Create instance of generic type whose constructor requires a parameter

Question

If BaseFruit has a constructor that accepts an int weight  can I instantiate a piece of fruit in a generic method like this   public void AddFruit lt T gt   where T  BaseFruit      BaseFruit fruit   new T weight     new Apple 150         fruit Enlist fruitManager       An example is added behind comments  It seems I can only do this if I give BaseFruit a parameterless constructor and then fill in everything through member variables  In my real code  not about fruit  this is rather impractical   -Update- So it seems it can t be solved by constraints in any way then  From the answers there are three candidate solutions    Factory Pattern Reflection Activator   I tend to think reflection is the least clean one  but I can t decide between the other two

User · Answer

You can t use any parameterised constructor  You can use a parameterless constructor if you have a  where T   new    constraint   It s a pain  but such is life     This is one of the things I d like to address with  static interfaces   You d then be able to constrain T to include static methods  operators and constructors  and then call them

User · Answer

You can do by using reflection   public void AddFruit lt T gt   where T  BaseFruit     ConstructorInfo constructor   typeof T  GetConstructor new Type     typeof int        if  constructor    null          throw new InvalidOperationException  Type     typeof T  Name     does not contain an appropriate constructor          BaseFruit fruit   constructor Invoke new object      int 150    as BaseFruit    fruit Enlist fruitManager       EDIT  Added constructor    null check   EDIT  A faster variant using a cache   public void AddFruit lt T gt   where T  BaseFruit     var constructor   FruitCompany lt T gt  constructor    if  constructor    null          throw new InvalidOperationException  Type     typeof T  Name     does not contain an appropriate constructor          var fruit   constructor Invoke new object      int 150    as BaseFruit    fruit Enlist fruitManager     private static class FruitCompany lt T gt      public static readonly ConstructorInfo constructor   typeof T  GetConstructor new Type     typeof int

User · Answer

You can use the following command   T instance    T typeof T  GetConstructor new Type 0   Invoke new object 0     Be sure to see the following reference

User · Answer

Yes  change your where to be   where T BaseFruit  new     However  this only works with parameterless constructors  You ll have to have some other means of setting your property  setting the property itself or something similar

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Recently I came across a very similar problem  Just wanted to share our solution with you all  I wanted to I created an instance of a Car lt CarA gt  from a json object using which had an enum   Dictionary lt MyEnum  Type gt  mapper   new Dictionary lt MyEnum  Type gt      mapper Add 1  typeof CarA    mapper Add 2  typeof BarB      public class Car lt T gt  where T   class              public T Detail   get  set        public Car T data               Detail   data          public class CarA         public int PropA   get  set        public CarA       public class CarB       public int PropB   get  set        public CarB        var jsonObj     Type   1   PropA   10   MyEnum t   GetTypeOfCar jsonObj   Type objectT   mapper t  Type genericType   typeof Car lt  gt    Type carTypeWithGenerics   genericType MakeGenericType objectT   Activator CreateInstance carTypeWithGenerics   new Object     JsonConvert DeserializeObject jsonObj  objectT

User · Answer

As Jon pointed out this is life for constraining a non-parameterless constructor   However a different solution is to use a factory pattern   This is easily constrainable   interface IFruitFactory lt T gt  where T   BaseFruit     T Create int weight      public void AddFruit lt T gt   IFruitFactory lt T gt  factory   where T  BaseFruit         BaseFruit fruit   factory Create weight     new Apple 150           fruit Enlist fruitManager       Yet another option is to use a functional approach   Pass in a factory method   public void AddFruit lt T gt  Func lt int T gt  factoryDel  where T   BaseFruit      BaseFruit fruit   factoryDel weight      new Apple 150        fruit Enlist fruitManager

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I created this method   public static V ConvertParentObjToChildObj lt T V gt   T obj  where V   new         Type typeT   typeof T       PropertyInfo   propertiesT   typeT GetProperties        V newV   new V        foreach  var propT in propertiesT                var nomePropT   propT Name          var valuePropT   propT GetValue obj  null            Type typeV   typeof V           PropertyInfo   propertiesV   typeV GetProperties            foreach  var propV in propertiesV                        var nomePropV   propV Name              if nomePropT    nomePropV                                propV SetValue newV  valuePropT                   break                                    return newV      I use that in this way   public class A        public int PROP1  get  set      public class B   A       public int PROP2  get  set       Code   A instanceA   new A    instanceA PROP1   1   B instanceB   new B    instanceB   ConvertParentObjToChildObj lt A B gt  instanceA

User · Answer

Additionally a simpler example   return  T Activator CreateInstance typeof T   new object     weight       Note that using the new   constraint on T is only to make the compiler check for a public parameterless constructor at compile time  the actual code used to create the type is the Activator class   You will need to ensure yourself regarding the specific constructor existing  and this kind of requirement may be a code smell  or rather something you should just try to avoid in the current version on c

User · Answer

It is still possible  with high performance  by doing the following              public List lt R gt  GetAllItems lt R gt    where R   IBaseRO  new             var list   new List lt R gt             using   var wl   new ReaderLock lt T gt   this                   foreach   var bo in this items                     T t   bo Value Data as T                  R r   new R                    r Initialize  t                    list Add  r                                    return list          and             lt summary gt Base class for read-only objects lt  summary gt  public partial interface IBaseRO        void Initialize  IDTO dto        void Initialize  object value        The relevant classes then have to derive from this interface and initialize accordingly  Please note  that in my case  this code is part of a surrounding class  which already has  lt T gt  as generic parameter  R  in my case  also is a read-only class  IMO  the public availability of Initialize   functions has no negative effect on the immutability  The user of this class could put another object in  but this would not modify the underlying collection

User · Answer

As an addition to user1471935 s suggestion    To instantiate a generic class by using a constructor with one or more parameters  you can now use the Activator class   T instance   Activator CreateInstance typeof T   new object             The list of objects are the parameters you want to supply  According to Microsoft      CreateInstance       creates an instance of the specified type using the constructor that best matches the specified parameters    There s also a generic version of CreateInstance  CreateInstance lt T gt     but that one also does not allow you to supply constructor parameters

User · Answer

Most simple solution Activator CreateInstance lt T gt

[c#] Create instance of generic type whose constructor requires a parameter?

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