是否有可能合并在C＃字符串的DBNull？ [英] Is it possible to coalesce string and DBNull in C#?

问题描述

我正在写一个C＃程序调用存储过程。在我传入的参数列表，它是可能的值中的一个可以合法地为空。所以我想我会用这样一行：

I'm writing a C# routine to call a stored proc. In the parameter list I'm passing in, it is possible that one of the values can legally be null. So I thought I'd use a line like this:

cmd.Parameters.Add(new SqlParameter("@theParam", theParam ?? DBNull.Value));

不幸的是，这将返回以下错误：

Unfortunately, this returns the following error:

CS0019：操作'??'不能应用于类型为'字符串'和'System.DBNull

CS0019: Operator '??' cannot be applied to operands of type 'string' and 'System.DBNull'

现在，这似乎不够清楚，但我不明白它背后的基本原理。为什么会这样不行？ （通常，当我不明白为什么有什么不工作，这不是它的无法的工作......这就是我做错了。）

Now, this seems clear enough, but I don't understand the rationale behind it. Why would this not work? (And often, when I don't understand why something isn't working, it's not that it can't work...it's that I'm doing it wrong.)

难道我真的要舒展了这一点成为一个较长的if-then语句

Do I really have to stretch this out into a longer if-then statement?

编辑： （顺便说一句，这些建议只使用零作为是，这是行不通的。我本来想出空会自动翻译成DBNull的太多，但它显然并非如此。（谁知道？））

(As an aside, to those suggesting to just use "null" as is, it doesn't work. I originally figured null would auto-translated into DBNull too, but it apparently does not. (Who knew?))

推荐答案

不是那样的，没有。该类型必须匹配。这同样适用于三元如此。

Not like that, no. The types have to match. The same is true for the ternary.

现在，通过匹配，我不是说他们必须是相同的。但他们必须分配兼容。基本上：在相同的继承树

Now, by "match", I don't mean they have to be the same. But they do have to be assignment compatible. Basically: in the same inheritance tree.

要解决这个问题的方法之一是投你的字符串对象：

One way to get around this is to cast your string to object:

var result = (object)stringVar ?? DBNull.Value;

但我不喜欢这样，因为这意味着你更多地依赖于SqlParameter的构造函数让你的类型的权利。相反，我喜欢做这样的：

But I don't like this, because it means you're relying more on the SqlParameter constructor to get your types right. Instead, I like to do it like this:

cmd.Parameters.Add("@theParam", SqlDbTypes.VarChar, 50).Value = theParam;
// ... assign other parameters as well, don't worry about nulls yet

// all parameters assigned: check for any nulls
foreach (var p in cmd.Parameters) 
{ 
    if (p.Value == null) p.Value = DBNull.Value; 
}

还请注意，我明确声明的参数类型。

Note also that I explicitly declared the parameter type.

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