C＃空合并运算符返回null [英] C# null coalescing operator returning null

问题描述

最近我的同事向我展示了code块，这不是正常工作：

Recently my co-worker showed me a block of code that was not working correctly:

public class SomeClass
{
    private IList<Category> _categories;

    public void SetCategories()
    {
        _categories = GetCategories() ?? new List<Category>();
        DoSomethingElse();
    }

    public IList<Category> GetCategories()
    {
        return RetrieveCategories().Select(Something).ToList();
    }
}

（我知道，运营商是多余的，因为LINQ了ToList总是会返回一个列表，但这是code的设置方式）。

(I am aware that the operator is superfluous since the linq ToList will always return a list, but that is how the code was set up).

但问题是，_categories是空。在调试器，设置在 _categories = GetCategories断点（）？新的List＆LT;类别＆GT;（），然后踩着过来DoSomethingElse（）_ 类别仍然为空的

The problem was that _categories was null. In the debugger, setting a breakpoint on _categories = GetCategories() ?? new List<Category>(), then stepping over to DoSomethingElse(), _categories would still be null.

直接设置_categories到GetCategories（）工作的罚款。拆分？在一个完整的if语句工作得很好。空合并经营者也没有。

Directly setting _categories to GetCategories() worked fine. Splitting the ?? in to a full if statement worked fine. The null coalescing operator did not.

这是一个ASP.NET应用程序，因此在不同的线程也可能会被干扰，但是这是他的机器上，只有他在一个浏览器连接。 _cateogories也不是一成不变的，或任何东西。

It is an ASP.NET application, so a different thread could possibly be interfering, but it was on his machine, with only him connected in a browser. _cateogories is not static, or anything.

我不知道是的怎么会这样可能发生？的

编辑：

我想补充的离奇， _categories 从未设置除了该功能在任何地方的（除了初始化类的）。

Just to add to the bizarreness, _categories is never set anywhere besides that function (apart from initializing the class).

确切的code是像这样：

The exact code is like so:

public class CategoryListControl
{
    private ICategoryRepository _repo;
    private IList<Category> _categories;

    public override string Render(/* args */)
    {
        _repo = ServiceLocator.Get<ICategoryRepository>();
        Category category = _repo.FindByUrl(url);
        _categories = _repo.GetChildren(category) ?? new List<Category>();
        Render(/* Some other rendering stuff */);
    }
}

public class CategoryRepository : ICategoryRepository
{
    private static IList<Category> _categories;

    public IList<Category> GetChildren(Category parent)
    {
        return _categories.Where(c => c.Parent == parent).ToList<Category>();
    }
}

即使它的GetChildren神奇地返回null，CategoryListControl._categories还是应该永远不会为null。的GetChildren应该也永远不会回来，因为IEnumerable.ToList（）的空。

Even it GetChildren magically returned null, CategoryListControl._categories still should never, ever be null. GetChildren should also never, ever return null because of IEnumerable.ToList().

编辑2：

试行@ smartcaveman的code，我发现这样的：

Trying out @smartcaveman's code, I found out this:

Category category = _repo.FindByUrl(url);

_categories = _repo.GetChildren(category) ?? new List<Category>();

_skins = skin; // When the debugger is here, _categories is null

Renderer.Render(output, _skins.Content, WriteContent); // When the debugger is here, _categories is fine.

同时，在测试时如果（_categories == NULL）抛出新的异常（），_categories是空的if语句，然后跨过的异常没有抛出

As well, when testing if(_categories == null) throw new Exception(), _categories was null on the if statement, then stepping over the exception was not thrown.

所以，看起来这是一个调试器的bug。

So, it seems like this is a debugger bug.

推荐答案

这可能是与调试器，而不是用code的一个问题。尝试打印出的价值或聚结运营商的声明后，做一个空检查。

This could be a problem with the debugger rather than with the code. Trying printing out the value or doing a null check after the statement with the coalesce operator.

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