C# 2.0: new() constraint on constructors having parameters?

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Axel Dahmen - 07 Mar 2008 15:09 GMT

Hi,

I've created a generic class having a type argument where I want to use
classes that have a parameterized constructor. I'm calling these constructors
from within the generic class.

Unfortunately I can't use this construct as the C# compiler requires me to
use the new() constraint, but the classes I'm using for the type argument
actually *don't* have a parameterless constructor. They just contain a
constructor having parameters.

Can someone please enlightening me on how to get this to work in C# without
having to implement useless (and invalid) parameterless constructors?

TIA,
Axel Dahmen

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Marc Gravell - 07 Mar 2008 15:22 GMT

new() is the only constructor constraint, so you have three options:

0: revise your design so that you *can* use the default ctor and new()
1: pass in (or otherwise obtain) a factory for creating the objects
2: "wing it"... i.e. hope that the constructor you want exists, and invoke
it anyway

1 might be via either a delegate or an interface, but is a nuicance. 2 gives
less compile-time safety. If this is for moderate usage, then
Activator.CreateInstance has an overload that allows you to pass in
arguments. I also have some .NET 3.5 code for creating constructor delegates
from the type, but you should probably think about the other options
first...

Marc

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Jon Skeet [C# MVP] - 07 Mar 2008 15:26 GMT

> I've created a generic class having a type argument where I want to use
> classes that have a parameterized constructor. I'm calling these constructors
[quoted text clipped - 4 lines]
> actually *don't* have a parameterless constructor. They just contain a
> constructor having parameters.

In that case the new() constraint is no use to you, and indeed there
aren't any constraints which will do this for you.

> Can someone please enlightening me on how to get this to work in C# without
> having to implement useless (and invalid) parameterless constructors?

You can't specify a constructor with parameters - you'll have to either
use reflection, or have a separate factory type (which could easily
have a parameterless constructor).

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Christopher Van Kirk - 07 Mar 2008 15:39 GMT

>Hi,
>
[quoted text clipped - 12 lines]
>TIA,
>Axel Dahmen

I'm struggling to understand what you're trying to do here. Why not
simply leave off the constraint? I have plenty of generic classes
which have no declared parameterless constructor.

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Jon Skeet [C# MVP] - 07 Mar 2008 15:41 GMT

On Mar 7, 3:39 pm, Christopher Van Kirk <chris.vank...@fdcjapan.com>
wrote:

<snip>

> I'm struggling to understand what you're trying to do here. Why not
> simply leave off the constraint? I have plenty of generic classes
> which have no declared parameterless constructor.

The problem is that there's then no compile-time-safe way of creating
new instances of the type parameter, which I assume is what the OP
wants. However, he wants a compile-time-safe way of doing it *with
parameters*.

Jon

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Axel Dahmen - 07 Mar 2008 16:11 GMT

Hi lads,

thanks for all your answers and trying to help.

Actually I didn't want to use that constraint in the first place but I get a
compiler error if I omit it. That's because in some of the generic class's
member functions return a new object based on another function's return value:

        public T Add(string name, string schemaClassName)
        {
            return new T(_collection.Add(name, schemaClassName));
        }

(I'm actually trying to create a typed version of
System.DirectoryServices.DirectoryEntries)

Axel Dahmen

---------------------

> On Mar 7, 3:39 pm, Christopher Van Kirk <chris.vank...@fdcjapan.com>
> wrote:
[quoted text clipped - 11 lines]
>
> Jon

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Duy Lam - 09 Mar 2008 15:17 GMT

> Hi lads,
>
[quoted text clipped - 13 lines]
>
> Axel Dahmen

I think you can work around with problem by do this: use where and new
keyword. All thing you need is add parameterless constructor and expose
parameters on constructor to public.

public class MyList<T> where T: SomeService,new()
{
    ......
    public T Add(string name,string schemaClassName)
    {
        T newObject = new T();
        T.NewItemIndex = _collection.Add(name, schemaClassName);
    }
    ......
}

class SomeService will have 2 constructor:

public SomeService() { }

public SomeService(int newIndex) { ... }

And it also have an public property NewItemIndex. I assumed that Adđ()
method will return an integer number.

Hope this help.

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Thanks,
Duy Lam Phuong

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Jon Skeet [C# MVP] - 09 Mar 2008 15:54 GMT

> I think you can work around with problem by do this: use where and new
> keyword. All thing you need is add parameterless constructor and expose
> parameters on constructor to public.

The OP's whole problem is that he *doesn't* want to have a
parameterless constructor - he wants to be able to use constructors
which do have parameters, but in a safe way.

Setting properties afterwards has distinct downsides, not least because
it means you can't make the type immutable.

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World class .NET training in the UK: http://iterativetraining.co.uk

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asdf asdf - 19 Jun 2008 05:46 GMT

Try this,

var t = typeof (T);
var genericObject = (T) Activator.CreateInstance(t, new object[2] {name,
schemaClassName});

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Christopher Van Kirk - 07 Mar 2008 16:16 GMT

>On Mar 7, 3:39 pm, Christopher Van Kirk <chris.vank...@fdcjapan.com>
>wrote:
[quoted text clipped - 11 lines]
>
>Jon

Ahh well, can't have everything I guess.

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Axel Dahmen - 07 Mar 2008 16:44 GMT

Thanks again guys,

I've created a suggestion for C# at Connect
https://connect.microsoft.com/VisualStudio/feedback/ViewFeedback.aspx?FeedbackID
=332050

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Cowboy (Gregory A. Beamer) - 07 Mar 2008 16:04 GMT

My advice would be to alter the architecture slightly so you can use the
parameterless constructor, even if you have to add one. This may require
re-architecting other parts of your application(s). Teh only other options I
can think of are a) reflection (it works in my mind, not sure in real life,
as you are working from a generic class) or b) fire and forget (which is
dangerous). Mentally, a chain of generics might work, but I have not played
in that playground either.

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Gregory A. Beamer
MVP, MCP: +I, SE, SD, DBA

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> Hi,
>
[quoted text clipped - 14 lines]
> TIA,
> Axel Dahmen

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C# 2.0: new() constraint on constructors having parameters?

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