问题

Sure, here is the translated code portion without any additional content:

我有一个在.NET Framework 4.8中的类，它同时实现了`IEnumerable<T>`和`IReadOnlyDictionary<U, T>`：

```csharp
public class Foo : IEnumerable<Bar>, IReadOnlyDictionary<int, Bar>
{
	public Bar this[int key] => /* ... */;

	public IEnumerable<int> Keys => /* ... */;

	public IEnumerable<Bar> Values => /* ... */;

	public int Count => /* ... */;

	public bool ContainsKey(int key) => /* ... */;

	public IEnumerator<Bar> GetEnumerator() => /* ... */;

	public bool TryGetValue(int key, out Bar value) => /* ... */;

	IEnumerator IEnumerable.GetEnumerator()	{ /* ... */; }

	IEnumerator<KeyValuePair<int, Bar>> IEnumerable<KeyValuePair<int, Bar>>.GetEnumerator()
    {
        /* ... */;
    }
}

在foreach循环中使用这个类是正常的：

var foo = new Foo();
foreach (var bar in foo)
{
    /* ... */
}

但如果我尝试使用IEnumerable<T>的扩展方法，例如

bool b = foo.Any();

我会得到以下错误：

CS1061 'Foo' does not contain a definition for 'Any' and no accessible extension method 'Any' accepting a first argument of type 'Foo' could be found (are you missing a using directive or an assembly reference?)

我的代码中已经包含了using System.Linq，如果我从类中删除IReadOnlyDictionary<int, Bar>，错误就消失了。为什么编译器不能看到IEnumerable<T>，当IReadOnlyDictionary<U, T>也存在时？

<details>
<summary>英文:</summary>

I have a class in .NET Framework 4.8 that implements both `IEnumerable&lt;T&gt;` and `IReadonlyDictionary&lt;U, T&gt;`:

public class Foo : IEnumerable<Bar>, IReadOnlyDictionary<int, Bar>
{
public Bar this[int key] => /* ... */;

public IEnumerable<int> Keys => /* ... */;

public IEnumerable<Bar> Values => /* ... */;

public int Count => /* ... */;

public bool ContainsKey(int key) => /* ... */;

public IEnumerator<Bar> GetEnumerator() => /* ... */;

public bool TryGetValue(int key, out Bar value) => /* ... */;

IEnumerator IEnumerable.GetEnumerator()	{ /* ... */; }

IEnumerator<KeyValuePair<int, Bar>> IEnumerable<KeyValuePair<int, Bar>>.GetEnumerator()
{
    /* ... */;
}

}

Using this class in a `foreach` loop works fine:

var foo = new Foo();
foreach (var bar in foo)
{
/* ... */
}

But if I try to use an extention method for `IEnumerable<T>`, for example

bool b = foo.Any();

I'm getting the error

> CS1061	'Foo' does not contain a definition for 'Any' and no accessible extension method 'Any' accepting a first argument of type 'Foo' could be found (are you missing a using directive or an assembly reference?)

My code already contains `using System.Linq`, and If I remove `IReadOnlyDictionary<int, Bar>` from my class the error goes away. Why can't the compiler see `IEnumerable<T>` when `IReadonlyDictionary<U, T>` is also present?

</details>


# 答案1
**得分**: 4

The compiler doesn't know whether you want to call `Any()` on:

* `IEnumerable<Bar>`, or
* `IReadOnlyDictionary<int, Bar> : IEnumerable<<KeyValuePair<int, Bar>>>`

So be explicit:

```csharp
IEnumerable<Bar> foo = ...
// or
IReadOnlyDictionary<int, Bar> foo = ...

if (foo.Any())
{
    // ...
}

But yeah, the compiler error looks confusing, it could be clearer about that ambiguity.

IEnumerable<Bar> foo = ...
// or
IReadOnlyDictionary<int, Bar> foo = ...

if (foo.Any())
{
    // ...
}

答案2

得分: 4

IReadOnlyDictionary<K, V> 继承自 IEnumerable<KeyValuePair<K, V>>。您的类同时实现了 IEnumerable<KeyValuePair<int, Bar>> 和 IEnumerable<Bar>。

扩展方法 IEnumerable.Any 是泛型的，并且是对所有 IEnumerable<T> 的扩展。

public static bool Any<TSource> (this System.Collections.Generic.IEnumerable<TSource> source);

编译器无法确定 TSource 应该是 Bar 还是 KeyValuePair<int, Bar>。

foreach 没有这个问题，只是因为您隐式地实现了 IEnumerable<Bar>。这使得编译器在成员查找时可以直接找到 Foo 上的 GetEnumerator。如果您以显式方式实现它，如下所示：

IEnumerator<Bar> IEnumerable<Bar>.GetEnumerator() => ...;

foreach 也无法确定您想要什么。按照语言规范中指定的编译器执行步骤，了解这是如何工作的。

除了将其转换为所需的 IEnumerable<T> 实例（就像在 CodeCaster 的答案中一样），您还可以通过指定 Any 的类型参数告诉编译器您想要什么 TSource：

bool b = foo.Any<Bar>();

public static bool Any<TSource> (this System.Collections.Generic.IEnumerable<TSource> source);

IEnumerator<Bar> IEnumerable<Bar>.GetEnumerator() => ...;

bool b = foo.Any<Bar>();