c# - 我什么时候应该或不应该使用泛型类型约束？

Question

我有一个基类:

public abstract class StuffBase
{
    public abstract void DoSomething();
}

还有两个派生类

public class Stuff1 : StuffBase
{
    public void DoSomething()
    {
        Console.WriteLine("Stuff 1 did something cool!");
    }
    public Stuff1()
    {
        Console.WriteLine("New stuff 1 reporting for duty!");
    }
}

public class Stuff2 : StuffBase
{
    public void DoSomething()
    {
        Console.WriteLine("Stuff 2 did something cool!");
    }
    public Stuff1()
    {
        Console.WriteLine("New stuff 2 reporting for duty!");
    }
}

好的，现在假设我有一个项目列表:

var items = new List<StuffBase>();
items.Add(new Stuff1());
items.Add(new Stuff2());

我希望他们都调用他们的 DoSomething() 方法。我可能期望只是迭代列表并调用他们的 DoSomething() 方法，所以假设我有一个方法来执行此操作，称为 AllDoSomething()，它只是迭代列表并完成工作:

public static void AllDoSomething(List<StuffBase> items)
{
    items.ForEach(i => i.DoSomething());
}

以下方法的实际区别是什么？

public static void AllDoSomething<T>(List<T> items) where T: StuffBase
{
    items.ForEach(i => i.DoSomething());
}

虽然语法不同，但实际上这两种方法都在做同样的事情。

它们只是做同一件事的不同方式吗？我了解泛型和类型约束，但不明白为什么在这种情况下我会使用一种方式而不是另一种方式。

Answer 1

这是因为到目前为止，C# 还不支持 Covariance .

More formally, in C# v2.0 if T is a subtype of U, then T[] is a subtype of U[], but G is not a subtype of G (where G is any generic type). In type-theory terminology, we describe this behavior by saying that C# array types are “covariant” and generic types are “invariant”.

引用:http://blogs.msdn.com/rmbyers/archive/2005/02/16/375079.aspx

如果你有以下方法:

public static void AllDoSomething(List<StuffBase> items)
{
    items.ForEach(i => i.DoSomething());
}

var items = new List<Stuff2>();
x.AllDoSomething(items); //Does not compile

如果您使用泛型类型约束，它会。

有关协变和逆变的更多信息]，请查看 Eric Lippert's series of posts .

---

其他值得一读的帖子:

• http://www.pabich.eu/blog/archive/2008/02/12/c-generics---parameter-variance-its-constraints-and-how-it.aspx
• http://blogs.msdn.com/rmbyers/archive/2006/06/01/613690.aspx
• http://msdn.microsoft.com/en-us/library/ms228359(VS.80).aspx
• http://www.csharp411.com/convert-between-generic-ienumerablet/
• http://research.microsoft.com/apps/pubs/default.aspx?id=64042
• Why can't List<parent> = List<child>?