[英]C# dictionary initializer compilation inconsistency
The following code compiles, but fails with a NullReferenceException
: 以下代码编译,但失败并出现NullReferenceException
:
class Test
{
public Dictionary<string, string> Dictionary { get; set; }
}
static void Main(string[] args)
{
var x = new Test
{
Dictionary = // fails
{
{ "key", "value" }, { "key2", "value2" }
}
};
}
If you replace the line marked 'fails' with the following, it works (as expected): 如果用以下内容替换标记为“失败”的行,则可以正常工作(如预期的那样):
Dictionary = new Dictionary<string, string>
Is there any purpose to the failing syntax--can it be used successfully in some other case? 失败的语法是否有任何目的 - 是否可以在其他情况下成功使用? Or is this an oversight in the compiler? 或者这是编译器的疏忽?
No, it's not a mistake... it's a flaw in your understanding of initialization syntax :) 不,这不是一个错误......这是你理解初始化语法的一个缺陷:)
The idea of the 的想法
Dictionary = { ... }
is for cases where the caller has read access to a collection property, but not write access. 适用于调用者具有对集合属性的读访问权但不具有写访问权的情况。 In other words, situations like this: 换句话说,这样的情况:
class Test
{
private readonly Dictionary<string, string> dictionary
= new Dictionary<string, string>();
public Dictionary<string, string> Dictionary { get { return dictionary; } }
}
Basically it ends up being calls to Add, but without creating a new collection first. 基本上它最终会调用Add,但不会先创建新的集合。 So this code: 所以这段代码:
Test test = new Test { Dictionary = { { "a", "b"}, {"c", "d" } };
is equivalent to: 相当于:
Test tmp = new Test();
Dictionary<string, string> tmpDictionary = tmp.Dictionary;
tmpDictionary.Add("a", "b");
tmpDictionary.Add("c", "d");
Test test = tmp;
A good example of where this is useful is with the Controls
collection for a UI. 这有用的一个很好的例子是UI的Controls
集合。 You can do this: 你可以这样做:
Form form = new Form
{
Controls =
{
new Button { Text = "Hi" },
new TextBox { Text = "There" }
}
};
but you couldn't actually set the Controls
property, because it's read-only. 但你实际上无法设置 Controls
属性,因为它是只读的。
You can still use the syntax you want in a constructor: 您仍然可以在构造函数中使用所需的语法:
Dictionary<string, string> dictionary = new Dictionary<string, string>
{
{"a", "b"},
{"c", "d"}
};
It fails with a null reference exception because you have declared a variable (Dictionary) that is unintialised, hence it is null. 它因空引用异常而失败,因为您声明了一个未初始化的变量(Dictionary),因此它为null。
When you attempt to add the entries into it using the initialiser syntax, you are trying to write data into a null object. 当您尝试使用初始化语法将条目添加到其中时,您尝试将数据写入空对象。
When you replace the line with a "= new Dictionary...", you are creating a new object for Dictionary to reference, and hence you are then able to add entries into it successfully. 当您使用“= new Dictionary ...”替换该行时,您正在为要引用的Dictionary创建一个新对象,因此您可以成功地向其中添加条目。
(In Jon Skeet's example, the Controls collection must already have been created by the Form, hence it works ok) (在Jon Skeet的例子中,Controls集合必须已经由Form创建,因此它可以正常工作)
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