Why can t I define a default constructor for a struct in NET

Question

In  NET  a value type  C  struct  can t have a constructor with no parameters  According to this post this is mandated by the CLI specification  What happens is that for every value-type a default constructor is created  by the compiler   which initialized all members to zero  or null    Why is it disallowed to define such a default constructor   One trivial use is for rational numbers   public struct Rational       private long numerator      private long denominator       public Rational long num  long denom           Todo  Find GCD etc            public Rational long num                numerator   num          denominator   1             public Rational      This is not allowed               numerator   0          denominator   1            Using current version of C   a default Rational is 0 0 which is not so cool   PS  Will default parameters help solve this for C  nbsp 4 0 or will the CLR-defined default constructor be called     Jon Skeet answered      To use your example  what would you want to happen when someone did    Rational   fractions   new Rational 1000         Should it run through your constructor 1000 times    Sure it should  that s why I wrote the default constructor in the first place  The CLR should use the default zeroing constructor when no explicit default constructor is defined  that way you only pay for what you use  Then if I want a container of 1000 non-default Rationals  and want to optimize away the 1000 constructions  I will use a List lt Rational gt  rather than an array   This reason  in my mind  is not strong enough to prevent definition of a default constructor

User · Answer

Shorter explanation    In C    struct and class were just two sides of the same coin  The only real difference is that one was public by default and the other was private   In  NET  there is a much greater difference between a struct and a class  The main thing is that struct provides value-type semantics  while class provides reference-type semantics  When you start thinking about the implications of this change  other changes start to make more sense as well  including the constructor behavior you describe

User · Answer

Just special-case it  If you see a numerator of 0 and a denominator of 0  pretend like it has the values you really want

User · Answer

public struct Rational        private long numerator      private long denominator       public Rational long num   0  long denom   1       This is allowed                  numerator     num          denominator   denom

User · Answer

Shorter explanation    In C    struct and class were just two sides of the same coin  The only real difference is that one was public by default and the other was private   In  NET  there is a much greater difference between a struct and a class  The main thing is that struct provides value-type semantics  while class provides reference-type semantics  When you start thinking about the implications of this change  other changes start to make more sense as well  including the constructor behavior you describe

User · Answer

Note  the answer below was written a long time prior to C  6  which is planning to introduce the ability to declare parameterless constructors in structs - but they still won t be called in all situations  e g  for array creation    in the end this feature was not added to C  6      EDIT  I ve edited the answer below due to Grauenwolf s insight into the CLR   The CLR allows value types to have parameterless constructors  but C  doesn t  I believe this is because it would introduce an expectation that the constructor would be called when it wouldn t  For instance  consider this   MyStruct   foo   new MyStruct 1000     The CLR is able to do this very efficiently just by allocating the appropriate memory and zeroing it all out  If it had to run the MyStruct constructor 1000 times  that would be a lot less efficient   In fact  it doesn t - if you do have a parameterless constructor  it doesn t get run when you create an array  or when you have an uninitialized instance variable    The basic rule in C  is  the default value for any type can t rely on any initialization   Now they could have allowed parameterless constructors to be defined  but then not required that constructor to be executed in all cases - but that would have led to more confusion   Or at least  so I believe the argument goes    EDIT  To use your example  what would you want to happen when someone did   Rational   fractions   new Rational 1000     Should it run through your constructor 1000 times    If not  we end up with 1000 invalid rationals If it does  then we ve potentially wasted a load of work if we re about to fill in the array with real values    EDIT   Answering a bit more of the question  The parameterless constructor isn t created by the compiler  Value types don t have to have constructors as far as the CLR is concerned - although it turns out it can if you write it in IL  When you write  new Guid    in C  that emits different IL to what you get if you call a normal constructor  See this SO question for a bit more on that aspect   I suspect that there aren t any value types in the framework with parameterless constructors  No doubt NDepend could tell me if I asked it nicely enough    The fact that C  prohibits it is a big enough hint for me to think it s probably a bad idea

User · Answer

I haven t seen equivalent to late solution I m going to give  so here it is   use offsets to move values from default 0 into any value you like  here properties must be used instead of directly accessing fields   maybe with possible c 7 feature you better define property scoped fields so they remain protected from being directly accessed in code    This solution works for simple structs with only value types  no ref type or nullable struct    public struct Tempo       const double DefaultBpm   120      private double  bpm     this field must not be modified other than with its property       public double BeatsPerMinute               get   gt   bpm   DefaultBpm          set   gt   bpm   value - DefaultBpm            This is different than this answer  this approach is not especial casing but its using offset which will work for all ranges   example with enums as field   public struct Difficaulty       Easy      Medium      Hard    public struct Level       const Difficaulty DefaultLevel   Difficaulty Medium      private Difficaulty  level     this field must not be modified other than with its property       public Difficaulty Difficaulty               get   gt   level   DefaultLevel          set   gt   level   value - DefaultLevel            As I said this trick may not work in all cases  even if struct has only value fields  only you know that if it works in your case or not  just examine  but you get the general idea

User · Answer

You can make a static property that initializes and returns a default  rational  number   public static Rational One   gt  new Rational 0  1      And use it like   var rat   Rational One

User · Answer

You can t define a default constructor because you are using C    Structs can have default constructors in  NET  though I don t know of any specific language that supports it

User · Answer

I haven t seen equivalent to late solution I m going to give  so here it is   use offsets to move values from default 0 into any value you like  here properties must be used instead of directly accessing fields   maybe with possible c 7 feature you better define property scoped fields so they remain protected from being directly accessed in code    This solution works for simple structs with only value types  no ref type or nullable struct    public struct Tempo       const double DefaultBpm   120      private double  bpm     this field must not be modified other than with its property       public double BeatsPerMinute               get   gt   bpm   DefaultBpm          set   gt   bpm   value - DefaultBpm            This is different than this answer  this approach is not especial casing but its using offset which will work for all ranges   example with enums as field   public struct Difficaulty       Easy      Medium      Hard    public struct Level       const Difficaulty DefaultLevel   Difficaulty Medium      private Difficaulty  level     this field must not be modified other than with its property       public Difficaulty Difficaulty               get   gt   level   DefaultLevel          set   gt   level   value - DefaultLevel            As I said this trick may not work in all cases  even if struct has only value fields  only you know that if it works in your case or not  just examine  but you get the general idea

User · Answer

What I use is the null-coalescing operator      combined with a backing field like this  public struct SomeStruct     private SomeRefType m MyRefVariableBackingField     public SomeRefType MyRefVariable       get   return m MyRefVariableBackingField     m MyRefVariableBackingField   new SomeRefType              Hope this helps    Note  the null coalescing assignment is currently a feature proposal for C  8 0

User · Answer

What I use is the null-coalescing operator      combined with a backing field like this  public struct SomeStruct     private SomeRefType m MyRefVariableBackingField     public SomeRefType MyRefVariable       get   return m MyRefVariableBackingField     m MyRefVariableBackingField   new SomeRefType              Hope this helps    Note  the null coalescing assignment is currently a feature proposal for C  8 0

User · Answer

A struct is a value type and a value type must have a default value as soon as it is declared   MyClass m  MyStruct m2    If you declare two fields as above without instantiating either  then break the debugger  m will be null but m2 will not  Given this  a parameterless constructor would make no sense  in fact all any constructor on a struct does is assign values  the thing itself already exists just by declaring it  Indeed m2 could quite happily be used in the above example and have its methods called  if any  and its fields and properties manipulated

User · Answer

Here s my solution to the no default constructor dilemma  I know this is a late solution  but I think it s worth noting this is a solution    public struct Point2D       public static Point2D NULL   new Point2D -1 -1       private int   Data       public int X           get               return this Data  0                      set               try                   this Data  0     value                catch  Exception                     this Data   new int  2                  finally                   this Data  0     value                                     public int Z           get               return this Data  1                      set               try                   this Data  1     value                catch  Exception                     this Data   new int  2                  finally                   this Data  1     value                                     public Point2D  int x   int z             this Data   new int  2     x   z               public static Point2D operator    Point2D A   Point2D B             return new Point2D  A X   B X   A Z   B Z               public static Point2D operator -  Point2D A   Point2D B             return new Point2D  A X - B X   A Z - B Z               public static Point2D operator    Point2D A   int B             return new Point2D  B   A X   B   A Z               public static Point2D operator    int A   Point2D B             return new Point2D  A   B Z   A   B Z               public override string ToString             return string Format     0   1      this X   this Z              ignoring the fact I have a static struct called null   Note  This is for all positive quadrant only   using get set  in C   you can have a try catch finally  for dealing with the errors where a particular data type is not initialized by the default constructor Point2D    I guess this is elusive as a solution to some people on this answer  Thats mostly why i m adding mine  Using the getter and setter functionality in C  will allow you to bypass this default constructor non-sense and put a try catch around what you dont have initialized  For me this works fine  for someone else you might want to add some if statements  So  In the case where you would want a Numerator Denominator setup  this code might help  I d just like to reiterate that this solution does not look nice  probably works even worse from an efficiency standpoint  but  for someone coming from an older version of C   using array data types gives you this functionality  If you just want something that works  try this   public struct Rational       private long   Data       public long Numerator           get               try                   return this Data  0                  catch  Exception                     this Data   new long  2     0   1                    return this Data  0                                    set               try                   this Data  0     value                catch  Exception                     this Data   new long  2     0   1                    this Data  0     value                                     public long Denominator           get               try                   return this Data  1                  catch  Exception                     this Data   new long  2     0   1                    return this Data  1                                    set               try                   this Data  1     value                catch  Exception                     this Data   new long  2     0   1                    this Data  1     value                                     public Rational  long num   long denom             this Data   new long  2     num   denom               Todo  Find GCD etc                public Rational  long num             this Data   new long  2     num   1            this Numerator   num          this Denominator   1

User · Answer

Just special-case it  If you see a numerator of 0 and a denominator of 0  pretend like it has the values you really want

User · Answer

Just special-case it  If you see a numerator of 0 and a denominator of 0  pretend like it has the values you really want

User · Answer

Just special-case it  If you see a numerator of 0 and a denominator of 0  pretend like it has the values you really want

User · Answer

Note  the answer below was written a long time prior to C  6  which is planning to introduce the ability to declare parameterless constructors in structs - but they still won t be called in all situations  e g  for array creation    in the end this feature was not added to C  6      EDIT  I ve edited the answer below due to Grauenwolf s insight into the CLR   The CLR allows value types to have parameterless constructors  but C  doesn t  I believe this is because it would introduce an expectation that the constructor would be called when it wouldn t  For instance  consider this   MyStruct   foo   new MyStruct 1000     The CLR is able to do this very efficiently just by allocating the appropriate memory and zeroing it all out  If it had to run the MyStruct constructor 1000 times  that would be a lot less efficient   In fact  it doesn t - if you do have a parameterless constructor  it doesn t get run when you create an array  or when you have an uninitialized instance variable    The basic rule in C  is  the default value for any type can t rely on any initialization   Now they could have allowed parameterless constructors to be defined  but then not required that constructor to be executed in all cases - but that would have led to more confusion   Or at least  so I believe the argument goes    EDIT  To use your example  what would you want to happen when someone did   Rational   fractions   new Rational 1000     Should it run through your constructor 1000 times    If not  we end up with 1000 invalid rationals If it does  then we ve potentially wasted a load of work if we re about to fill in the array with real values    EDIT   Answering a bit more of the question  The parameterless constructor isn t created by the compiler  Value types don t have to have constructors as far as the CLR is concerned - although it turns out it can if you write it in IL  When you write  new Guid    in C  that emits different IL to what you get if you call a normal constructor  See this SO question for a bit more on that aspect   I suspect that there aren t any value types in the framework with parameterless constructors  No doubt NDepend could tell me if I asked it nicely enough    The fact that C  prohibits it is a big enough hint for me to think it s probably a bad idea

User · Answer

public struct Rational        private long numerator      private long denominator       public Rational long num   0  long denom   1       This is allowed                  numerator     num          denominator   denom

User · Answer

You can make a static property that initializes and returns a default  rational  number   public static Rational One   gt  new Rational 0  1      And use it like   var rat   Rational One

User · Answer

Here s my solution to the no default constructor dilemma  I know this is a late solution  but I think it s worth noting this is a solution    public struct Point2D       public static Point2D NULL   new Point2D -1 -1       private int   Data       public int X           get               return this Data  0                      set               try                   this Data  0     value                catch  Exception                     this Data   new int  2                  finally                   this Data  0     value                                     public int Z           get               return this Data  1                      set               try                   this Data  1     value                catch  Exception                     this Data   new int  2                  finally                   this Data  1     value                                     public Point2D  int x   int z             this Data   new int  2     x   z               public static Point2D operator    Point2D A   Point2D B             return new Point2D  A X   B X   A Z   B Z               public static Point2D operator -  Point2D A   Point2D B             return new Point2D  A X - B X   A Z - B Z               public static Point2D operator    Point2D A   int B             return new Point2D  B   A X   B   A Z               public static Point2D operator    int A   Point2D B             return new Point2D  A   B Z   A   B Z               public override string ToString             return string Format     0   1      this X   this Z              ignoring the fact I have a static struct called null   Note  This is for all positive quadrant only   using get set  in C   you can have a try catch finally  for dealing with the errors where a particular data type is not initialized by the default constructor Point2D    I guess this is elusive as a solution to some people on this answer  Thats mostly why i m adding mine  Using the getter and setter functionality in C  will allow you to bypass this default constructor non-sense and put a try catch around what you dont have initialized  For me this works fine  for someone else you might want to add some if statements  So  In the case where you would want a Numerator Denominator setup  this code might help  I d just like to reiterate that this solution does not look nice  probably works even worse from an efficiency standpoint  but  for someone coming from an older version of C   using array data types gives you this functionality  If you just want something that works  try this   public struct Rational       private long   Data       public long Numerator           get               try                   return this Data  0                  catch  Exception                     this Data   new long  2     0   1                    return this Data  0                                    set               try                   this Data  0     value                catch  Exception                     this Data   new long  2     0   1                    this Data  0     value                                     public long Denominator           get               try                   return this Data  1                  catch  Exception                     this Data   new long  2     0   1                    return this Data  1                                    set               try                   this Data  1     value                catch  Exception                     this Data   new long  2     0   1                    this Data  1     value                                     public Rational  long num   long denom             this Data   new long  2     num   denom               Todo  Find GCD etc                public Rational  long num             this Data   new long  2     num   1            this Numerator   num          this Denominator   1

User · Answer

Shorter explanation    In C    struct and class were just two sides of the same coin  The only real difference is that one was public by default and the other was private   In  NET  there is a much greater difference between a struct and a class  The main thing is that struct provides value-type semantics  while class provides reference-type semantics  When you start thinking about the implications of this change  other changes start to make more sense as well  including the constructor behavior you describe

User · Answer

You can make a static property that initializes and returns a default  rational  number   public static Rational One   gt  new Rational 0  1      And use it like   var rat   Rational One

User · Answer

You can t define a default constructor because you are using C    Structs can have default constructors in  NET  though I don t know of any specific language that supports it

User · Answer

Note  the answer below was written a long time prior to C  6  which is planning to introduce the ability to declare parameterless constructors in structs - but they still won t be called in all situations  e g  for array creation    in the end this feature was not added to C  6      EDIT  I ve edited the answer below due to Grauenwolf s insight into the CLR   The CLR allows value types to have parameterless constructors  but C  doesn t  I believe this is because it would introduce an expectation that the constructor would be called when it wouldn t  For instance  consider this   MyStruct   foo   new MyStruct 1000     The CLR is able to do this very efficiently just by allocating the appropriate memory and zeroing it all out  If it had to run the MyStruct constructor 1000 times  that would be a lot less efficient   In fact  it doesn t - if you do have a parameterless constructor  it doesn t get run when you create an array  or when you have an uninitialized instance variable    The basic rule in C  is  the default value for any type can t rely on any initialization   Now they could have allowed parameterless constructors to be defined  but then not required that constructor to be executed in all cases - but that would have led to more confusion   Or at least  so I believe the argument goes    EDIT  To use your example  what would you want to happen when someone did   Rational   fractions   new Rational 1000     Should it run through your constructor 1000 times    If not  we end up with 1000 invalid rationals If it does  then we ve potentially wasted a load of work if we re about to fill in the array with real values    EDIT   Answering a bit more of the question  The parameterless constructor isn t created by the compiler  Value types don t have to have constructors as far as the CLR is concerned - although it turns out it can if you write it in IL  When you write  new Guid    in C  that emits different IL to what you get if you call a normal constructor  See this SO question for a bit more on that aspect   I suspect that there aren t any value types in the framework with parameterless constructors  No doubt NDepend could tell me if I asked it nicely enough    The fact that C  prohibits it is a big enough hint for me to think it s probably a bad idea

User · Answer

A struct is a value type and a value type must have a default value as soon as it is declared   MyClass m  MyStruct m2    If you declare two fields as above without instantiating either  then break the debugger  m will be null but m2 will not  Given this  a parameterless constructor would make no sense  in fact all any constructor on a struct does is assign values  the thing itself already exists just by declaring it  Indeed m2 could quite happily be used in the above example and have its methods called  if any  and its fields and properties manipulated

User · Answer

Shorter explanation    In C    struct and class were just two sides of the same coin  The only real difference is that one was public by default and the other was private   In  NET  there is a much greater difference between a struct and a class  The main thing is that struct provides value-type semantics  while class provides reference-type semantics  When you start thinking about the implications of this change  other changes start to make more sense as well  including the constructor behavior you describe

User · Answer

A struct is a value type and a value type must have a default value as soon as it is declared   MyClass m  MyStruct m2    If you declare two fields as above without instantiating either  then break the debugger  m will be null but m2 will not  Given this  a parameterless constructor would make no sense  in fact all any constructor on a struct does is assign values  the thing itself already exists just by declaring it  Indeed m2 could quite happily be used in the above example and have its methods called  if any  and its fields and properties manipulated

User · Answer

You can make a static property that initializes and returns a default  rational  number   public static Rational One   gt  new Rational 0  1      And use it like   var rat   Rational One

User · Answer

You can t define a default constructor because you are using C    Structs can have default constructors in  NET  though I don t know of any specific language that supports it

User · Answer

A struct is a value type and a value type must have a default value as soon as it is declared   MyClass m  MyStruct m2    If you declare two fields as above without instantiating either  then break the debugger  m will be null but m2 will not  Given this  a parameterless constructor would make no sense  in fact all any constructor on a struct does is assign values  the thing itself already exists just by declaring it  Indeed m2 could quite happily be used in the above example and have its methods called  if any  and its fields and properties manipulated

User · Answer

You can t define a default constructor because you are using C    Structs can have default constructors in  NET  though I don t know of any specific language that supports it

User · Answer

Note  the answer below was written a long time prior to C  6  which is planning to introduce the ability to declare parameterless constructors in structs - but they still won t be called in all situations  e g  for array creation    in the end this feature was not added to C  6      EDIT  I ve edited the answer below due to Grauenwolf s insight into the CLR   The CLR allows value types to have parameterless constructors  but C  doesn t  I believe this is because it would introduce an expectation that the constructor would be called when it wouldn t  For instance  consider this   MyStruct   foo   new MyStruct 1000     The CLR is able to do this very efficiently just by allocating the appropriate memory and zeroing it all out  If it had to run the MyStruct constructor 1000 times  that would be a lot less efficient   In fact  it doesn t - if you do have a parameterless constructor  it doesn t get run when you create an array  or when you have an uninitialized instance variable    The basic rule in C  is  the default value for any type can t rely on any initialization   Now they could have allowed parameterless constructors to be defined  but then not required that constructor to be executed in all cases - but that would have led to more confusion   Or at least  so I believe the argument goes    EDIT  To use your example  what would you want to happen when someone did   Rational   fractions   new Rational 1000     Should it run through your constructor 1000 times    If not  we end up with 1000 invalid rationals If it does  then we ve potentially wasted a load of work if we re about to fill in the array with real values    EDIT   Answering a bit more of the question  The parameterless constructor isn t created by the compiler  Value types don t have to have constructors as far as the CLR is concerned - although it turns out it can if you write it in IL  When you write  new Guid    in C  that emits different IL to what you get if you call a normal constructor  See this SO question for a bit more on that aspect   I suspect that there aren t any value types in the framework with parameterless constructors  No doubt NDepend could tell me if I asked it nicely enough    The fact that C  prohibits it is a big enough hint for me to think it s probably a bad idea

[c#] Why can't I define a default constructor for a struct in .NET?

Examples related to c#

Examples related to .net

Examples related to struct