blocks|key|700536|text|我非常肯定，这个限制是创建包装器的功能原因之一。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|700537|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

I am pretty sure that this restriction is one of the functional reasons for the creation of the wrapper in the first place.

blocks|key|352591|text|如果您使用Reflector查看代码，您将看到Dictionary试图查找键，如果它找不到键，则显式地引发异常。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|352592|public+TValue+get_Item(TKey+key)
{
++++int+index+=+this.FindEntry(key);
++++if+(index+>=+0)
++++{
++++++++return+this.entries[index].value;
++++}
++++ThrowHelper.ThrowKeyNotFoundException();
++++return+default(TValue);
}|code-block|syntax|javascript|352593|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

If you look to the code using Reflector you'll see that Dictionary tries to find the key and explicitly raises an exception if it doesn't find the key.

<pre><code>public TValue get_Item(TKey key)
{
 int index = this.FindEntry(key);
 if (index &gt;= 0)
 {
 return this.entries[index].value;
 }
 ThrowHelper.ThrowKeyNotFoundException();
 return default(TValue);
}
</code></pre>

blocks|key|700571|text|在你的岗位上有一个误解。字典不是Hashtable的包装器。这是一个完全不同的实现。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|700572|之所以进行此更改，主要是因为有一个断言:+Null是哈希表的一个有效值。如果不进行这种更改，就不可能使用[]访问方法区分不存在的键和具有空值的值键。字典把这个弄清楚了。|700573|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|F|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|G|8|@]|9|@]|A|$]]|$1|D|3|-4|5|6|7|H|8|@]|9|@]|A|$]]]|E|$]]

There is one misconception in your post. Dictionary is not a wrapper around Hashtable. It's a completely different implementation. 

The reason why this change was made is mainly justified by one assertion: Null is a valid value for a hashtable. Without this change it's not possible to distinguish between a non-exsistent key and a value key with a null value using the [] method of access. Dictionary clears this up.

blocks|key|713990|text|Dictionary.ContainsKey可能比TryGetValue更适合你。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|713991|但是为什么我不知道。|713992|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|F|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|G|8|@]|9|@]|A|$]]|$1|D|3|-4|5|6|7|H|8|@]|9|@]|A|$]]]|E|$]]

Dictionary.ContainsKey is probably better for you than TryGetValue.

But as to why, no idea.

blocks|key|351649|text|我为这个写了一个扩展。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|351650|public+static+class+DictionaryExtension
{
++++public+static+TValue+GetValueOrDefault<TKey,+TValue>(this+Dictionary<TKey,+TValue>+items,+string+key)
++++{
++++++++if+(items+!=+null+&&+items.ContainsKey(key))
++++++++{
++++++++++++return+items[key];
++++++++}

++++++++return+default(TValue);
++++}
}|code-block|syntax|javascript|351651|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

I wrote an extension for this.

<pre><code>public static class DictionaryExtension
{
 public static TValue GetValueOrDefault&lt;TKey, TValue&gt;(this Dictionary&lt;TKey, TValue&gt; items, string key)
 {
 if (items != null &amp;&amp; items.ContainsKey(key))
 {
 return items[key];
 }

 return default(TValue);
 }
}
</code></pre>

blocks|key|351658|text|Dictionary<T>和Hashtable的区别在于，Dictionary<T>是一种泛型类型，可以专门存储引用类型的值类型。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|351659|哈希表只能存储引用类型(通过引用传递的Object+)和仅存储装箱的值类型(也可以通过引用传递)。|351660|当字典对值类型进行专门化时，它必须返回那些值，而不是reference.So返回的值，因此，Dictionary<T>不能返回null，因为null是而不是值类型的有效值。|BOLD|351661|entityMap^0|0|D|E|9|T|D|0|J|6|0|1A|D|23|A|0^^$0|@$1|2|3|4|5|6|7|M|8|@$9|N|A|O|B|C]|$9|P|A|Q|B|C]|$9|R|A|S|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|T|8|@$9|U|A|V|B|C]]|D|@]|E|$]]|$1|H|3|I|5|6|7|W|8|@$9|X|A|Y|B|C]|$9|Z|A|10|B|J]]|D|@]|E|$]]|$1|K|3|-4|5|6|7|11|8|@]|D|@]|E|$]]]|L|$]]

The difference between a <code>Dictionary&lt;T&gt;</code> and a <code>Hashtable</code> is that a <code>Dictionary&lt;T&gt;</code> is a generic type that can specialize to store value types along reference types.

The hashtable can only store reference types (a <code>Object</code> passed by reference) and only value types that are boxed (also passed by reference).

When a dictionary is specialized with value types, it must return those values 'by value' not by reference.So, therefore, a <code>Dictionary&lt;T&gt;</code> can't return null, as null is not a valid value for value types.

If I'm using a <code>Hashtable</code>, I can write code like this:

<pre><code>object item = hashtable[key] ?? default_value;
</code></pre>

That works whether or not <code>key</code> appears in the <code>Hashtable</code>.

I can't do that with a <code>Dictionary&lt;TKey. TValue&gt;</code>. If the key's not present in the dictionary, that will throw a <code>KeyNotFoundException</code>. So I have to write code like this:

<pre><code>MyClass item;
if (!(dict.TryGetValue(key, out item))
{
 item = default_value;
}
</code></pre>

I'm wondering why this is. <code>Dictionary&lt;TKey, TValue&gt;</code> is just a wrapper around <code>Hashtable</code>. Why has this restriction been added to it?

Edit:

For another perspective on PopCatalin's answer (see below), the code I'd written above won't work if the dictionary's values are of a value type. If I'm using a <code>Dictionary&lt;int, int&gt;</code>, then the code I would like to use looks like this:

<pre><code>int i = dict[key] ?? default_value;
</code></pre>

And that won't compile, because <code>dict[key]</code> isn't a nullable or reference type.

Why doesn't a Dictionary access nonexistent keys like a Hashtable does?

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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如果我使用的是Hashtable，我可以编写如下代码：object item = hashtable[key] ?? default_value;无论key是否出现在Hashtable中，它都能工作。我不能用Dictionary<TKey. TValue>做这件事。如果字典中没有密钥，就会抛出一个KeyNotFound...

问为什么字典不像Hashtable那样访问不存在的键？
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问为什么字典不像Hashtable那样访问不存在的键？EN