blocks|key|235840|text|您可以先使用DefaultIfEmpty，然后使用First|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|235841|T+customDefault+=+...;
IEnumerable<T>+mySequence+=+...;
mySequence.DefaultIfEmpty(customDefault).First();|code-block|syntax|javascript|235842|entityMap|0|LINK|mutability|MUTABLE|url|http://msdn.microsoft.com/en-us/library/vstudio/bb355419%2528v=vs.100%2529.aspx|1|http://msdn.microsoft.com/en-us/library/vstudio/bb291976%2528v=vs.100%2529.aspx^0|6|E|0|P|5|1|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@$A|T|B|U|1|V]|$A|W|B|X|1|Y]]|C|$]]|$1|D|3|E|5|F|7|Z|8|@]|9|@]|C|$G|H]]|$1|I|3|-4|5|6|7|10|8|@]|9|@]|C|$]]]|J|$K|$5|L|M|N|C|$O|P]]|Q|$5|L|M|N|C|$O|R]]]]

You can use <a href="http://msdn.microsoft.com/en-us/library/vstudio/bb355419%28v=vs.100%29.aspx">DefaultIfEmpty</a> followed by <a href="http://msdn.microsoft.com/en-us/library/vstudio/bb291976%28v=vs.100%29.aspx">First</a>:

<pre><code>T customDefault = ...;
IEnumerable&lt;T&gt; mySequence = ...;
mySequence.DefaultIfEmpty(customDefault).First();
</code></pre>

blocks|key|235865|text|您也可以这样做|type|unstyled|depth|inlineStyleRanges|entityRanges|data|235866|++++Band[]+objects+=+{+new+Band+{+Name+=+"Iron+Maiden"+}+};
++++first+=+objects.Where(o+=>+o.Name+==+"Slayer")
++++++++.DefaultIfEmpty(new+Band+{+Name+=+"Black+Sabbath"+})
++++++++.FirstOrDefault();+++//+returns+"Black+Sabbath"+|code-block|syntax|javascript|235867|这只使用linq+-+yipee！|235868|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

You can also do this 

<pre><code> Band[] objects = { new Band { Name = "Iron Maiden" } };
 first = objects.Where(o =&gt; o.Name == "Slayer")
 .DefaultIfEmpty(new Band { Name = "Black Sabbath" })
 .FirstOrDefault(); // returns "Black Sabbath" 
</code></pre>

This uses only linq - yipee!

blocks|key|235899|text|@sloth从评论中复制过来|type|unstyled|depth|inlineStyleRanges|entityRanges|data|235900|例如，您可以使用YourCollection.DefaultIfEmpty(YourDefault).First()代替YourCollection.FirstOrDefault()。|offset|length|style|CODE|235901|示例：|235902|var+viewModel+=+new+CustomerDetailsViewModel
++++{
++++++++++++MainResidenceAddressSection+=+(MainResidenceAddressSection)addresses.DefaultIfEmpty(new+MainResidenceAddressSection()).FirstOrDefault(+o+=>+o+is+MainResidenceAddressSection),
++++++++++++RiskAddressSection+=+addresses.DefaultIfEmpty(new+RiskAddressSection()).FirstOrDefault(o+=>+!(o+is+MainResidenceAddressSection)),
++++};|code-block|syntax|javascript|235903|entityMap^0|0|8|1E|1O|V|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|R|8|@$D|S|E|T|F|G]|$D|U|E|V|F|G]]|9|@]|A|$]]|$1|H|3|I|5|6|7|W|8|@]|9|@]|A|$]]|$1|J|3|K|5|L|7|X|8|@]|9|@]|A|$M|N]]|$1|O|3|-4|5|6|7|Y|8|@]|9|@]|A|$]]]|P|$]]

Copied over from comment by @sloth

Instead of <code>YourCollection.FirstOrDefault()</code>, you could use <code>YourCollection.DefaultIfEmpty(YourDefault).First()</code> for example.

Example:

<pre><code>var viewModel = new CustomerDetailsViewModel
 {
 MainResidenceAddressSection = (MainResidenceAddressSection)addresses.DefaultIfEmpty(new MainResidenceAddressSection()).FirstOrDefault( o =&gt; o is MainResidenceAddressSection),
 RiskAddressSection = addresses.DefaultIfEmpty(new RiskAddressSection()).FirstOrDefault(o =&gt; !(o is MainResidenceAddressSection)),
 };
</code></pre>

blocks|key|235911|text|使用DefaultIfEmpty()而不是FirstOrDefault()。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|235912|entityMap^0|2|G|L|G|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]|$9|K|A|L|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|M|8|@]|D|@]|E|$]]]|G|$]]

Use <code>DefaultIfEmpty()</code> instead of <code>FirstOrDefault()</code>.

blocks|key|453638|text|例如，您可以使用YourCollection.DefaultIfEmpty(YourDefault).First()代替YourCollection.FirstOrDefault()。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|453639|entityMap^0|8|1E|1O|V|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]|$9|K|A|L|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|M|8|@]|D|@]|E|$]]]|G|$]]

Instead of <code>YourCollection.FirstOrDefault()</code>, you could use <code>YourCollection.DefaultIfEmpty(YourDefault).First()</code> for example.

blocks|key|235966|text|实际上，在处理集合时，我使用两种方法来避免NullReferenceException：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|235967|public+class+Foo
{
++++public+string+Bar{get;+set;}
}
void+Main()
{
++++var+list+=+new+List<Foo>();
++++//before+C#+6.0
++++string+barCSharp5+=+list.DefaultIfEmpty(new+Foo()).FirstOrDefault().Bar;
++++//C#+6.0+or+later
++++var+barCSharp6+=+list.FirstOrDefault()?.Bar;
}|code-block|syntax|javascript|235968|对于C#+6.0或更高版本：|235969|在执行成员访问Null-conditional+Operators+documentation之前，使用?.或?[测试是否为null|235970|示例：var+barCSharp6+=+list.FirstOrDefault()?.Bar;|235971|C#旧版本：|235972|如果序列为空，则使用DefaultIfEmpty()检索默认值。MSDN+Documentation|235973|示例：string+barCSharp5+=+list.DefaultIfEmpty(new+Foo()).FirstOrDefault().Bar;|235974|entityMap|0|LINK|mutability|MUTABLE|url|https://docs.microsoft.com/en-us/dotnet/articles/csharp/language-reference/operators/null-conditional-operators|1|https://msdn.microsoft.com/en-us/library/bb360530(v=vs.110).aspx^0|L|M|0|0|0|1G|2|1J|2|7|14|0|0|3|18|0|0|A|G|W|I|1|0|3|20|0^^$0|@$1|2|3|4|5|6|7|16|8|@$9|17|A|18|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|19|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|1A|8|@]|D|@]|E|$]]|$1|M|3|N|5|6|7|1B|8|@$9|1C|A|1D|B|C]|$9|1E|A|1F|B|C]]|D|@$9|1G|A|1H|1|1I]]|E|$]]|$1|O|3|P|5|6|7|1J|8|@$9|1K|A|1L|B|C]]|D|@]|E|$]]|$1|Q|3|R|5|6|7|1M|8|@]|D|@]|E|$]]|$1|S|3|T|5|6|7|1N|8|@$9|1O|A|1P|B|C]]|D|@$9|1Q|A|1R|1|1S]]|E|$]]|$1|U|3|V|5|6|7|1T|8|@$9|1U|A|1V|B|C]]|D|@]|E|$]]|$1|W|3|-4|5|6|7|1W|8|@]|D|@]|E|$]]]|X|$Y|$5|Z|10|11|E|$12|13]]|14|$5|Z|10|11|E|$12|15]]]]

Actually, I use two approaches to avoid <code>NullReferenceException</code> when I'm working with collections:


<pre><code>public class Foo
{
 public string Bar{get; set;}
}
void Main()
{
 var list = new List&lt;Foo&gt;();
 //before C# 6.0
 string barCSharp5 = list.DefaultIfEmpty(new Foo()).FirstOrDefault().Bar;
 //C# 6.0 or later
 var barCSharp6 = list.FirstOrDefault()?.Bar;
}
</code></pre>

<h2>For C# 6.0 or later:</h2>

Use <code>?.</code> or <code>?[</code> to test if is null before perform a member access <a href="https://docs.microsoft.com/en-us/dotnet/articles/csharp/language-reference/operators/null-conditional-operators" rel="noreferrer">Null-conditional Operators documentation</a>

Example:
<code>var barCSharp6 = list.FirstOrDefault()?.Bar;</code>

<h2>C# older version:</h2>

Use <code>DefaultIfEmpty()</code> to retrieve a default value if the sequence is empty.<a href="https://msdn.microsoft.com/en-us/library/bb360530(v=vs.110).aspx" rel="noreferrer">MSDN Documentation</a>

Example:
<code>string barCSharp5 = list.DefaultIfEmpty(new Foo()).FirstOrDefault().Bar;</code>

blocks|key|236024|text|我刚刚遇到了类似的情况，正在寻找一种解决方案，允许我返回一个替代默认值，而无需在每次需要时在调用方处理它。如果Linq不支持我们想要的东西，我们通常会做的是编写一个新的扩展来处理它。我就是这么做的。这是我想出来的(虽然没有经过测试)：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|236025|public+static+class+EnumerableExtensions
{
++++public+static+T+FirstOrDefault<T>(this+IEnumerable<T>+items,+T+defaultValue)
++++{
++++++++foreach+(var+item+in+items)
++++++++{
++++++++++++return+item;
++++++++}
++++++++return+defaultValue;
++++}

++++public+static+T+FirstOrDefault<T>(this+IEnumerable<T>+items,+Func<T,+bool>+predicate,+T+defaultValue)
++++{
++++++++return+items.Where(predicate).FirstOrDefault(defaultValue);
++++}

++++public+static+T+LastOrDefault<T>(this+IEnumerable<T>+items,+T+defaultValue)
++++{
++++++++return+items.Reverse().FirstOrDefault(defaultValue);
++++}

++++public+static+T+LastOrDefault<T>(this+IEnumerable<T>+items,+Func<T,+bool>+predicate,+T+defaultValue)
++++{
++++++++return+items.Where(predicate).LastOrDefault(defaultValue);
++++}
}|code-block|syntax|javascript|236026|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 just had a similar situation and was looking for a solution that allows me to return an alternative default value without taking care of it at the caller side every time I need it. What we usually do in case Linq does not support what we want, is to write a new extension that takes care of it. That´s what I did. Here is what I came up with (not tested though): 

<pre><code>public static class EnumerableExtensions
{
 public static T FirstOrDefault&lt;T&gt;(this IEnumerable&lt;T&gt; items, T defaultValue)
 {
 foreach (var item in items)
 {
 return item;
 }
 return defaultValue;
 }

 public static T FirstOrDefault&lt;T&gt;(this IEnumerable&lt;T&gt; items, Func&lt;T, bool&gt; predicate, T defaultValue)
 {
 return items.Where(predicate).FirstOrDefault(defaultValue);
 }

 public static T LastOrDefault&lt;T&gt;(this IEnumerable&lt;T&gt; items, T defaultValue)
 {
 return items.Reverse().FirstOrDefault(defaultValue);
 }

 public static T LastOrDefault&lt;T&gt;(this IEnumerable&lt;T&gt; items, Func&lt;T, bool&gt; predicate, T defaultValue)
 {
 return items.Where(predicate).LastOrDefault(defaultValue);
 }
}
</code></pre>

blocks|key|236088|text|我知道这已经有一段时间了，但我会补充这一点，基于最流行的答案，但有一点扩展我想分享以下：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|236089|static+class+ExtensionsThatWillAppearOnIEnumerables
{
++++public+static+T+FirstOr<T>(this+IEnumerable<T>+source,+Func<T,+bool>+predicate,+Func<T>+alternate)
++++{
++++++++var+thing+=+source.FirstOrDefault(predicate);
++++++++if+(thing+!=+null)
++++++++++++return+thing;
++++++++return+alternate();
++++}
}|code-block|syntax|javascript|236090|这使得我可以通过我自己的例子调用它，因为我遇到了问题：|236091|_controlDataResolvers.FirstOr(x+=>+x.AppliesTo(item.Key),+()+=>+newDefaultResolver()).GetDataAsync(conn,+item.ToList())|236092|所以对我来说，我只是想要一个默认的解析器被内联使用，我可以做我通常的检查，然后传入一个函数，这样一个类即使没有使用也不会被实例化，它是一个在需要的时候执行的函数！|236093|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 know its been a while but Ill add to this, based on the most popular answer but with a little extension Id like to share the below:

<pre><code>static class ExtensionsThatWillAppearOnIEnumerables
{
 public static T FirstOr&lt;T&gt;(this IEnumerable&lt;T&gt; source, Func&lt;T, bool&gt; predicate, Func&lt;T&gt; alternate)
 {
 var thing = source.FirstOrDefault(predicate);
 if (thing != null)
 return thing;
 return alternate();
 }
}
</code></pre>

This allows me to call it inline as such with my own example I was having issues with:

<pre><code>_controlDataResolvers.FirstOr(x =&gt; x.AppliesTo(item.Key), () =&gt; newDefaultResolver()).GetDataAsync(conn, item.ToList())
</code></pre>

So for me I just wanted a default resolver to be used inline, I can do my usual check and then pass in a function so a class isn't instantiated even if unused, its a function to execute when required instead!

blocks|key|453775|text|如果项列表不是可以为空的类型，请考虑强制转换为可为空的类型，然后使用firstordefault和null合并运算符。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|453776|示例：|453777|static+class+MyClass
{
++++public+enum+Level+{+Low,+Medium,+High+}

++++public+static+void+Dosomething()
++++{
++++++++var+levels+=+Enum.GetValues<Level>();

++++++++var+myLevel+=+levels.Select(x+=>+(Level?)x).FirstOrDefault()+??+Level.Medium;+//+The+Default+value+other+than+null
++++}
}|code-block|syntax|javascript|453778|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|$]]

If the list of items is not a nullable type consider casting to a nullable type then firstordefault and null coalescing operator with what ever default value you want.
Example:
<pre><code>static class MyClass
{
 public enum Level { Low, Medium, High }

 public static void Dosomething()
 {
 var levels = Enum.GetValues&lt;Level&gt;();

 var myLevel = levels.Select(x =&gt; (Level?)x).FirstOrDefault() ?? Level.Medium; // The Default value other than null
 }
}
</code></pre>

blocks|key|236125|text|这对我们很有效|type|unstyled|depth|inlineStyleRanges|entityRanges|data|236126|int+valueOrDefault+=+myList.Find(x+=>+x.Id+==+1)?.Value+??+-1;|code-block|syntax|javascript|236127|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|$]]

This worked for us
<pre><code>int valueOrDefault = myList.Find(x =&gt; x.Id == 1)?.Value ?? -1;
</code></pre>

blocks|key|453829|text|.NET6+/+c#10解决方案|type|unstyled|depth|inlineStyleRanges|entityRanges|data|453830|.NET6+/+c#10通过向*OrDefault+LINQ方法添加新功能来解决此问题。新的重载允许您在序列为空时指定要使用的默认值。|453831|public+static+TSource+FirstOrDefault<TSource>+(this+System.Collections.Generic.IEnumerable<TSource>+source,+Func<TSource,bool>+predicate,+TSource+defaultValue);|code-block|syntax|javascript|453832|返回序列中满足条件的第一个元素，如果找不到这样的元素，则返回指定的默认值。|453833|下面是一个例子；|453834|var+nums+=+new+List<int>+{+1,+2,+3+};
var+target+=+4;

var+value+=+nums.FirstOrDefault(x+=>+x+==+target,+-1);+//+value+becomes+-1.|453835|来源：https://docs.microsoft.com/en-us/dotnet/api/system.linq.enumerable.firstordefault?view=net-6.0#System_Linq_Enumerable_FirstOrDefault__1_System_Collections_Generic_IEnumerable___0____0_|offset|length|453836|entityMap|0|LINK|mutability|MUTABLE|url|https://docs.microsoft.com/en-us/dotnet/api/system.linq.enumerable.firstordefault?view=net-6.0#System_Linq_Enumerable_FirstOrDefault__1_System_Collections_Generic_IEnumerable___0____0^0|0|0|0|0|0|0|3|53|0|0^^$0|@$1|2|3|4|5|6|7|10|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|11|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|12|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|13|8|@]|9|@]|A|$]]|$1|K|3|L|5|6|7|14|8|@]|9|@]|A|$]]|$1|M|3|N|5|F|7|15|8|@]|9|@]|A|$G|H]]|$1|O|3|P|5|6|7|16|8|@]|9|@$Q|17|R|18|1|19]]|A|$]]|$1|S|3|-4|5|6|7|1A|8|@]|9|@]|A|$]]]|T|$U|$5|V|W|X|A|$Y|Z]]]]

<h1>.NET6 / c#10 Solution</h1>
.NET6 / c#10 solves this issue by adding new features to *OrDefault LINQ methods. New overloads let you specify a default value to use if the sequence is empty.
<pre><code>public static TSource FirstOrDefault&lt;TSource&gt; (this System.Collections.Generic.IEnumerable&lt;TSource&gt; source, Func&lt;TSource,bool&gt; predicate, TSource defaultValue);
</code></pre>
Returns the first element of the sequence that satisfies a condition, or a specified default value if no such element is found.
Here's an example;
<pre><code>var nums = new List&lt;int&gt; { 1, 2, 3 };
var target = 4;

var value = nums.FirstOrDefault(x =&gt; x == target, -1); // value becomes -1.
</code></pre>
Source: <a href="https://docs.microsoft.com/en-us/dotnet/api/system.linq.enumerable.firstordefault?view=net-6.0#System_Linq_Enumerable_FirstOrDefault__1_System_Collections_Generic_IEnumerable___0____0" rel="nofollow noreferrer">https://docs.microsoft.com/en-us/dotnet/api/system.linq.enumerable.firstordefault?view=net-6.0#System_Linq_Enumerable_FirstOrDefault__1_System_Collections_Generic_IEnumerable___0____0</a>_

As I understand it, in Linq the method <code>FirstOrDefault()</code> can return a <code>Default</code> value of something other than null. What I haven't worked out is what kind of things other than null can be returned by this (and similar) method when there are no items in the query result. Is there any particular way that this can be set up so that if there is no value for a particular query some predefined value is returned as the default value?

FirstOrDefault: Default value other than null

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。

文章

问答

视频

学习中心

腾讯云实验室

直播

竞赛

腾讯云架构师技术同盟交流圈

腾讯云数据库专区

腾讯云顾问专区

腾讯云原生专区

腾讯混元专区

腾讯云TCE专区

腾讯云Lighthouse专区

腾讯云HAI专区

腾讯云Edgeone专区

腾讯云存储专区

腾讯云智能专区

腾讯轻联专区 

腾讯云开发专区

TAPD专区

腾讯轻量云游戏服专区

腾讯云最具价值专家

腾讯云架构师技术同盟

腾讯云创作之星

腾讯云开发者先锋 

腾讯云代码助手

CODING DevOps

Cloud Studio

SDK中心

API中心

命令行工具

据我所知，在Linq中，方法FirstOrDefault()可以返回非Default的null值。我还没有弄清楚的是，当查询结果中没有项目时，这个(和类似的)方法可以返回除null之外的哪些东西。有没有什么特别的方法可以将其设置为，如果某个特定查询没有值，则返回某个预定义的值作为默认值？

问FirstOrDefault:非null的默认值
EN

Stack Overflow用户

社区

活动

圈层

关于

腾讯云开发者

热门产品

热门推荐

更多推荐

问FirstOrDefault:非null的默认值EN