What does where T class new mean

Question

Can you please explain to me what where T   class  new   means in the following line of code    void Add lt T gt  T item  where T   class  new

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This is part of the Generics mechanism  where the where keyword add constraints to what types must implement in order to be used as type parameters

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when using the class in constraints it s mean you can only use Reference type  another thing to add is when to use the constraint new    it s must be the last thing you write in the Constraints terms

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where T   struct  The type argument must be a value type  Any value type except Nullable can be specified  See Using Nullable Types  C  Programming Guide  for more information   where T   class  The type argument must be a reference type  including any class  interface  delegate  or array type   See note below    where T   new   The type argument must have a public parameterless constructor  When used in conjunction with other constraints  the new   constraint must be specified last   where T    base class name   The type argument must be or derive from the specified base class   where T    interface name   The type argument must be or implement the specified interface  Multiple interface constraints can be specified  The constraining interface can also be generic   where T   U  The type argument supplied for T must be or derive from the argument supplied for U  This is called a naked type constraint

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class  amp  new are 2 constraints on the generic type parameter T  Respectively they ensure   class     The type argument must be a reference type  this applies also to any class  interface  delegate  or array type    new     The type argument must have a public parameterless constructor  When used together with other constraints  the new   constraint must be specified last    Their combination means that the type T must be a Reference Type  can t be a Value Type   and must have a parameterless constructor   Example   struct MyStruct        structs are value types  class MyClass1        no constructors defined  so the class implicitly has a parameterless one  class MyClass2    parameterless constructor explicitly defined       public MyClass2          class MyClass3    only non-parameterless constructor defined       public MyClass3 object parameter         class MyClass4    both parameterless  amp  non-parameterless constructors defined       public MyClass4           public MyClass4 object parameter         interface INewable lt T gt      where T   new        interface INewableReference lt T gt      where T   class  new        class Checks       INewable lt int gt  cn1     ALLOWED  has parameterless ctor     INewable lt string gt  n2     NOT ALLOWED  no parameterless ctor     INewable lt MyStruct gt  n3     ALLOWED  has parameterless ctor     INewable lt MyClass1 gt  n4     ALLOWED  has parameterless ctor     INewable lt MyClass2 gt  n5     ALLOWED  has parameterless ctor     INewable lt MyClass3 gt  n6     NOT ALLOWED  no parameterless ctor     INewable lt MyClass4 gt  n7     ALLOWED  has parameterless ctor      INewableReference lt int gt  nr1     NOT ALLOWED  not a reference type     INewableReference lt string gt  nr2     NOT ALLOWED  no parameterless ctor     INewableReference lt MyStruct gt  nr3     NOT ALLOWED  not a reference type     INewableReference lt MyClass1 gt  nr4     ALLOWED  has parameterless ctor     INewableReference lt MyClass2 gt  nr5     ALLOWED  has parameterless ctor     INewableReference lt MyClass3 gt  nr6     NOT ALLOWED  no parameterless ctor     INewableReference lt MyClass4 gt  nr7     ALLOWED  has parameterless ctor

User · Answer

What comes after the  Where  is a constraint on the generic type T you declared  so    class means that the T should be a class and not a value type or a struct  new   indicates that the T class should have a public parameter-free default constructor defined

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Those are generic type constraints  In your case there are two of them   where T   class   Means that the type T must be a reference type  not a value type    where T   new     Means that the type T must have a parameter-less constructor  Having this constraint will allow you to do something like T field   new T    in your code which you wouldn t be able to do otherwise   You then combine the two using a comma to get   where T   class  new

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new    Specifying the new   constraint means type T must use a parameterless constructor  so an object can be instantiated from it - see Default constructors   class  Means T must be a reference type so it can t be an int  float  double  DateTime or other struct  value type     public void MakeCars           This won t compile as researchEngine doesn t have a public constructor and so can t be instantiated      CarFactory lt ResearchEngine gt  researchLine   new CarFactory lt ResearchEngine gt         var researchEngine   researchLine MakeEngine           Can instantiate new object of class with default public constructor     CarFactory lt ProductionEngine gt  productionLine   new CarFactory lt ProductionEngine gt         var productionEngine   productionLine MakeEngine       public class ProductionEngine     public class ResearchEngine       private ResearchEngine          public class CarFactory lt TEngine gt  where TEngine   class  new         public TEngine MakeEngine                 return new TEngine

User · Answer

That is a constraint on the generic parameter T  It must be a class  reference type  and must have a public parameter-less default constructor   That means T can t be an int  float  double  DateTime or any other struct  value type    It could be a string  or any other custom reference type  as long as it has a default or parameter-less constructor

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That means that type T must be a class and have a constructor that does not take any arguments   For example  you must be able to do this   T t   new T

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It s called a  constraint  on the generic parameter T  It means that T must be a reference type  a class  and that it must have a public default constructor

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where  C  Reference      The new   Constraint lets the compiler know that any type argument supplied must have an accessible parameterless--or default-- constructor   So it should be  T must be a class  and have an accessible parameterless--or default constructor

[c#] What does "where T : class, new()" mean?

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