在净2.5我通常可以得到一个价值，它的类型默认

 如果（MyString的==默认（字符串））
 

不过，我当我尝试运行在默认的相等比较得到下面的异常KeyValuePair和KeyValuePair

code样品（从pre-扩展方法，原拉姆达静态ListUtilities类:)）

 公共静态TKEY的
        FirstKeyOrDefault＆LT; TKEY的，TValue＆GT;（词典＆LT; TKEY的，TValue＆GT;查找，
                   predicate＆LT; KeyValuePair＆LT; TKEY的，TValue＆GT;＆GT; predicate）
{
    KeyValuePair＆LT; TKEY的，TValue＆GT;对= FirstOrDefault（查找，predicate）;

    回归对==默认（KeyValuePair＆LT; TKEY的，TValue＆GT;）？
                   默认（TKEY的）：pair.Key;
}
 

例外：

算'=='不能被应用到   类型的操作数   System.Collections.Generic.KeyValuePair＆LT;字符串，对象>   和   System.Collections.Generic.KeyValuePair＆LT;字符串，对象>

是因为，作为一个结构，该KeyValuePair不能为空？如果是这样的话，为什么，因为，presumably，默认情况下实施，以处理不空类型？

修改

有关的记录，我选择了@克里斯汉农作为选择的答案，因为他给了我什么，我一直在寻找，最优雅的选择，和一个简洁的解释，然而，我鼓励阅读@Dasuraga一个非常COM prehensive解释，为什么是这样的话

这是因为 KeyValuePair＆LT; TKEY的，TValue＆GT; 不定义一个定制的==操作符，而不是包括在值类型的predefined列表可以使用它。

下面是一个链接 MSDN文档为该运营商。

有关predefined值类型，如果它的操作数的值相等等号（= =）返回true，否则为false。

您最好在这种情况下，同性检查的赌注，因为这不是一个结构，你可以控制，是调用默认（KeyValuePair＆LT; TKEY的，TValue＆GT;）。等于（对）代替。

In .Net 2.5 I can usually get an equality comparison (==) between a value and its type default

if (myString == default(string))

However I get the following exception when I try to run an equality comparison on a default KeyValuePair and a KeyValuePair

Code Sample (from a pre-extension method, proto-lambda static ListUtilities class :) )

public static TKey 
        FirstKeyOrDefault<TKey, TValue>(Dictionary<TKey, TValue> lookups, 
                   Predicate<KeyValuePair<TKey, TValue>> predicate)
{
    KeyValuePair<TKey, TValue> pair = FirstOrDefault(lookups, predicate);

    return pair == default(KeyValuePair<TKey, TValue>) ? 
                   default(TKey) : pair.Key;
}

Exception:

Operator '==' cannot be applied to operands of type 'System.Collections.Generic.KeyValuePair<string,object>' and 'System.Collections.Generic.KeyValuePair<string,object>'

Is it because, as a struct, the KeyValuePair is not nullable? If this is the case, why, as, presumably, default was implemented to handle not nullable types?

EDIT

For the record, I chose @Chris Hannon as selected answer, as he gave me what I was looking for, the most elegant option, and a succinct explanation, however I do encourage reading @Dasuraga for a very comprehensive explanation as to why this is the case

This happens because KeyValuePair<TKey, TValue> does not define a custom == operator and is not included in the predefined list of value types that can use it.

Here is a link to the MSDN documentation for that operator.

For predefined value types, the equality operator (==) returns true if the values of its operands are equal, false otherwise.

Your best bet for an equality check in this case, because this is not a struct you have control over, is to call default(KeyValuePair<TKey,TValue>).Equals(pair) instead.