基地> C＃不能自IEnumerable&LT转换;到了IEnumerable<衍生GT; [英] C# Cannot convert from IEnumerable<Base> to IEnumerable<Derived>

问题描述

我想的AddRange（IEnumerable的）时将列出最近遇到了麻烦。可能是一个经典的问题，但我真的不明白呢。

I recently run into trouble when trying to AddRange(IEnumerable) to a List. Probably a classic issue, but I do not really get it yet.

据我了解，期待一个列表参数的方法不满意列表，因为他们可能会增加一个基地的名单，这显然是不可能的。

I understand that methods expecting a List parameter are not satisfied with a List, because they might try to add a Base to the List, which is obviously impossible.

但是，如果我得到这个正确的，因为IEnumerables本身不能改变，它应该在这种情况下工作。

But if i get this correctly, since IEnumerables themselves cannot be changed, it ought to work in this case.

我想看起来像这样的代码：

The code i thought of looks like this:

class Foo
{
}

class Bar : Foo
{
}

class FooCol
{
	private List<Foo> m_Foos = new List<Foo> ();

	public void AddRange1(IEnumerable<Foo> foos)
	{
		m_Foos.AddRange (foos); // does work
	}

	public void AddRange2<T>(IEnumerable<T> foos) where T : Foo
	{
		m_Foos.AddRange (foos); // does not work
	}
}

class Program
{
	static void Main(string[] args)
	{
		FooCol fooCol = new FooCol ();

		List<Foo> foos = new List<Foo> ();
		List<Bar> bars = new List<Bar> ();

		fooCol.AddRange1 (foos); // does work
		fooCol.AddRange1 (bars); // does not work

		fooCol.AddRange2 (foos); // does work
		fooCol.AddRange2 (bars); // does work
	}
}

我试图通过一个提示编译器在AddRange2方法，但是这只是转移到周围的问题。

I tried to pass a hint to the compiler in the AddRange2 method, but this just moved to problem around.

时我的思维方式存在缺陷？这是语言的限制还是设计？

Is my way of thinking flawed? Is this a limitation of the language or is it by design?

IIRC，对于这种操作的支持添加到Java 1.5，所以也许它会被添加到C＃在未来的某个时刻，太...？

IIRC, support for this kind of operations was added to Java 1.5, so maybe it will be added to C# at some point in the future, too...?

推荐答案

这是协方差，并且将其固定在C＃4.0 /。 NET 4.0。目前，通用的选择是最好的答案（对于的IEnumerable< T> - 不是的IList< T> 等）

This is covariance, and will be fixed in C# 4.0 / .NET 4.0. For now, the generic option is the best answer (for IEnumerable<T> - not IList<T> etc).

但泛型方法中，你必须要考虑在 T 的条款。你也可以使用演员LT; T> 或 OfType< T> 使用LINQ来实现类似的东西。

But within the generic method, you have to think in terms of T. You could also use Cast<T> or OfType<T> with LINQ to achieve something similar.

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