Generic constraints to a specific class, why? [duplicate]

问题

I've been reading on leveraging generic constraints, I've found that generics can be constrained to struct, class, new, Class and Interface. The reasons behind the first three are pretty obvious. But I can't really understand why and when to constraint to a class. i.e

class Foo<T> where T : Bar 
class Foo<T> where T : IBar

This allows us to constrain to Bar and it's children, and IBar and implementers respectively. Can't we just program to the class, or the interface ? Because that's basically what polymorphism is for, and Microsoft Engineers are far from dumb to implement a useless feature.

What am I missing ?

Update:

My question has been marked as duplicate for : Why use generic constraints in C#

I don't think it's the case.

I went through this question thoroughly, and didn't really find the answer: It discussed the use of generic constraints in general, while here I'm asking about why do we constrain to a class or an interface, why not just program to it directly ?

回答1:

Perhaps this simple example might help.

If I have these classes:

public class ListOfCars<T> : List<T> where T : Car { }

public abstract class Car { }
public class Porsche : Car { }
public class Bmw : Car { }

...and then if I write this code:

var porsches = new ListOfCars<Porsche>();

// OK
porsches.Add(new Porsche());

//Error - Can't add BMW's to Porsche List
porsches.Add(new Bmw());

You can see that I can't add a BMW to a Porsche list, but if I just programmed off of the base class it would be allowed.

来源：https://stackoverflow.com/questions/38249592/generic-constraints-to-a-specific-class-why

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