C＃.net 4.5：泛型类从具有where约束的List继承

Question

Working on a big project, I realized the following snippet :

public interface Mother 
{
    void CoolFeature();
}

public interface Daughter : Mother 
{
}

public class YouJustLostTheGame<T> : List<T> where T : Mother 
{
    public void Crowd(Mother item) 
    {
       this.Add(item); 
    }

    public void useFeature() 
    {
       this.Find(a => { return true; }).CoolFeature(); 
    }
}

fails to compile the Crowd(Mother) function with the message "Unable to convert 'Test.Mother' to 'T'". Of course this seems deeply wrong to me, and the useFeature() is perfectly fine. So what am I missing ?

NB: VS2012, win7 x64, .NET 4.5

Answer 1

The reason it doesn't compile is because it cannot work. Consider

public class MotherClass : Mother
{
    // ...
}

public class DaughterClass : Daughter
{
    // ...
}

void breakThings()
{
    new YouJustLostTheGame<Daughter>().Crowd(new MotherClass());
}

YouJustLostTheGame<Daughter> derives from List<Daughter> . List<Daughter> can only store Daughter s. Your Crowd takes a Mother as its parameter, so new MotherClass() is a valid argument. It then attempts to store an item in the list that the list isn't capable of holding.

Answer 2

You need to change the method signature to accept T instead of mother. crowd(T item)....