blocks|key|1307195|text|没有一种内置的方法可以做到这一点。但是，您可以让扩展方法返回一个字符串或null，这将允许合并运算符工作。然而，这会很奇怪，我个人更喜欢你目前的方法。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1307196|既然您已经在使用扩展方法，为什么不直接创建一个返回值或默认值的方法：|1307197|string+result+=+s.SiteNumber.ConvertNullOrEmptyTo("No+Number");|code-block|syntax|javascript|1307198|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|L|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|M|8|@]|9|@]|A|$G|H]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

There isn't a built-in way to do this. You could make your extension method return a string or null, however, which would allow the coalescing operator to work. This would be odd, however, and I personally prefer your current approach.

Since you're already using an extension method, why not just make one that returns the value or a default:

<pre><code>string result = s.SiteNumber.ConvertNullOrEmptyTo("No Number");
</code></pre>

blocks|key|1092030|text|字符串扩展方法ValueOrDefault()怎么样？|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1092031|public+static+string+ValueOrDefault(this+string+s,+string+sDefault)
{
++++if+(string.IsNullOrEmpty(s))
++++++++return+sDefault;
++++return+s;
}|code-block|syntax|javascript|1092032|如果字符串为空，则返回null：|1092033|public+static+string+Value(this+string+s)
{
++++if+(string.IsNullOrEmpty(s))
++++++++return+null;
++++return+s;
}|1092034|虽然没有尝试这些解决方案。|1092035|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Q|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|R|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|S|8|@]|9|@]|A|$]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

how about a string extension method ValueOrDefault()

<pre><code>public static string ValueOrDefault(this string s, string sDefault)
{
 if (string.IsNullOrEmpty(s))
 return sDefault;
 return s;
}
</code></pre>

or return null if string is Empty:

<pre><code>public static string Value(this string s)
{
 if (string.IsNullOrEmpty(s))
 return null;
 return s;
}
</code></pre>

Didn't try these solutions though.

blocks|key|1307265|text|一种比之前提出的更快的扩展方法：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1307266|public+static+string+Fallback(this+string+@this,+string+@default+=+"")
{
++++return+(@this+==+null+%7C%7C+@this.Trim().Length+==+0)+?+@default+:+@this;
}|code-block|syntax|javascript|1307267|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|$]]

A slightly faster extension method than proposed earlier perhaps:

<pre><code>public static string Fallback(this string @this, string @default = "")
{
 return (@this == null || @this.Trim().Length == 0) ? @default : @this;
}
</code></pre>

blocks|key|1307333|text|我正在使用我自己的字符串合并扩展方法。由于这里的人正在使用LINQ，并且在时间密集型操作上绝对浪费资源(我在紧凑的循环中使用它)，我将分享我的：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1307334|public+static+class+StringCoalesceExtension
{
++++public+static+string+Coalesce(this+string+s1,+string+s2)
++++{
++++++++return+string.IsNullOrWhiteSpace(s1)+?+s2+:+s1;
++++}
}|code-block|syntax|javascript|1307335|我认为这非常简单，您甚至不需要为空字符串值而烦恼。像这样使用它：|1307336|string+s1+=+null;
string+s2+=+"";
string+s3+=+"loudenvier";
string+s+=+s1.Coalesce(s2.Coalesce(s3));
Assert.AreEqual("loudenvier",+s);|1307337|我经常使用它。在第一次使用之后，你离不开那些“实用工具”函数之一:-)|1307338|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Q|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|R|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|S|8|@]|9|@]|A|$]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

I'm using a string Coalesce extension method of my own. Since those here are using LINQ, and absolutelly wasting resources for time intensive operations (I'm using it in tight loops), I'll share mine:

<pre><code>public static class StringCoalesceExtension
{
 public static string Coalesce(this string s1, string s2)
 {
 return string.IsNullOrWhiteSpace(s1) ? s2 : s1;
 }
}
</code></pre>

I think it is quite simple, and you don't even need to bother with null string values. Use it like this:

<pre><code>string s1 = null;
string s2 = "";
string s3 = "loudenvier";
string s = s1.Coalesce(s2.Coalesce(s3));
Assert.AreEqual("loudenvier", s);
</code></pre>

I use it a lot. One of those "utility" functions you can't live without after first using it :-)

blocks|key|1307372|text|我喜欢下面的扩展方法QQQ的简洁性，当然，像这样的操作符？那就更好了。但我们可以通过不只是两个而是三个字符串选项值的比较来实现这一点，这其中一个需要时不时地处理(参见下面的第二个函数)。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1307373|#region+QQ

[DebuggerStepThrough]
public+static+string+QQQ(this+string+str,+string+value2)
{
++++return+(str+!=+null+&&+str.Length+>+0)
++++++++?+str
++++++++:+value2;
}

[DebuggerStepThrough]
public+static+string+QQQ(this+string+str,+string+value2,+string+value3)
{
++++return+(str+!=+null+&&+str.Length+>+0)
++++++++?+str
++++++++:+(value2+!=+null+&&+value2.Length+>+0)
++++++++++++?+value2
++++++++++++:+value3;
}


//+Following+is+only+two+QQ,+just+checks+null,+but+allows+more+than+1+string+unlike+??+can+do:

[DebuggerStepThrough]
public+static+string+QQ(this+string+str,+string+value2,+string+value3)
{
++++return+(str+!=+null)
++++++++?+str
++++++++:+(value2+!=+null)
++++++++++++?+value2
++++++++++++:+value3;
}

#endregion|code-block|syntax|javascript|1307374|entityMap^0|A|3|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@$9|N|A|O|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|P|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|Q|8|@]|D|@]|E|$]]]|L|$]]

I like the brevity of the following extension method <code>QQQ</code> for this, though of course an operator like? would be better. But we can 1 up this by allowing not just two but three string option values to be compared, which one encounters the need to handle every now and then (see second function below).

<pre><code>#region QQ

[DebuggerStepThrough]
public static string QQQ(this string str, string value2)
{
 return (str != null &amp;&amp; str.Length &gt; 0)
 ? str
 : value2;
}

[DebuggerStepThrough]
public static string QQQ(this string str, string value2, string value3)
{
 return (str != null &amp;&amp; str.Length &gt; 0)
 ? str
 : (value2 != null &amp;&amp; value2.Length &gt; 0)
 ? value2
 : value3;
}


// Following is only two QQ, just checks null, but allows more than 1 string unlike ?? can do:

[DebuggerStepThrough]
public static string QQ(this string str, string value2, string value3)
{
 return (str != null)
 ? str
 : (value2 != null)
 ? value2
 : value3;
}

#endregion
</code></pre>

blocks|key|1307433|text|null-coalescing运算符的优点之一是它短路。如果第一部分不为空，则不计算第二部分。当回退需要开销很大的操作时，这会很有用。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1307434|我最终得到了：|1307435|public+static+string+Coalesce(this+string+s,+Func<string>+func)
{
++++return+String.IsNullOrEmpty(s)+?+func()+:+s;
}|code-block|syntax|javascript|1307436|用法：|1307437|string+navigationTitle+=+model?.NavigationTitle.
++++Coalesce(()+=>+RemoteTitleLookup(model?.ID)).+//+Expensive!
++++Coalesce(()+=>+model?.DisplayName);|1307438|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|P|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|Q|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|R|8|@]|9|@]|A|$]]|$1|K|3|L|5|F|7|S|8|@]|9|@]|A|$G|H]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

One of the advantages of the null-coalescing operator is that it short-circuits. When the first part isn't null, the second part isn't evaluated. This can be useful when the fallback requires an expensive operation.

I ended up with:

<pre><code>public static string Coalesce(this string s, Func&lt;string&gt; func)
{
 return String.IsNullOrEmpty(s) ? func() : s;
}
</code></pre>

Usage:

<pre><code>string navigationTitle = model?.NavigationTitle.
 Coalesce(() =&gt; RemoteTitleLookup(model?.ID)). // Expensive!
 Coalesce(() =&gt; model?.DisplayName);
</code></pre>

blocks|key|1307485|text|我简单地使用了一个NullIfEmpty扩展方法，如果字符串为空，它将始终返回null，从而允许?？(空合并运算符)正常使用。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1307486|public+static+string+NullIfEmpty(this+string+s)
{
++++return+string.IsNullOrEmpty(s)+?+null+:+s;
}|code-block|syntax|javascript|1307487|这就允许?？像平常一样使用，并使链接易于阅读。|1307488|string+string1+=+string2.NullIfEmpty()+??+string3.NullIfEmpty()+??+string4;|1307489|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|N|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

I simply use a NullIfEmpty extension method which will always return null if the string is empty allowing ?? (Null Coalescing Operator) to be used as normal.
<pre><code>public static string NullIfEmpty(this string s)
{
 return string.IsNullOrEmpty(s) ? null : s;
}
</code></pre>
This then allows ?? to be used as normal and makes chaining easy to read.
<pre><code>string string1 = string2.NullIfEmpty() ?? string3.NullIfEmpty() ?? string4;
</code></pre>

blocks|key|1307540|text|可以很容易地将一些答案转换为具有泛型的助手扩展类，以获得更广泛的用法：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1307541|注:有关短路方法的说明，请参阅wensveen+answer|offset|length|1307542|//+classic
public+static+string+Coalesce(this+string+s,+params+string[]+strings)
++=>+s.Coalesce(string.IsNullOrEmpty,+strings);

//+short-circuit+compatible,+for+expensive+string+getting
public+static+string+Coalesce(this+string+s,+params+Func<string>[]+getters)
++=>+s.Coalesce(string.IsNullOrEmpty,+getters);

//+generic
public+static+T+Coalesce<T>(this+T+value,+Func<T,+bool>+isEmpty,+params+T[]+values)+where+T+:+class
++=>+isEmpty(value)+?+values.FirstOrDefault(val+=>+!isEmpty(val))+:+value;

//+generic,+short-circuit+compatible
public+static+T+Coalesce<T>(this+T+value,+Func<T,+bool>+isEmpty,+params+Func<T>[]+getters)+where+T+:+class+{
++if+(isEmpty(value))
++++return+getters
++++++.Select(getter+=>+new+Lazy<T>(getter))
++++++.FirstOrDefault(val+=>+!isEmpty(val.Value))
++++++?.Value;

++return+value;
}|code-block|syntax|javascript|1307543|示例用法：|1307544|string+result+=+s.SiteNumber.Coalesce(s.AltSiteNumber,+"No+Number");

string+result+=+s.SiteNumber.Coalesce(string.IsNullOrWhiteSpace,+s.AltSiteNumber,+"No+Number");

string+navigationTitle+=+model?.NavigationTitle.
++Coalesce(()+=>+RemoteTitleLookup(model?.ID),+()+=>+model?.DisplayName);

Player+player+=+player1.Coalesce(p+=>+p?.Score+>+0,+player2,+player3);|1307545|(PS:我认为我在这里使用泛型有点离题。我是不是想多了？)|1307546|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/a/44717244/4249111^0|0|F|F|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Y|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|Z|8|@]|9|@$D|10|E|11|1|12]]|A|$]]|$1|F|3|G|5|H|7|13|8|@]|9|@]|A|$I|J]]|$1|K|3|L|5|6|7|14|8|@]|9|@]|A|$]]|$1|M|3|N|5|H|7|15|8|@]|9|@]|A|$I|J]]|$1|O|3|P|5|6|7|16|8|@]|9|@]|A|$]]|$1|Q|3|-4|5|6|7|17|8|@]|9|@]|A|$]]]|R|$S|$5|T|U|V|A|$W|X]]]]

It's easy to turn some of the answers in to a helper extension class with generics for a even broader usage:
NOTE: for an explanation of short-circuit method the see the <a href="https://stackoverflow.com/a/44717244/4249111">wensveen answer</a>
<pre><code>// classic
public static string Coalesce(this string s, params string[] strings)
 =&gt; s.Coalesce(string.IsNullOrEmpty, strings);

// short-circuit compatible, for expensive string getting
public static string Coalesce(this string s, params Func&lt;string&gt;[] getters)
 =&gt; s.Coalesce(string.IsNullOrEmpty, getters);

// generic
public static T Coalesce&lt;T&gt;(this T value, Func&lt;T, bool&gt; isEmpty, params T[] values) where T : class
 =&gt; isEmpty(value) ? values.FirstOrDefault(val =&gt; !isEmpty(val)) : value;

// generic, short-circuit compatible
public static T Coalesce&lt;T&gt;(this T value, Func&lt;T, bool&gt; isEmpty, params Func&lt;T&gt;[] getters) where T : class {
 if (isEmpty(value))
 return getters
 .Select(getter =&gt; new Lazy&lt;T&gt;(getter))
 .FirstOrDefault(val =&gt; !isEmpty(val.Value))
 ?.Value;

 return value;
}
</code></pre>
Example usage:
<pre><code>string result = s.SiteNumber.Coalesce(s.AltSiteNumber, &quot;No Number&quot;);

string result = s.SiteNumber.Coalesce(string.IsNullOrWhiteSpace, s.AltSiteNumber, &quot;No Number&quot;);

string navigationTitle = model?.NavigationTitle.
 Coalesce(() =&gt; RemoteTitleLookup(model?.ID), () =&gt; model?.DisplayName);

Player player = player1.Coalesce(p =&gt; p?.Score &gt; 0, player2, player3);
</code></pre>
(PS: I think I'm getting bit off topic here using generics. Am I overthinking this?)

Something I find myself doing more and more is checking a string for empty (as in <code>""</code> or null) and a conditional operator.

A current example:

<pre><code>s.SiteNumber.IsNullOrEmpty() ? "No Number" : s.SiteNumber;
</code></pre>

This is just an extension method, it's equivalent to:

<pre><code>string.IsNullOrEmpty(s.SiteNumber) ? "No Number" : s.SiteNumber;
</code></pre>

Since it's empty and not null, <code>??</code> won't do the trick. A <code>string.IsNullOrEmpty()</code> version of <code>??</code> would be the perfect solution. I'm thinking there has to be a cleaner way of doing this (I hope!), but I've been at a loss to find it. 

Does anyone know of a better way to do this, even if it's only in .Net 4.0?

?? Coalesce for empty string?

我发现自己越来越多地在检查字符串是否为空(如在""或null中)和条件运算符。当前示例：s.SiteNumber.IsNullOrEmpty() ? "No Number" : s.SiteNumber;这只是一个扩展方法，它等同于：string.IsNullOrEmpty(s.SiteNumber) ? "No Nu...

问?？是否合并为空字符串？
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