blocks|key|5117882|text|简短的答案是因为规范上说这是不合法的。;-)|type|unstyled|depth|inlineStyleRanges|entityRanges|data|5117883|一般来说，这不是你想要做的。成员的类型应该是IDictionary<string,+string>而不是Dictionary<string,+string>。它很小，但通常最好在外部可见的对象中使用接口，这样您可以在以后更改类型，而不会影响代码的客户端。编译器只是给你一个小小的推动，引导你朝那个方向前进。|offset|length|style|CODE|5117884|entityMap^0|0|M|R|1G|Q|0^^$0|@$1|2|3|4|5|6|7|J|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|K|8|@$D|L|E|M|F|G]|$D|N|E|O|F|G]]|9|@]|A|$]]|$1|H|3|-4|5|6|7|P|8|@]|9|@]|A|$]]]|I|$]]

The short answer is because the spec says it's not legal. ;-)

Generally, this is not what you want to do anyway. The type of the member should be <code>IDictionary&lt;string, string&gt;</code> not <code>Dictionary&lt;string, string&gt;</code>. It's a small nit but generally it's better to use an interface in an externally visible object so you can change the type later without affecting the clients of the code. The compiler is just giving you a little nudge to guide you that direction.

blocks|key|3707269|text|我关于这个主题的文章：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3707270|Why+no+var+on+fields?|offset|length|3707271|总结一下：|3707272|3707273|如果我们有"var“字段，那么在分析表达式之前不能确定字段的类型，这发生在我们已经需要知道字段的类型之后，|ordered-list-item|3707274|如果这些引用中有很长的链，甚至循环怎么办？所有这些算法都必须重新编写和测试，在这种情况下，顶级类型信息是从中确定的，而不是被所使用。|style|BOLD|3707275|如果你有"var“字段，那么初始化器可能是匿名类型的。假设该字段是公共的。在CLR或CLS中还没有任何标准来说明公开匿名类型字段的正确方式是什么。|3707276|3707277|entityMap|0|LINK|mutability|MUTABLE|url|http://blogs.msdn.com/ericlippert/archive/2009/01/26/why-no-var-on-fields.aspx^0|0|0|L|0|0|0|0|0|1H|D|0|0|21|0|0^^$0|@$1|2|3|4|5|6|7|10|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|11|8|@]|9|@$D|12|E|13|1|14]]|A|$]]|$1|F|3|G|5|6|7|15|8|@]|9|@]|A|$]]|$1|H|3|-4|5|6|7|16|8|@]|9|@]|A|$]]|$1|I|3|J|5|K|7|17|8|@]|9|@]|A|$]]|$1|L|3|M|5|K|7|18|8|@$D|19|E|1A|N|O]]|9|@]|A|$]]|$1|P|3|Q|5|K|7|1B|8|@$D|1C|E|1D|N|O]]|9|@]|A|$]]|$1|R|3|-4|5|6|7|1E|8|@]|9|@]|A|$]]|$1|S|3|-4|5|6|7|1F|8|@]|9|@]|A|$]]]|T|$U|$5|V|W|X|A|$Y|Z]]]]

My article on the subject:

<a href="http://blogs.msdn.com/ericlippert/archive/2009/01/26/why-no-var-on-fields.aspx" rel="noreferrer">Why no var on fields?</a>

To summarize:

<ol>
<li>If we have "var" fields then the type of the field cannot be determined until the expression is analyzed, and that happens after we already need to know the type of the field.</li>
<li>What if there are long chains, or even cycles in those references? All of those algorithms would have to be rewritten and tested in a world where top-level type information is being determined from them rather than being consumed by them.</li>
<li>If you have "var" fields then the initializer could be of anonymous type. Suppose the field is public. There is not yet any standard in the CLR or the CLS about what the right way to expose a field of anonymous type is.</li>
</ol>

blocks|key|3707301|text|从C#+reference|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|3707302|3707303|从Visual+C#+3.0开始，在方法作用域中声明为的变量可以具有隐式类型var。|unordered-list-item|style|BOLD|3707304|3707305|也来自The+C#+Programming+Reference|3707306|仅当在同一语句中声明并初始化局部变量时，才能使用|3707307|var；该变量不能初始化为null，也不能初始化为方法组或匿名函数。|3707308|变量不能用于类范围内的字段。|3707309|3707310|它并不是为你想要的用途而设计的。|3707311|它的主要目标是允许在代码中支持匿名类型，另外的优点是允许以一种简洁的方式指定局部变量。|3707312|entityMap|0|LINK|mutability|MUTABLE|url|http://msdn.microsoft.com/en-us/library/bb383973.aspx|1|http://msdn.microsoft.com/en-us/library/bb384061.aspx^0|1|C|0|0|0|R|F|0|0|0|V|3|S|1|0|0|O|0|0|Y|0|0|E|0|0|0|G|0|0|17|0^^$0|@$1|2|3|4|5|6|7|17|8|@]|9|@$A|18|B|19|1|1A]]|C|$]]|$1|D|3|-4|5|6|7|1B|8|@]|9|@]|C|$]]|$1|E|3|F|5|G|7|1C|8|@$A|1D|B|1E|H|I]]|9|@]|C|$]]|$1|J|3|-4|5|6|7|1F|8|@]|9|@]|C|$]]|$1|K|3|L|5|6|7|1G|8|@$A|1H|B|1I|H|I]]|9|@$A|1J|B|1K|1|1L]]|C|$]]|$1|M|3|N|5|6|7|1M|8|@$A|1N|B|1O|H|I]]|9|@]|C|$]]|$1|O|3|P|5|G|7|1P|8|@$A|1Q|B|1R|H|I]]|9|@]|C|$]]|$1|Q|3|R|5|G|7|1S|8|@$A|1T|B|1U|H|I]]|9|@]|C|$]]|$1|S|3|-4|5|6|7|1V|8|@]|9|@]|C|$]]|$1|T|3|U|5|6|7|1W|8|@$A|1X|B|1Y|H|I]]|9|@]|C|$]]|$1|V|3|W|5|6|7|1Z|8|@$A|20|B|21|H|I]]|9|@]|C|$]]|$1|X|3|-4|5|6|7|22|8|@]|9|@]|C|$]]]|Y|$Z|$5|10|11|12|C|$13|14]]|15|$5|10|11|12|C|$13|16]]]]

From the <a href="http://msdn.microsoft.com/en-us/library/bb383973.aspx" rel="noreferrer">C# reference</a>

<ul>
<li>Beginning in Visual C# 3.0, variables
that are declared at method scope
can have an implicit type var.</li>
</ul>

Also from <a href="http://msdn.microsoft.com/en-us/library/bb384061.aspx" rel="noreferrer">The C# Programming Reference</a>

<ul>
<li>var can only be used when a local variable is declared and initialized in the same statement; the variable cannot be initialized to null, or to a method group or an anonymous function. </li>
<li>var cannot be used on fields at class scope.</li>
</ul>

It just isn't intended for the usage you have in mind. 

It's primary aim is to allow the support of anonymous types in your code, with the added advantage of allowing a nice terse way of specifying local variables.

I wanted to use the <code>var</code> keyword to declare a field in my class however <code>var</code> only seems to work inside methods.

The code I have looks like:

<pre><code>public static Dictionary&lt;string, string&gt; CommandList = new Dictionary&lt;string, string&gt;{};
</code></pre>

and I wanted to have:

<pre><code>public static var CommandList = new Dictionary&lt;string, string&gt;
</code></pre>

How come this isn't possible?

Using var outside of a method

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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我想使用var关键字在我的类中声明一个字段，但是var似乎只在方法中起作用。我的代码看起来像这样：public static Dictionary<string, string> CommandList = new Dictionary<string, string>{};我想要有：public static var ...

问在方法外部使用var
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