C＃无法从IEnumerable转换< Base>到IEnumerable< Derived> [英] C# Cannot convert from IEnumerable<Base> to IEnumerable<Derived>

问题描述

我最近遇到麻烦，当尝试AddRange（IEnumerable）到一个列表。

我理解，期望List参数的方法不能满足列表，因为他们可能尝试添加

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

p>

我认为代码看起来像这样：

  class Foo 
 {
} 
 
 class Bar：Foo 
 {
} 
 
 class FooCol 
 {
 private列表< 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）其中T：Foo 
 {
 m_Foos.AddRange（foos）; //不工作
} 
} 
 
类程序
 {
 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）; //不工作
 
 fooCol.AddRange2（foos）; // does work 
 fooCol.AddRange2（bars）; // does work 
} 
} 
  

我尝试传递一个提示编译器在AddRange2方法，但这只是转移到问题。

我的思维方式有缺陷吗？这是对语言的限制还是由设计？

IIRC，对这种操作的支持被添加到Java 1.5，因此也许它将被添加到C＃

$ p

解决方案

这是协方差，将在C＃4.0 /中修复。 NET 4.0。现在，通用选项是最好的答案（ IEnumerable< T> - IList< T> 等）。

但是在通用方法中，你必须考虑 T 。您也可以使用LINQ来使用 Cast< T> 或 OfType< T>

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.

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.

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

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
    }
}

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

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

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...?

解决方案

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).

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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