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

User · Answer

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.

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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.

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

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

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