blocks|key|1280221|text|不-这不安全。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1280222|这行代码：|1280223|bool+b+=+foo.bar.Value+??+false;|code-block|syntax|javascript|1280224|如果foo.bar没有值，将抛出InvalidOperationException。|1280225|相反，请使用|1280226|var+b+=+foo.bar+??+false;|1280227|1280228|更新-我刚从其他答案中了解到.GetValueOrDefault();+-这看起来是一个很好的建议！|offset|length|style|BOLD|CODE|1280229|1280230|更新2+-@ProgrammingHero的答案也是正确的(增加了%2B1！)-这行：|1280231|bool+b+=+foo.bar.Value+??+false|1280232|实际上不会编译-因为Error++50++Operator+'??'+cannot+be+applied+to+operands+of+type+'bool'+and+'bool'|1280233|entityMap^0|0|0|0|0|0|0|0|0|2|E|L|0|0|0|3|0|0|A|28|0^^$0|@$1|2|3|4|5|6|7|15|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|16|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|17|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|18|8|@]|9|@]|A|$]]|$1|K|3|L|5|6|7|19|8|@]|9|@]|A|$]]|$1|M|3|N|5|F|7|1A|8|@]|9|@]|A|$G|H]]|$1|O|3|-4|5|6|7|1B|8|@]|9|@]|A|$]]|$1|P|3|Q|5|6|7|1C|8|@$R|1D|S|1E|T|U]|$R|1F|S|1G|T|V]]|9|@]|A|$]]|$1|W|3|-4|5|6|7|1H|8|@]|9|@]|A|$]]|$1|X|3|Y|5|6|7|1I|8|@$R|1J|S|1K|T|U]]|9|@]|A|$]]|$1|Z|3|10|5|F|7|1L|8|@]|9|@]|A|$G|H]]|$1|11|3|12|5|6|7|1M|8|@$R|1N|S|1O|T|V]]|9|@]|A|$]]|$1|13|3|-4|5|6|7|1P|8|@]|9|@]|A|$]]]|14|$]]

No - this is not safe.

The line:

<pre><code>bool b = foo.bar.Value ?? false;
</code></pre>

will throw an InvalidOperationException if foo.bar has no value.

Instead use 

<pre><code>var b = foo.bar ?? false;
</code></pre>

<hr>

Update - I just learned about <code>.GetValueOrDefault();</code> from the other answers - that looks like a very good suggestion to use!

<hr>

Update 2 - @ProgrammingHero's answer is also correct (+1 added!) - the line:

<pre><code>bool b = foo.bar.Value ?? false
</code></pre>

actually won't compile - because of <code>Error 50 Operator '??' cannot be applied to operands of type 'bool' and 'bool'</code>

blocks|key|1280251|text|不是的。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1280252|1280253|如果赋值，则Value属性返回一个值，否则将引发System.InvalidOperationException。|blockquote|1280254|1280255|1280256|来自：http://msdn.microsoft.com/en-us/library/1t3y8s4s%2528v=vs.80%2529.aspx|offset|length|1280257|因此，如果hasValue为false，那么当您尝试运行第二个程序时，将在第二个程序中抛出异常。|1280258|entityMap|0|LINK|mutability|MUTABLE|url|http://msdn.microsoft.com/en-us/library/1t3y8s4s%2528v=vs.80%2529.aspx^0|0|0|0|0|0|3|1U|0|0|0^^$0|@$1|2|3|4|5|6|7|V|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|W|8|@]|9|@]|A|$]]|$1|C|3|D|5|E|7|X|8|@]|9|@]|A|$]]|$1|F|3|-4|5|6|7|Y|8|@]|9|@]|A|$]]|$1|G|3|-4|5|6|7|Z|8|@]|9|@]|A|$]]|$1|H|3|I|5|6|7|10|8|@]|9|@$J|11|K|12|1|13]]|A|$]]|$1|L|3|M|5|6|7|14|8|@]|9|@]|A|$]]|$1|N|3|-4|5|6|7|15|8|@]|9|@]|A|$]]]|O|$P|$5|Q|R|S|A|$T|U]]]]

Nope.

<blockquote>
 The Value property returns a value if
 one is assigned, otherwise a
 System.InvalidOperationException is
 thrown.
</blockquote>

From: <a href="http://msdn.microsoft.com/en-us/library/1t3y8s4s%28v=vs.80%29.aspx" rel="nofollow">http://msdn.microsoft.com/en-us/library/1t3y8s4s%28v=vs.80%29.aspx</a>

So if hasValue is false then you will get an exception thrown in your second one when you try to run it.

blocks|key|1280273|text|使用ffoo.bar+??+false会更安全|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1280274|entityMap^0|2|H|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|K|8|@]|D|@]|E|$]]]|G|$]]

<code>ffoo.bar ?? false</code> would be more safer to use

blocks|key|2895651|text|你想要的是：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2895652|bool+b+=+foo.bar+??+false;|code-block|syntax|javascript|2895653|这(令人惊讶地)是安全的，并且是null-coalescing运算符的预期用途。|2895654|2895655|+The+?？运算符称为空合并运算符，用于定义、可为空值类型、以及引用类型的默认值。如果左操作数不为空，则返回左操作数；否则返回右操作数。|blockquote|offset|length|style|BOLD|2895656|2895657|2895658|来源：http://msdn.microsoft.com/en-us/library/ms173224.aspx|2895659|在Nullable<T>的情况下，它在功能上等同于Nullable.GetValueOrDefault(T+defaultValue)。|CODE|2895660|代码：|2895661|bool+b+=+foo.bar.Value+??+false;|2895662|将导致编译器错误，因为您不能将运算符应用于值类型，并且Nullable<T>.Value总是返回值类型(或者在没有值时抛出异常)。|2895663|entityMap|0|LINK|mutability|MUTABLE|url|http://msdn.microsoft.com/en-us/library/ms173224.aspx|1|http://msdn.microsoft.com/en-us/library/3d6d4f1d.aspx^0|0|0|0|0|N|8|0|0|0|3|1H|0|0|1|B|P|16|P|16|1|0|0|0|R|H|0^^$0|@$1|2|3|4|5|6|7|1D|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|1E|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|1F|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|1G|8|@]|9|@]|A|$]]|$1|J|3|K|5|L|7|1H|8|@$M|1I|N|1J|O|P]]|9|@]|A|$]]|$1|Q|3|-4|5|6|7|1K|8|@]|9|@]|A|$]]|$1|R|3|-4|5|6|7|1L|8|@]|9|@]|A|$]]|$1|S|3|T|5|6|7|1M|8|@]|9|@$M|1N|N|1O|1|1P]]|A|$]]|$1|U|3|V|5|6|7|1Q|8|@$M|1R|N|1S|O|W]|$M|1T|N|1U|O|W]]|9|@$M|1V|N|1W|1|1X]]|A|$]]|$1|X|3|Y|5|6|7|1Y|8|@]|9|@]|A|$]]|$1|Z|3|10|5|D|7|1Z|8|@]|9|@]|A|$E|F]]|$1|11|3|12|5|6|7|20|8|@$M|21|N|22|O|W]]|9|@]|A|$]]|$1|13|3|-4|5|6|7|23|8|@]|9|@]|A|$]]]|14|$15|$5|16|17|18|A|$19|1A]]|1B|$5|16|17|18|A|$19|1C]]]]

What you want is:

<pre><code>bool b = foo.bar ?? false;
</code></pre>

This is (surprisingly) safe and an intended use for the null-coalescing operator.

<blockquote>
 The ?? operator is called the null-coalescing operator and is used to define a default value for a nullable value types as well as reference types. It returns the left-hand operand if it is not null; otherwise it returns the right operand.
</blockquote>

Source: <a href="http://msdn.microsoft.com/en-us/library/ms173224.aspx">http://msdn.microsoft.com/en-us/library/ms173224.aspx</a>

In the case of <code>Nullable&lt;T&gt;</code>, it is functionally equivalent to <a href="http://msdn.microsoft.com/en-us/library/3d6d4f1d.aspx"><code>Nullable&lt;T&gt;.GetValueOrDefault(T defaultValue)</code></a>.

The code:

<pre><code>bool b = foo.bar.Value ?? false;
</code></pre>

Will cause a compiler-error, because you cannot apply the operator to value types, and <code>Nullable&lt;T&gt;.Value</code> always returns a value-type (or throws an exception when there is no value).

blocks|key|3181164|text|当存在不可为空的值时，foo.bar.Value表示不可为空的值；如果没有实际值，则抛出InvalidOperationException。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3181165|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|$]]

foo.bar.Value represents the non-nullable value when there is one, and throws an InvalidOperationException if there’s no real value.

blocks|key|2895787|text|最简单的修复方法是|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2895788|bool+b+=+foo.bar.GetValueOrDefault();|code-block|syntax|javascript|2895789|这实际上也比.Value更便宜，因为它省略了has-value检查。它将默认为default(T)，在这里它实际上是false+(它只返回底层T字段的值，而不进行任何检查)。|offset|length|style|CODE|2895790|如果您需要不同于default(T)的默认值，则：|2895791|var+value+=+yourNullable.GetValueOrDefault(yourPreferredValue);|2895792|entityMap^0|0|0|6|6|13|A|1M|5|1Z|1|0|8|A|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|T|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|U|8|@$I|V|J|W|K|L]|$I|X|J|Y|K|L]|$I|Z|J|10|K|L]|$I|11|J|12|K|L]]|9|@]|A|$]]|$1|M|3|N|5|6|7|13|8|@$I|14|J|15|K|L]]|9|@]|A|$]]|$1|O|3|P|5|D|7|16|8|@]|9|@]|A|$E|F]]|$1|Q|3|-4|5|6|7|17|8|@]|9|@]|A|$]]]|R|$]]

The easiest fix there is

<pre><code>bool b = foo.bar.GetValueOrDefault();
</code></pre>

which is also actually cheaper than <code>.Value</code> as it omits the has-value check. It will default to <code>default(T)</code> , which is indeed <code>false</code> here (it just returns the value of the underlying <code>T</code> field, without any checks at all).

If you need a different default to <code>default(T)</code>, then:

<pre><code>var value = yourNullable.GetValueOrDefault(yourPreferredValue);
</code></pre>

blocks|key|3181364|text|bar.Value+??+false|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|3181365|有编译错误，因为左操作数是?？运算符应为引用类型或可为nullable类型。|unstyled|3181366|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$B|C]]|$1|D|3|E|5|F|7|J|8|@]|9|@]|A|$]]|$1|G|3|-4|5|F|7|K|8|@]|9|@]|A|$]]]|H|$]]

<pre><code>bar.Value ?? false
</code></pre>

has compilation error, because left operand of ?? operator should be of reference or nullable type.

blocks|key|4966934|text|也许你也看过Stackoverflow中的this文章--这是一个优雅的obj.IfNotNull(lambdaExpression)风格的null检查--如果obj不是null，则返回您想要的对象，否则只返回null+(但不会抛出异常)。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|4966935|如果你正在访问一个被引用的实体，我将它与实体框架一起使用。|4966936|var+str=Entity1.Entity2.IfNotNull(x=>x.EntityDescription)+??+string.Empty+|code-block|syntax|javascript|4966937|它返回Entity1引用的Entity2中包含的EntityDescription+-如果任何对象Entity1或Entity2为null，则返回一个空字符串。如果没有IfNotNull，你会得到一个又长又难看的表达式。|4966938|扩展方法IfNotNull的定义如下：|4966939|public+static+TOut+IfNotNull<TIn,+TOut>(this+TIn+v,+Func<TIn,+TOut>+f)
++++where+TIn+:+class+
++++where+TOut:+class
{
++++if+(v+==+null)+return+null;+else+return+f(v);
}|4966940|4966941|更新：如果您将代码更新到C#版本6.0+(.NET+Framework4.6-但seems+to+support+older+frameworks+4.x+too)，则有一个新的运算符可以简化此任务："elvis“operator+Framework|BOLD|4966942|它的工作原理如下：|4966943|var+str=(Entity1.Entity2?.EntityDescription)+??+string.Empty+|4966944|在这种情况下，如果Entity2为null，则计算停止，(...)变为null+-之后，??+string.empty部件将null替换为string.Empty。换句话说，它的工作方式与.IfNotNull(...)相同。|4966945|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/a/4244582/1016343|1|2|https://stackoverflow.com/q/28921701/1016343^0|10|V|L|4|0|0|0|0|3|7|D|7|O|H|1D|7|1L|7|1T|4|2C|9|2C|9|1|0|4|9|0|0|0|0|3|C|7|2S|F|14|15|2|0|0|0|9|7|H|4|S|5|Z|4|18|F|1Q|4|1X|C|2M|F|0^^$0|@$1|2|3|4|5|6|7|1D|8|@$9|1E|A|1F|B|C]]|D|@$9|1G|A|1H|1|1I]]|E|$]]|$1|F|3|G|5|6|7|1J|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|1K|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|1L|8|@$9|1M|A|1N|B|C]|$9|1O|A|1P|B|C]|$9|1Q|A|1R|B|C]|$9|1S|A|1T|B|C]|$9|1U|A|1V|B|C]|$9|1W|A|1X|B|C]|$9|1Y|A|1Z|B|C]]|D|@$9|20|A|21|1|22]]|E|$]]|$1|O|3|P|5|6|7|23|8|@$9|24|A|25|B|C]]|D|@]|E|$]]|$1|Q|3|R|5|J|7|26|8|@]|D|@]|E|$K|L]]|$1|S|3|-4|5|6|7|27|8|@]|D|@]|E|$]]|$1|T|3|U|5|6|7|28|8|@$9|29|A|2A|B|V]|$9|2B|A|2C|B|V]|$9|2D|A|2E|B|V]]|D|@$9|2F|A|2G|1|2H]]|E|$]]|$1|W|3|X|5|6|7|2I|8|@]|D|@]|E|$]]|$1|Y|3|Z|5|J|7|2J|8|@]|D|@]|E|$K|L]]|$1|10|3|11|5|6|7|2K|8|@$9|2L|A|2M|B|C]|$9|2N|A|2O|B|C]|$9|2P|A|2Q|B|C]|$9|2R|A|2S|B|C]|$9|2T|A|2U|B|C]|$9|2V|A|2W|B|C]|$9|2X|A|2Y|B|C]|$9|2Z|A|30|B|C]]|D|@]|E|$]]|$1|12|3|-4|5|6|7|31|8|@]|D|@]|E|$]]]|13|$14|$5|15|16|17|E|$18|19]]|1A|$5|15|16|17|E|$18|19]]|1B|$5|15|16|17|E|$18|1C]]]]

Maybe you have a look at <a href="https://stackoverflow.com/a/4244582/1016343">this</a> article in Stackoverflow too - it is an elegant null checking in the style <code>obj.IfNotNull(lambdaExpression)</code> - returning the object you want if obj is not null otherwise just null (but without throwing an exception).

I used it with the Entity Framework if you're accessing a referenced entity, e.g. 

<pre><code>var str=Entity1.Entity2.IfNotNull(x=&gt;x.EntityDescription) ?? string.Empty 
</code></pre>

which returns <code>EntityDescription</code> contained in <code>Entity2</code> which is referenced by <code>Entity1</code> - or an empty string if any object <code>Entity1</code> or <code>Entity2</code> is <code>null</code>. Without <a href="https://stackoverflow.com/a/4244582/1016343"><code>IfNotNull</code></a> you would get a long and ugly expression.

The extension method <code>IfNotNull</code> is defined there as follows:

<pre><code>public static TOut IfNotNull&lt;TIn, TOut&gt;(this TIn v, Func&lt;TIn, TOut&gt; f)
 where TIn : class 
 where TOut: class
{
 if (v == null) return null; else return f(v);
}
</code></pre>

<hr>

Update:
If you update the code to C# version 6.0 (.NET Framework 4.6 - but <a href="https://stackoverflow.com/q/28921701/1016343">seems to support older frameworks 4.x too</a>), there is a new operator available which makes this task easy: The "elvis" operator <code>?.</code>.

It works as follows:

<pre><code>var str=(Entity1.Entity2?.EntityDescription) ?? string.Empty 
</code></pre>

In this case, if <code>Entity2</code> is <code>null</code>, evaluation stops and <code>(...)</code> becomes <code>null</code> - after which the <code>?? string.empty</code> part replaces <code>null</code> by <code>string.Empty</code>. In other words, it works the same way as <code>.IfNotNull(...)</code> would.

Is it safe to rewrite the following code:

<pre><code>bool b = foo.bar.HasValue ? foo.bar.Value : false;
</code></pre>

to

<pre><code>bool b = foo.bar.Value ?? false;
</code></pre>

where <code>bar</code> is the nullable type <code>bool?</code>

Rewrite HasValue to the ?? Operators

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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重写以下代码是否安全：bool b = foo.bar.HasValue ? foo.bar.Value : false;至bool b = foo.bar.Value ?? false;其中，bar是可以为空的类型bool?

问将HasValue重写为?？运算符
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问将HasValue重写为?？运算符EN