一个使用（）{}块中yield return语句执行前作主 [英] yield return statement inside a using() { } block Disposes before executing

问题描述

我写我自己的自定义数据层坚持到一个特定的文件，我已经有一个自定义的DataContext图案抽象的。

I've written my own custom data layer to persist to a specific file and I've abstracted it with a custom DataContext pattern.

这是所有基于.NET 2.0框架（目标服务器给定约束），所以尽管它的一些可能看起来像LINQ到SQL，它不是！我刚刚实施了类似的数据模式。

This is all based on the .NET 2.0 Framework (given constraints for the target server), so even though some of it might look like LINQ-to-SQL, its not! I've just implemented a similar data pattern.

请参见下面的例子，我还不能说明情况的例子。

See example below for example of a situation that I cannot yet explain.

要获得动物的所有实例 - 我做到这一点，它工作得很好。

To get all instances of Animal - I do this and it works fine

public static IEnumerable<Animal> GetAllAnimals() {
        AnimalDataContext dataContext = new AnimalDataContext();
            return dataContext.GetAllAnimals();
}

而GetAllAnimals在AnimalDataContext（）低于

And the implementation of the GetAllAnimals() method in the AnimalDataContext() below

public IEnumerable<Animal> GetAllAnimals() {
        foreach (var animalName in AnimalXmlReader.GetNames())
        {
            yield return GetAnimal(animalName);
        }
}

该AnimalDataContext（）实现IDisposable，因为我有在那里一个XmlTextReader，我想确保它得到迅速清理。

The AnimalDataContext() implements IDisposable because I've got an XmlTextReader in there and I want to make sure it gets cleaned up quickly.

现在，如果我换第一次调用using语句里面像这样

Now if I wrap the first call inside a using statement like so

public static IEnumerable<Animal> GetAllAnimals() {
        using(AnimalDataContext dataContext = new AnimalDataContext()) {
            return dataContext.GetAllAnimals();
        }
}

和把一个断点在AnimalDataContext.GetAllAnimals（）方法和在该AnimalDataContext.Dispose（）方法中的第一线另一断点的第一行，并执行...

and put a break-point at the first line of the AnimalDataContext.GetAllAnimals() method and another break-point at the first line in the AnimalDataContext.Dispose() method, and execute...

的Dispose（）方法，首先是让AnimalXmlReader.GetNames（）给出了对象引用未设置为对象实例的异常，因为AnimalXmlReader已经在Dispose（）方法???

任何想法？我有一种预感，其相关的收益回报率没有被允许一个try-catch块内被调用，它的使用有效地重新presents，编译一次..

Any ideas? I have a hunch that its related to yield return not being allowed to be called inside a try-catch block, which using effectively represents, once compiled...

推荐答案

当你调用 GetAllAnimals 它实际上并没有，直到你枚举执行任何code返回在IEnumerable的foreach循环。

When you call GetAllAnimals it doesn't actually execute any code until you enumerate the returned IEnumerable in a foreach loop.

在DataContext正在尽快处置的包装方法返回时，你列举了IEnumerable之前。

The dataContext is being disposed as soon as the wrapper method returns, before you enumerate the IEnumerable.

最简单的解决办法是使包装方法的迭代器为好，像这样的：

The simplest solution would be to make the wrapper method an iterator as well, like this:

public static IEnumerable<Animal> GetAllAnimals() {
    using (AnimalDataContext dataContext = new AnimalDataContext()) {
        foreach (var animalName in dataContext.GetAllAnimals()) {
            yield return GetAnimal(animalName);
        }
    }
}

这样的话，使用的语句将在外部迭代器进行编译，外迭代处置时，将只进行处理。

This way, the using statement will be compiled in the outer iterator, and it will only be disposed when the outer iterator is disposed.

另一种解决方案将是列举了IEnumerable在包装。要做到这一点，最简单的方法是返回一个列表与LT;动物＆GT; ，就像这样：

Another solution would be to enumerate the IEnumerable in the wrapper. The simplest way to do that would be to return a List<Animal>, like this:

public static IEnumerable<Animal> GetAllAnimals() {
    using (AnimalDataContext dataContext = new AnimalDataContext()) {
        return new List<Animal>(dataContext.GetAllAnimals());
    }
}

请注意，这个失去了延迟执行的好处，因此它会得到所有的动物，即使你不需要他们。

Note that this loses the benefit of deferred execution, so it will get all of the animals even if you don't need them.

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