有没有办法在没有通用约束的情况下强制执行无参数构造函数

Question

I have an interface ISnack which when implemented by a class, it should have a default parameterless constructor. Basically this:

public interface ISnack<T> where T : new()
{

}

I use <T> where T : new() just to enforce the parameterless constructor.

I would then implement the interface this way:

public class Cutlet : ISnack<Cutlet>
{

}

This works and it simply ensures Cutlet class has a parameterless constructor.

Now I have an abstract base class Kitchen :

public abstract class Kitchen<T> where T : ISnack
{

}

The requirement is that Kitchen should have constraint where T should be an ISnack . But this wont work because there exists no ISnack , but only ISnack<T> .

If I tried this

public abstract class Kitchen<T> where T : ISnack<T>
{

}

it wouldn't compile ( '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 'ISnack<T>' ) and also wouldn't make sense in my context.

If I could force ISnack s to have a parameterless constructor without constraining by a T type parameter, then T in Kitchen<T> could easily be an ISnack . How to go about it?

Answer 1

You can't unless you add the constraint; generic constraints are cumulative, so to make the compiler happy you would have to have:

public abstract class Kitchen<T> where T : ISnack<T>, new()

If that is fine, then do that . If it isn't fine, then you'll have to remove the : new from the original, and make do without it. This isn't as bad as it sounds, but it means you push validation down to execution rather than compilation. But: Activator.CreateInstance<T>() still does what you would need, anyway - even without the new() constraint. So you can replace:

T newObj = new T(); // validated by the compiler

with:

T newObj = Activator.CreateInstance<T>(); // not validated until executed

A handy trick when removing constraints can be: add a unit/integration test that finds the candidate types via reflection, and validate the missing constraint as part of your test suite .

Answer 2

You can use a second generic parameter:

abstact class Kitchen<T, S> 
    where T : ISnack<S> 
    where S : new()
....

This will solve your issue.

Adding a second parameter to a class also can cause some issues which i've faced since .NET 2.0 is available. Some complex situations may require to add more generic parameters to classes than you like to. Normally i break down the generic chain by adding more direct casts ( like (SpecificType)base.MyTypeTProperty ). Comment: I try to find a sample later

Answer 3

只需再次将约束添加到T.

public abstract class Kitchen<T> where T : ISnack<T>, new()   {      }