这正是C＃编译器做的，但它隐藏在幕后。

要注意的一点，这可能不是很明显，否则：的IEnumerator＆LT; T＆GT; 工具的IDisposable ，和的foreach 循环部署的迭代器结束。这是非常重要的 - 所以，如果你的不的创建自己执行这一（和我建议你不这样做，坦率地说），你需要调用处置从迭代器返回 source.GetEnumerator（）在自己的处置方法。

What's the translation of this snippet in VB .NET?

public static IEnumerable<TSource> Where<TSource>(
this IEnumerable<TSource> source,
Func<TSource, Boolean> predicate)
{
  foreach (TSource element in source)
  {
    if (predicate(element))
    yield return element;
  }
}

The problem here isn't converting an extension method - it's converting an iterator block (the method uses yield return. VB doesn't have any equivalent language construct - you'd have to create your own implementation of IEnumerable<T> which did the filtering, then return an instance of the class from the extension method.

That's exactly what the C# compiler does, but it's hidden behind the scenes.

One point to note, which might not be obvious otherwise: IEnumerator<T> implements IDisposable, and a foreach loop disposes of the iterator at the end. This can be very important - so if you do create your own implementation of this (and I'd recommend that you don't, frankly) you'll need to call Dispose on the iterator returned from source.GetEnumerator() in your own Dispose method.