Why can't I enforce derived classes to have parameterless constructors?

Question

I am trying to do the following:

class Program
{
    static void Main(string[] args)
    {
        foo<baz> fooObject = new foo<baz>();
        AnotherClass<baz> _baz = new AnotherClass<baz>();
        _baz.testMethod(fooObject);
    }
}

public class AnotherClass<T> where T : bar
{
    public void testMethod(foo<T> dummy)
    {
        foobar = dummy;
    }

    private foo<T> foobar = null;
}

public class foo<T> where T : bar, new()
{
    public foo()
    {
        _t = new T();
    }

    private T _t;

}

public abstract class bar
{
    public abstract void someMethod();
    // Some implementation
}

public class baz : bar
{
    public override void someMethod()
    {
        //Implementation
    }
}

And I get an error explaining that 'T' must be a non-abstract type with a public parameterless constructor in order to use it as parameter 'T' in the generic type or method. I fully understand why this must be, and also understand that I could pass a pre-initialized object of type 'T' in as a constructor argument to avoid having to 'new' it, but is there any way around this? any way to enforce classes that derive from 'bar' to supply parameterless constructors?

Fixed - I was missing a 'new()' constraint on AnotherClass().

score 7 · Answer 1 · answered Apr 16 '10 at 09:01

7

The correct syntax is

public class foo<T> where T : bar, new()

answered Apr 16 '10 at 09:01

Gerrie Schenck

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Sorry, I typed that in a rush. – Ben Heymink Apr 16 '10 at 09:05

score 5 · Accepted Answer · answered Apr 16 '10 at 18:48

5

Aren't you missing a new() constraint on your AnotherClass?

public class AnotherClass<T> where T : bar, new()

Without that VS2010 refuses to compile, with that it works just fine.

answered Apr 16 '10 at 18:48

Ian Mercer

38,490
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Thanks, this seems to have fixed it! – Ben Heymink Apr 16 '10 at 21:49

score 1 · Answer 3 · answered Apr 16 '10 at 09:31

Your code compiles in VS2010.

You probably want to use interfaces instead:

public class foo<T> where T : ibar, new()
...

public interface ibar
{
    void someMethod();
}

public abstract class bar : ibar
{
    public abstract void someMethod();
    // Some implementation
}

score 0 · Answer 4 · answered Mar 08 '13 at 00:13

0

First remove "new()" from keyword "where" of class "Foo" After must alter constructor as

public foo()
{
    _t = Activator.CreateInstance<T>();
}

For me worked!

answered Mar 08 '13 at 00:13

Marcal

1

score 0 · Answer 5 · edited May 23 '17 at 09:57

0

Isn't the generic syntax new() rather than just new?

Also, you're doing this code in the middle of a class rather than in a function, which is sure to baffle it:

_t = new T();

Edit:

Ok, so after the syntax cleanup, I'm guessing the issue is that 'baz' does not have a parameterless constructor. I don't know of any way in which you can enforce this.

There is a similar question here, which may help out a little.

edited May 23 '17 at 09:57

Community

1
1

answered Apr 16 '10 at 08:58

Fiona - myaccessible.website

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"In the middle of the class": this is just a field declaration with a initial value. – Stefan Steinegger Apr 16 '10 at 09:03
Fixed, thanks. My bad habit of half-pseudocode, half real code! – Ben Heymink Apr 16 '10 at 09:04

score 0 · Answer 6 · answered Apr 16 '10 at 09:03

0

Like workaround - You can pass delegate creating the instance of a type you need:

  Func<T> creator = ...

  _t = creator();

answered Apr 16 '10 at 09:03

Andrew Bezzub

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Why can't I enforce derived classes to have parameterless constructors?

6 Answers6