来自 IEnumerable 的 foreach 中特定类型的对象 [英] Objects of a specific type in foreach from an IEnumerable

问题描述

我正在使用仅实现非通用 IEnumerable 和 ICollection 的遗留集合对象.当我尝试将这个对象与 foreach 一起使用时，这个对象究竟会发生什么，在 foreach 表达式的 LHS 上给出更具体的类型?

I'm working with a legacy collection object that only implements non-generic IEnumerable and ICollection. What exactly happens with this object when I try to use this object with a foreach giving a more specific type on the LHS of the foreach expression?

// LegacyFooCollection implements non-generic IEnumerable
LegacyFooCollection collection = GetFooCollection();
foreach (Foo f in collection)
{
    // etc.
}

我知道(因为我已经尝试过了)当 collection 中的所有内容都属于 Foo 类型时，这是安全的，但如果失败会怎样?

I know (because I've tried it) that this is safe when everything in collection really is of type Foo, but what happens if that fails?

推荐答案

C# 编译器会为您隐式执行强制转换.在转换方面(但仅在这些方面¹)它相当于:

The C# compiler performs the cast implicitly for you. In terms of the casting (but only in those terms¹) it's equivalent to:

foreach (object tmp in collection)
{
    Foo f = (Foo) tmp;
    ...
}

请注意，泛型集合也会发生这种情况:

Note that this will happen with generic collections too:

List<object> list = new List<object> { "hello", "there", 12345 };

// This will go bang on the last element
foreach (string x in list) 
{
}

这在 C# 4 规范的第 8.8.4 节中有详细说明.

This is all detailed in section 8.8.4 of the C# 4 spec.

如果您使用 .NET 3.5 或更高版本并且只想选择适当类型的项目，则可以使用 Enumerable.OfType:

If you're using .NET 3.5 or higher and you want to only select items of the appropriate type, you can use Enumerable.OfType:

LegacyFooCollection collection = GetFooCollection();
foreach (Foo f in collection.OfType<Foo>())
{
    // etc.
}

这对于 LegacyFooCollection 可能不是必需的，但是当您尝试在表单中查找(例如)所有 TextBox 控件时，它会很有用.

That may not be necessary for a LegacyFooCollection, but it can be useful when you're trying to find (say) all the TextBox controls in a form.

¹ 区别在于:

• 在你的原始代码中，f 是只读的；在转换"中它是可写的
• 在您的原始代码中，如果您捕获 f，您(当前)将在所有迭代中捕获单个变量，而不是在转换"中每次迭代捕获一个单独的变量

• In your original code, f is read-only; in the "conversion" it's writable
• In your original code, if you capture f you will (currently) capture a single variable across all iterations, as opposed to a separate variable per iteration in the "conversion"

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