where the value should not change.
  • Use static when you have data that you want all instances or consumers of a class to have access to where the value can change.
  • Use static readonly when you have a variable of a type that you cannot know at runtime (objects) that you want all instances or consumers of a class to have access to where the value should not change.
  • Use readonly when you have an instance level variable you will know at the time of object creation that should not change.

One final note: a const field is static, but the inverse is not true.

@Michael Blackburn 2016-04-27 21:02:23

I think you mean "converse." The inverse would be "a non-const field is not static." Which may or may not be true. The converse, "a static field is (always) const" is not true.

@user1333 2008-09-11 10:37:04

Just to add, ReadOnly for reference types only makes the reference readonly not the values. For example:

public class Const_V_Readonly
  public const int I_CONST_VALUE = 2;
  public readonly char[] I_RO_VALUE = new Char[]{'a', 'b', 'c'};

  public UpdateReadonly()
     I_RO_VALUE[0] = 'V'; //perfectly legal and will update the value
     I_RO_VALUE = new char[]{'V'}; //will cause compiler error

@springy76 2016-08-19 13:02:57

Is there any other reference type than string that you could use as a constant?

@Mike Rosoft 2018-04-27 14:28:40

You can have const with reference types other than string, but the constant can only have the value null.

@Mark T 2008-09-17 13:28:01

Another gotcha.

Since const really only works with basic data types, if you want to work with a class, you may feel "forced" to use ReadOnly. However, beware of the trap! ReadOnly means that you can not replace the object with another object (you can't make it refer to another object). But any process that has a reference to the object is free to modify the values inside the object!

So don't be confused into thinking that ReadOnly implies a user can't change things. There is no simple syntax in C# to prevent an instantiation of a class from having its internal values changed (as far as I know).

@Gishu 2008-09-17 13:31:27

Yes that is more of a general theme. If you have a get only property exposing an arraylist, you can still modify the arraylist. You cannot set a different arraylist to that property, but you cannot stop the user from altering the arraylist.

@AlanR 2008-09-15 21:30:19

The key difference is that Const is the C equivalent of #DEFINE. The number literally gets substituted a-la precompiler. Readonly is actually treated as a variable.

This distinction is especially relevant when you have Project A depending on a Public constant from Project B. Suppose the public constant changes. Now your choice of const/readonly will impact the behavior on project A:

Const: project A does not catch the new value (unless it is recompiled with the new const, of course) because it was compiled with the constants subtituted in.

ReadOnly: Project A will always ask project B for it's variable value, so it will pick up the new value of the public constant in B.

Honestly, I would recommend you use readonly for nearly everything except truly universal constants ( e.g. Pi, Inches_To_Centimeters). For anything that could possibly change, I say use readonly.

Hope this helps, Alan.

@Anthony 2008-09-15 10:14:17

One thing to add to what people have said above. If you have an assembly containing a readonly value (e.g. readonly MaxFooCount = 4; ), you can change the value that calling assemblies see by shipping a new version of that assembly with a different value (e.g. readonly MaxFooCount = 5;)

But with a const, it would be folded into the caller's code when the caller was compiled.

If you've reached this level of C# proficiency, you are ready for Bill Wagner's book, Effective C#: 50 Specific Ways to Improve Your C# Which answers this question in detail, (and 49 other things).

@Vinko Vrsalovic 2008-09-11 08:04:07

This explains it. Summary: const must be initialized at declaration time, readonly can be initialized on the constructor (and thus have a different value depending on the constructor used).

EDIT: See Gishu's gotcha above for the subtle difference

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