Create instance of Class using Activator with parameter-less constructor in base class

Question

I have a base class with many descendants. In the base class a parameter-less constructor is declared. Descendants may declare specialized constructors.

public class A 
{
   public A()
   {
   }
}

public class B : A
{
  string Stuff { get; set; }
  public B(string stuff)
  {
    Stuff = stuff;
  }
}

At some point I want to instantiate a B using the Activator. This instance needs no initialization of it's "Stuff". So I want to use the A's default constructor:

public object InstantiateType(Type type)
{
   return Activator.CreateInstance(
      type,
      BindingFlags.CreateInstance |
      BindingFlags.Public |
      BindingFlags.NonPublic
   );
}

This sample call results in an exception as many other variations on this theme, is it at all possible, and if so how do I instruct the Activator to do so?

You can't bypass a missing constructor. You could use `FormatterServices.GetUninitializedObject` but then the `A` constructor wouldn't be called. — xanatos, Mar 26 '15 at 15:09
possible duplicate: http://stackoverflow.com/questions/12051/calling-base-constructor-in-c-sharp — Binkan Salaryman, Mar 26 '15 at 15:14

D Stanley · Answer 1 · 2015-03-26T15:17:45.477

3

At some point I want to instantiate a B using ... A's default constructor

You can't. Constructors aren't inherited, and there is no parameterless constructor for B that you can use. You will need to create a constructur that passes through to the base constructor:

public class B : A
{
  string Stuff{get;set}
  public B(string stuff)
  {
    Stuff = stuff;
  }
  public B() : base()  // technically the ": base()" is redundant since A only has one constructor
  {
  }
}

You could also change your CreateInstance call to pass in null as the string parameter.

edited Mar 26 '15 at 15:17

answered Mar 26 '15 at 15:07

D Stanley

149,601
11
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I have a lot of descendants, and they declare different constructors. I could add a parameter less constructor to all sub classes, but this is the thing I'm trying to avoid – k.c. Mar 26 '15 at 15:11
Try all you want, but you can't call constructors that don't exist. – D Stanley Mar 26 '15 at 15:14
After defining a parameterized constructor the default constructor is not generated. – Dustin Kingen Mar 26 '15 at 15:15
So the answer is: No you can't. Thanks – k.c. Mar 26 '15 at 15:16
@k.c. Correct. I've edited my answer to make that more clear. Sorry if it was ambiguous before. – D Stanley Mar 26 '15 at 15:18

score 2 · Accepted Answer · answered Mar 26 '15 at 15:21

2

Here is a workaround for you:

public class A {
    ///<summary>Creates an instance of A, or of a subclass of A</summary>
    public static A Create<T>() where T : A {
        A result = (A)FormatterServices.GetUninitializedObject(typeof(T));
        result.defaultInit();
        return result;
    }

    public A() {
        defaultInit();
    }

    private void defaultInit() {
        // field assignments ...
        // default constructor code ...
    }
}

public class B : A {
    string Stuff { get; set; }
    public B(string stuff) {
        Stuff = stuff;
    }
}

answered Mar 26 '15 at 15:21

Binkan Salaryman

3,008
1
17
29

1

I will try this, as the default constructor doesn't perform any work, If it works the green tick is for you – k.c. Mar 26 '15 at 15:27
It works even without the default init. I needed an instance to invoke a method that builds some delegates. No data is used. For normal use it would not be very safe according to the documentation, and implementing a defaultinit in each class would be no better than adding a constructor, but for this your out of the box answer is great! – k.c. Mar 26 '15 at 15:39

Create instance of Class using Activator with parameter-less constructor in base class

2 Answers2