What is the difference between const and readonly in C

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What is the difference between const and readonly in C     When would you use one over the other

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Const  Absolute constant value during the application life time   Readonly  It can be changed in running time

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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

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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   Example   public static class Text     public const string ConstDescription    This can be used      public readonly static string ReadonlyDescription    Cannot be used       public class Foo       Description Text ConstDescription     public int BarThatBuilds         get  set            Description Text ReadOnlyDescription     public int BarThatDoesNotBuild         get  set

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Read Only   Value can be changed through Ctor at runtime  But not through member Function Constant   By default static  Value cannot be changed from anywhere   Ctor  Function  runtime etc no-where

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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

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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          or       public readonly double PI3        public MyClass2                   PI3   3 14159             const   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   readonly   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

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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

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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             Console WriteLine A C             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

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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

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CONST   const keyword can be applied to fields or local variables We must assign const field at time of declaration  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 Const in C  are by default static   The value is constant for all objects 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   Only C  built-in types can be declared as constant Const field can not be passed as ref or out parameter   ReadOnly   readonly keyword applies only to fields not local variables We can assign readonly field at the time of declaration or in constructor not in any other methods  dynamic memory allocated for readonly fields and we can get the value at run time  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  The value may be different depending upon constructor used  as it belongs to object of the class  If you declare a non-primitive types  reference type  as readonly only reference is immutable not the object it contains  Since the value is obtained at run time  there is no dll versioning problem with readonly fields  properties  We can pass readonly field as ref or out parameters in the constructor context

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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    quot set quot              public Sample string value    this             ro   value     this works even though it was set in the no-arg ctor

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I believe a const value is the same for all objects  and must be initialized with a literal expression   whereas readonly can be different for each instantiation

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const  Can t be changed anywhere   readonly  This value can only be changed in the constructor  Can t be changed in normal functions

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Yet another gotcha  readonly values can be changed by  devious  code via reflection   var fi   this GetType                 BaseType               GetField   someField                           BindingFlags Instance   BindingFlags NonPublic   fi SetValue this  1     Can I change a private readonly inherited field in C  using reflection

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A const is a compile-time constant whereas readonly allows a value to be calculated at run-time and set in the constructor or field initializer  So  a  const  is always constant but  readonly  is read-only once it is assigned  Eric Lippert of the C  team has more information on different types of immutability

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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

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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

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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

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

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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

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Just to add  readonly for reference types only makes the reference read only 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

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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

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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  http   blogs msdn com csharpfaq archive 2004 12 03 274791 aspx

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Here s another link demonstrating how const isn t version safe  or relevant for reference types   Summary    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

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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

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

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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

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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

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Constants   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

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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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