blocks|key|1152541|text|您不能直接更改它。但你有几个选择。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1152542|如果您的序列不包含可以使用的null：|offset|length|style|CODE|1152543|MyClass+result+=+seq.FirstOrDefault()+??+new+MyClass("HelloWorld");|code-block|syntax|javascript|1152544|或者，您可以实现自己的版本，该版本接受一个参数作为默认值：|1152545|++++public+static+T+FirstOrDefault<T>(this+IEnumerable<T>+source,+T+defaultValue)
++++{
++++++++using+(IEnumerator<T>+enumerator+=+source.GetEnumerator())
++++++++{
++++++++++++if+(enumerator.MoveNext())
++++++++++++++++return+enumerator.Current;
++++++++++++else
++++++++++++++++return+defaultValue;
++++++++}
++++}

++++public+static+T+FirstOrDefault<T>(this+IEnumerable<T>+source,+Func<T,+bool>+predicate,+T+defaultValue)
++++{
++++++++using+(IEnumerator<T>+enumerator+=+source.Where(predicate).GetEnumerator())
++++++++{
++++++++++++if+(enumerator.MoveNext())
++++++++++++++++return+enumerator.Current;
++++++++++++else
++++++++++++++++return+defaultValue;
++++++++}
++++}|1152546|您也可以为FirstOrDefault的this实现使用不同的名称，但由于它不会与任何现有重载冲突，所以我只使用了相同的名称。|1152547|https://github.com/CodesInChaos/ChaosUtil/blob/master/Chaos.Util/LinqExtensions.cs|1152548|entityMap|0|LINK|mutability|MUTABLE|url^0|0|E|4|0|0|0|0|5|E|0|0|2A|0|0^^$0|@$1|2|3|4|5|6|7|11|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|12|8|@$D|13|E|14|F|G]]|9|@]|A|$]]|$1|H|3|I|5|J|7|15|8|@]|9|@]|A|$K|L]]|$1|M|3|N|5|6|7|16|8|@]|9|@]|A|$]]|$1|O|3|P|5|J|7|17|8|@]|9|@]|A|$K|L]]|$1|Q|3|R|5|6|7|18|8|@$D|19|E|1A|F|G]]|9|@]|A|$]]|$1|S|3|T|5|6|7|1B|8|@]|9|@$D|1C|E|1D|1|1E]]|A|$]]|$1|U|3|-4|5|6|7|1F|8|@]|9|@]|A|$]]]|V|$W|$5|X|Y|Z|A|$10|T]]]]

You can't directly change it. But you have a few alternatives.

If your sequence contains no <code>null</code>s you can use:

<pre><code>MyClass result = seq.FirstOrDefault() ?? new MyClass("HelloWorld");
</code></pre>

Or you can implement your own version which takes a parameter for the default value:

<pre><code> public static T FirstOrDefault&lt;T&gt;(this IEnumerable&lt;T&gt; source, T defaultValue)
 {
 using (IEnumerator&lt;T&gt; enumerator = source.GetEnumerator())
 {
 if (enumerator.MoveNext())
 return enumerator.Current;
 else
 return defaultValue;
 }
 }

 public static T FirstOrDefault&lt;T&gt;(this IEnumerable&lt;T&gt; source, Func&lt;T, bool&gt; predicate, T defaultValue)
 {
 using (IEnumerator&lt;T&gt; enumerator = source.Where(predicate).GetEnumerator())
 {
 if (enumerator.MoveNext())
 return enumerator.Current;
 else
 return defaultValue;
 }
 }
</code></pre>

You could also use a different name for your this implementation of <code>FirstOrDefault</code>, but since it doesn't collide with any of the existing overloads, I just used the same name.

<a href="https://github.com/CodesInChaos/ChaosUtil/blob/master/Chaos.Util/LinqExtensions.cs" rel="noreferrer">https://github.com/CodesInChaos/ChaosUtil/blob/master/Chaos.Util/LinqExtensions.cs</a>

blocks|key|2768202|text|您可以像这样定义一个扩展方法：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2768203|public+static+T+FirstOrDefault<T>(this+IEnumerable<T>+source,+T+defaultValue)
{
++++foreach+(T+x+in+source)
++++{
++++++++return+x;
++++}
++++return+defaultValue;
}|code-block|syntax|javascript|2768204|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|$]]

You could define an extension method like this:

<pre><code>public static T FirstOrDefault&lt;T&gt;(this IEnumerable&lt;T&gt; source, T defaultValue)
{
 foreach (T x in source)
 {
 return x;
 }
 return defaultValue;
}
</code></pre>

blocks|key|1152601|text|作为CodeAsChaos的好答案的等效替代方案，您也可以使用稍微短一些的：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1152602|public+static+T+MyFirstOrDefault<T>(
++++this+IEnumerable<T>+sequence,+
++++T+myDefault)
{
++++foreach(T+item+in+sequence)
++++++++return+item;
++++return+myDefault;
}

public+static+T+MyFirstOrDefault<T>(
++++this+IEnumerable<T>+sequence,+
++++Func<T,+bool>+predicate,+
++++T+myDefault)
{
++++return+sequence.Where(predicate).MyFirstOrDefault(myDefault);
}|code-block|syntax|javascript|1152603|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|$]]

As an equivalent alternative to CodeAsChaos's good answer, you could also do the somewhat shorter:

<pre><code>public static T MyFirstOrDefault&lt;T&gt;(
 this IEnumerable&lt;T&gt; sequence, 
 T myDefault)
{
 foreach(T item in sequence)
 return item;
 return myDefault;
}

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

blocks|key|4841085|text|我将不使用泛型，只需将其绑定到您的类的可枚举：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4841086|public+static+MyClass+FirstOrDefault(this+IEnumerable<MyClass>+source)
{
++++foreach+(var+x+in+source)
++++{
++++++++return+x;
++++}
++++return+new+MyClass("hello+world");+
++++//OR,+could+be+made+a+bit+nicer+by+making+a+static+method+in+your+class
++++return+MyClass.Default();
}|code-block|syntax|javascript|4841087|这将完成您想要的操作，但仅适用于您的类。如果这是可以接受的，那么这是到目前为止最简单和最快捷的方法。|4841088|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|$]]

I'm going to instead of using generics, just tie it to an enumerable of your class:

<pre><code>public static MyClass FirstOrDefault(this IEnumerable&lt;MyClass&gt; source)
{
 foreach (var x in source)
 {
 return x;
 }
 return new MyClass("hello world"); 
 //OR, could be made a bit nicer by making a static method in your class
 return MyClass.Default();
}
</code></pre>

This will do what you want, but only for your class. If this is acceptable, then this is by far the easiest and shortest way of doing it.

blocks|key|3054082|text|为什么不直接使用DefaultIfEmtpy()|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|3054083|例如：|3054084|var+result+=+this.MyClasses
++++.Where(c+=>+c.SomeCondition)
++++.Select(c+=>+c)
++++.DefaultIfEmpty(new+DefaultClass())
++++.First();|code-block|syntax|javascript|3054085|entityMap|0|LINK|mutability|MUTABLE|url|http://msdn.microsoft.com/en-us/library/bb382577.aspx^0|8|G|0|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@$A|T|B|U|1|V]]|C|$]]|$1|D|3|E|5|6|7|W|8|@]|9|@]|C|$]]|$1|F|3|G|5|H|7|X|8|@]|9|@]|C|$I|J]]|$1|K|3|-4|5|6|7|Y|8|@]|9|@]|C|$]]]|L|$M|$5|N|O|P|C|$Q|R]]]]

Why don't just use <a href="http://msdn.microsoft.com/en-us/library/bb382577.aspx" rel="nofollow">DefaultIfEmtpy()</a>?

For instance:

<pre><code>var result = this.MyClasses
 .Where(c =&gt; c.SomeCondition)
 .Select(c =&gt; c)
 .DefaultIfEmpty(new DefaultClass())
 .First();
</code></pre>

<pre><code>public class MyClass
{
 public string myString;

 public MyClass(string s)
 {
 this.myString = s;
 }
}
</code></pre>

In <code>IEnumerable&lt;MyClass&gt;</code> how can I change the default value of <code>FirstOrDefault()</code> method 
For example I want to return <code>new MyClass("HelloWorld");</code>

Edit: Is there a way to override default value of a Class? 
Like Default Value of <code>MyClass</code> is <code>new MyClass("Default");</code>

Change Default Value of a Class

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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public class MyClass{ public string myString; public MyClass(string s) { this.myString = s; }}在IEnumerable<MyClass>中如何更改FirstOrDefault()方法的默认...

问更改类的默认值
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