Passing arguments to C generic new of templated type

Question

I m trying to create a new object of type T via its constructor when adding to the list   I m getting a compile error  The error message is       T   cannot provide arguments when creating an instance of a variable   But my classes do have a constructor argument  How can I make this work   public static string GetAllItems lt T gt       where T   new               List lt T gt  tabListItems   new List lt T gt        foreach  ListItem listItem in listCollection               tabListItems Add new T listItem       error here

User · Answer

This will not work in your situation  You can only specify the constraint that it has an empty constructor   public static string GetAllItems lt T gt       where T  new     What you could do is use property injection by defining this interface   public interface ITakesAListItem      ListItem Item   set        Then you could alter your method to be this   public static string GetAllItems lt T gt       where T   ITakesAListItem  new               List lt T gt  tabListItems   new List lt T gt        foreach  ListItem listItem in listCollection               tabListItems Add new T     Item   listItem                      The other alternative is the Func method described by JaredPar

User · Answer

In order to create an instance of a generic type in a function you must constrain it with the  new  flag     public static string GetAllItems lt T gt       where T   new     However that will only work when you want to call the constructor which has no parameters   Not the case here   Instead you ll have to provide another parameter which allows for the creation of object based on parameters   The easiest is a function     public static string GetAllItems lt T gt       Func lt ListItem T gt  del            List lt T gt  tabListItems   new List lt T gt       foreach  ListItem listItem in listCollection           tabListItems Add del listItem                  You can then call it like so  GetAllItems lt Foo gt       l   gt  new Foo l

User · Answer

If you simply want to initialise a member field or property with the constructor parameter  in C     3 you can do it very easier   public static string GetAllItems lt T gt       where T   InterfaceOrBaseClass  new                  List lt T gt  tabListItems   new List lt T gt         foreach  ListItem listItem in listCollection                 tabListItems Add new T  BaseMemberItem   listItem        No error  BaseMemberItem owns to InterfaceOrBaseClass                       This is the same thing Garry Shutler said  but I d like to put an aditional note   Of course you can use a property trick to do more stuff than just setting a field value  A property  set    can trigger any processing needed to setup its related fields and any other need for the object itself  including a check to see if a full initialization is to take place before the object is used  simulating a full contruction  yes  it is an ugly workaround  but it overcomes M  s new   limitation    I can t be assure if it is a planned hole or an accidental side effect  but it works   It is very funny how MS people adds new features to the language and seems to not do a full side effects analysis  The entire generic thing is a good evidence of this

User · Answer

This is kind of mucky  and when I say kind of mucky I may mean revolting  but supposing you can furnish your parameterised type with an empty constructor  then   public static T GetTInstance lt T gt    where T  new         var constructorTypeSignature   new Type    typeof  object        var constructorParameters   new object     Create a T        return  T  new T   GetType   GetConstructor constructorTypeSignature  Invoke constructorParameters       Will effectively allow you to construct an object from a parameterised type with an argument  In this case I am assuming the constructor I want has a single argument of type object  We create a dummy instance of T using the constraint permitted empty constructor and then use reflection to get one of its other constructors

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If all is you need is convertion from ListItem to your type T you can implement this convertion in T class as conversion operator   public class T       public static implicit operator T ListItem listItem    gt                public static string GetAllItems                    List lt T gt  tabListItems   new List lt T gt         foreach  ListItem listItem in listCollection                 tabListItems Add listItem

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I found that I was getting an error  cannot provide arguments when creating an instance of type parameter T  so I needed to do this   var x   Activator CreateInstance typeof T   args  as T

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Very old question  but new answer  -   The ExpressionTree version   I think the fastests and cleanest solution   Like Welly Tambunan said   we could also use expression tree to build the object   This will generate a  constructor   function  for the type parameters given  It returns a delegate and accept the parameter types as an array of objects   Here it is      this delegate is just  so you don t have to pass an object array    params   public delegate object ConstructorDelegate params object   args    public static ConstructorDelegate CreateConstructor Type type  params Type   parameters           Get the constructor info for these parameters     var constructorInfo   type GetConstructor parameters           define a object   parameter     var paramExpr   Expression Parameter typeof Object              To feed the constructor with the right parameters  we need to generate an array         of parameters that will be read from the initialize object array argument      var constructorParameters   parameters Select  paramType  index    gt             convert the object index  to the right constructor parameter type          Expression Convert                 read a value from the object index              Expression ArrayAccess                  paramExpr                  Expression Constant index                paramType   ToArray            just call the constructor      var body   Expression New constructorInfo  constructorParameters        var constructor   Expression Lambda lt ConstructorDelegate gt  body  paramExpr       return constructor Compile          Example MyClass   public class MyClass       public int TestInt   get  private set        public string TestString   get  private set         public MyClass int testInt  string testString                TestInt   testInt          TestString   testString              Usage      you should cache this  constructor  var myConstructor   CreateConstructor typeof MyClass   typeof int   typeof string        Call the  myConstructor  function to create a new instance  var myObject   myConstructor 10   test message          Another example  passing the types as an array  var type   typeof MyClass   var args   new Type     typeof int   typeof string         you should cache this  constructor  var myConstructor   CreateConstructor type  args       Call the  myConstructor  fucntion to create a new instance  var myObject   myConstructor 10   test message        DebugView of Expression   Lambda  Lambda1 lt TestExpressionConstructor MainWindow ConstructorDelegate gt  System Object    var1         New TestExpressionConstructor MainWindow MyClass           System Int32  var1 0            System String  var1 1         This is equivalent to the code that is generated   public object myConstructor object   var1        return new MyClass           System Int32 var1 0            System String var1 1          Small downside  All valuetypes parameters are boxed when they are passed like an object array     Simple performance test   private void TestActivator         Stopwatch sw   Stopwatch StartNew        for  int i   0  i  lt  1024   1024   10  i                  var myObject   Activator CreateInstance typeof MyClass   10   test message              sw Stop        Trace WriteLine  Activator      sw Elapsed      private void TestReflection         var constructorInfo   typeof MyClass  GetConstructor new     typeof int   typeof string           Stopwatch sw   Stopwatch StartNew        for  int i   0  i  lt  1024   1024   10  i                  var myObject   constructorInfo Invoke new object     10   test message                 sw Stop        Trace WriteLine  Reflection      sw Elapsed      private void TestExpression         var myConstructor   CreateConstructor typeof MyClass   typeof int   typeof string         Stopwatch sw   Stopwatch StartNew         for  int i   0  i  lt  1024   1024   10  i                  var myObject   myConstructor 10   test message               sw Stop        Trace WriteLine  Expression      sw Elapsed      TestActivator    TestReflection    TestExpression      Results   Activator  00 00 13 8210732 Reflection  00 00 05 2986945 Expression  00 00 00 6681696   Using Expressions is   - 8 times faster than Invoking the ConstructorInfo and   - 20 times faster than using the Activator

User · Answer

You need to add where T  new   to let the compiler know that T is guaranteed to provide a default constructor    public static string GetAllItems lt T gt       where T  new

User · Answer

I believe you have to constraint T with a where statement to only allow objects with a new constructor   RIght now it accepts anything including objects without it

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Object initializer  If your constructor with the parameter isn t doing anything besides setting a property  you can do this in C  3 or better using an object initializer rather than calling a constructor  which is impossible  as has been mentioned    public static string GetAllItems lt T gt       where T   new               List lt T gt  tabListItems   new List lt T gt        foreach  ListItem listItem in listCollection               tabListItems Add new T     YourPropertyName   listItem         Now using object initializer                  Using this  you can always put any constructor logic in the default  empty  constructor  too   Activator CreateInstance    Alternatively  you could call Activator CreateInstance   like so   public static string GetAllItems lt T gt       where T   new               List lt T gt  tabListItems   new List lt T gt        foreach  ListItem listItem in listCollection                object   args   new object     listItem            tabListItems Add  T Activator CreateInstance typeof T   args       Now using Activator CreateInstance                  Note that Activator CreateInstance can have some performance overhead that you may want to avoid if execution speed is a top priority and another option is maintainable to you

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Since nobody bothered to post the  Reflection  answer  which I personally think is the best answer   here goes   public static string GetAllItems lt T gt       where T   new               List lt T gt  tabListItems   new List lt T gt        foreach  ListItem listItem in listCollection               Type classType   typeof T          ConstructorInfo classConstructor   classType GetConstructor new Type     listItem GetType              T classInstance    T classConstructor Invoke new object     listItem             tabListItems Add classInstance                    Edit  This answer is deprecated due to  NET 3 5 s Activator CreateInstance  however it is still useful in older  NET versions

User · Answer

If you have access to the class you re going to use  you can use this approach which I used   Create an interface that has an alternative creator   public interface ICreatable1Param       void PopulateInstance object Param       Make your classes with an empty creator and implement this method   public class MyClass   ICreatable1Param       public MyClass       do something or nothing       public void PopulateInstance  object Param                  populate the class here           Now use your generic methods   public void MyMethod lt T gt       where T   ICreatable1Param  new           do stuff     T newT   new T        T PopulateInstance Param         If you don t have access  wrap the target class   public class MyClass   ICreatable1Param       public WrappedClass WrappedInstance  get  private set        public MyClass       do something or nothing       public void PopulateInstance  object Param                WrappedInstance   new WrappedClass Param

User · Answer

in  Net 3 5 and after you could use the activator class    T Activator CreateInstance typeof T   args

User · Answer

I sometimes use an approach that resembles to the answers using property injection  but keeps the code cleaner  Instead of having a base class interface with a set of properties  it only contains a  virtual  Initialize  -method that acts as a  poor man s constructor   Then you can let each class handle it s own initialization just as a constructor would  which also adds a convinient way of handling inheritance chains   If often find myself in situations where I want each class in the chain to initialize its unique properties  and then call its parent s Initialize  -method which in turn initializes the parent s unique properties and so forth  This is especially useful when having different classes  but with a similar hierarchy  for example business objects that are mapped to from DTO s   Example that uses a common Dictionary for initialization   void Main         var values   new Dictionary lt string  int gt       BaseValue   1       DerivedValue   2           Console WriteLine CreateObject lt Base gt  values  ToString          Console WriteLine CreateObject lt Derived gt  values  ToString        public T CreateObject lt T gt  IDictionary lt string  int gt  values      where T   Base  new         var obj   new T        obj Initialize values       return obj     public class Base       public int BaseValue   get  set         public virtual void Initialize IDictionary lt string  int gt  values                BaseValue   values  BaseValue               public override string ToString                 return  BaseValue       BaseValue           public class Derived   Base       public int DerivedValue   get  set         public override void Initialize IDictionary lt string  int gt  values                base Initialize values           DerivedValue   values  DerivedValue               public override string ToString                        return base ToString        DerivedValue       DerivedValue

[c#] Passing arguments to C# generic new() of templated type

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