blocks|key|924353|text|您要避免的实际上是执行查询，直到您准备就绪：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|924354|public+List<Widget>+GetWidgets(string+cond1,+string+cond2,+string+cond3)
{
++++MyDataContext+db+=+new+MyDataContext();
++++var+widgets+=+db.Widgets;

++++if(cond1+!=+null)
++++++++widgets+=+widgets.Where(w+=>+w.condition1+==+cond1);

++++if(cond2+!=+null)
++++++++widgets+=+widgets.Where(w+=>+w.condition2+==+cond2);

++++if(cond3+!=+null)
++++++++widgets+=+widgets.Where(w+=>+w.condition3+==+cond3);

++++return+widgets.ToList();
}|code-block|syntax|javascript|924355|注意ToList调用是如何被删除的。直到您开始迭代该查询，才会执行该查询。调用ToList将强制执行此操作，因此可以将结果放入List<>中并返回。我甚至建议将该方法的返回值更改为IEnumerable<Widget>，并在最后跳过ToList调用：|offset|length|style|CODE|924356|public+IEnumerable<Widget>+GetWidgets(string+cond1,+string+cond2,+string+cond3)
{
++++MyDataContext+db+=+new+MyDataContext();
++++var+widgets+=+db.Widgets;

++++if(cond1+!=+null)
++++++++widgets+=+widgets.Where(w+=>+w.condition1+==+cond1);

+++//+[...]

++++return+widgets;
}|924357|这样，调用代码就可以决定何时执行查询(它甚至可以在执行查询之前添加更多条件)。|924358|entityMap^0|0|0|2|6|13|6|1R|6|2I|J|38|6|0|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]|$I|11|J|12|K|L]|$I|13|J|14|K|L]]|9|@]|A|$]]|$1|M|3|N|5|D|7|15|8|@]|9|@]|A|$E|F]]|$1|O|3|P|5|6|7|16|8|@]|9|@]|A|$]]|$1|Q|3|-4|5|6|7|17|8|@]|9|@]|A|$]]]|R|$]]

What you want to avoid is actually executing the query until you are ready:

<pre><code>public List&lt;Widget&gt; GetWidgets(string cond1, string cond2, string cond3)
{
 MyDataContext db = new MyDataContext();
 var widgets = db.Widgets;

 if(cond1 != null)
 widgets = widgets.Where(w =&gt; w.condition1 == cond1);

 if(cond2 != null)
 widgets = widgets.Where(w =&gt; w.condition2 == cond2);

 if(cond3 != null)
 widgets = widgets.Where(w =&gt; w.condition3 == cond3);

 return widgets.ToList();
}
</code></pre>

Note how the <code>ToList</code> calls are removed. The query is not executed until you start iterating over it. Invoking <code>ToList</code> will force that to happen, so that the result can be put into a <code>List&lt;&gt;</code> and returned. I would even suggest to change the return value of the method to <code>IEnumerable&lt;Widget&gt;</code> and skipping the <code>ToList</code> call in the end:

<pre><code>public IEnumerable&lt;Widget&gt; GetWidgets(string cond1, string cond2, string cond3)
{
 MyDataContext db = new MyDataContext();
 var widgets = db.Widgets;

 if(cond1 != null)
 widgets = widgets.Where(w =&gt; w.condition1 == cond1);

 // [...]

 return widgets;
}
</code></pre>

That way the calling code gets to decide when to execute the query (it may even add more conditions before doing so).

blocks|key|924369|text|像这样怎么样？|type|unstyled|depth|inlineStyleRanges|entityRanges|data|924370|++++++++IEnumerable<Widget>+condQuery+=+(from+w+in+db.Widgets);
++++++++if(cond1+!=+null+)+condQuery+=+condQuery.Where(w=>+w.condition1+==+cond1);
++++++++if(cond2+!=+null+)+condQuery+=+condQuery.Where(w=>+w.condition2+==+cond2);|code-block|syntax|javascript|924371|等等...？|924372|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|$]]

How about something like this?

<pre><code> IEnumerable&lt;Widget&gt; condQuery = (from w in db.Widgets);
 if(cond1 != null ) condQuery = condQuery.Where(w=&gt; w.condition1 == cond1);
 if(cond2 != null ) condQuery = condQuery.Where(w=&gt; w.condition2 == cond2);
</code></pre>

etc...?

blocks|key|924418|text|使用“或门”：在每个小部件条件测试前面加上一个"%7C%7C“，并检查我们是否使用了该条件。如果不是，则不会计算"or“的后半部分。这就是为什么它是一个门--如果第一部分的计算结果为真，我们就不再前进了。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|924419|如果是我写的，我会像下面这样写。我使用var语法糖来保存LINQ+query，并将ToList()移到最后。|924420|public+List<Widget>+GetWidgets(string+cond1,+string+cond2,+string+cond3)+
{+
++++MyDataContext+db+=+new+MyDataContext();+
++++var+widgets+=+from+w+in+db.Widgets+
++++++++++++++++++where+(cond1+==+null+%7C%7C+w.condition1+==+cond1)
+++++++++++++++++++++&&+(cond2+==+null+%7C%7C+w.condition2+==+cond2)
+++++++++++++++++++++&&+(cond3+==+null+%7C%7C+w.condition3+==+cond3)
++++++++++++++++++select+w;
++++return+widgets.ToList();
}+|code-block|syntax|javascript|924421|编辑:语法|924422|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|N|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|O|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|P|8|@]|9|@]|A|$]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

Use an "or gate": preface every widget condition test with an "||" and a check to see if we're using that condition or not. If we're not, the second half of the "or" isn't evaluated. That's why it's a gate -- we don't go any further if the first part evaluates to true. 

If I were writing it, I'd do it like below. I used the var syntatic sugar to hold LINQ query and moved the ToList() to the end.

<pre><code>public List&lt;Widget&gt; GetWidgets(string cond1, string cond2, string cond3) 
{ 
 MyDataContext db = new MyDataContext(); 
 var widgets = from w in db.Widgets 
 where (cond1 == null || w.condition1 == cond1)
 &amp;&amp; (cond2 == null || w.condition2 == cond2)
 &amp;&amp; (cond3 == null || w.condition3 == cond3)
 select w;
 return widgets.ToList();
} 
</code></pre>

edit: grammar

blocks|key|1138945|text|我们可以使用非常简单的方法，如下所示。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1138946|(from+e+in+employee
join+d+in+departments+on+e.departmentId+equals+d.departmentId
Select+new+{
e.name,
d.name,
getEmployeeContacts(e)
}
//return+active+contact+if+not+return+first+.+This+is+same+like+if+else+along+with+null+check
private+contact+getEmployeeContacts(Employee+e+)
{
+return+e.Contacts.FirstOrDefault(x+=>+x.Active+==+1)+??+e.Contacts.FirstOrDefault();
}|code-block|syntax|javascript|1138947|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|$]]

we can use very simple way like below.

<pre><code>(from e in employee
join d in departments on e.departmentId equals d.departmentId
Select new {
e.name,
d.name,
getEmployeeContacts(e)
}
//return active contact if not return first . This is same like if else along with null check
private contact getEmployeeContacts(Employee e )
{
 return e.Contacts.FirstOrDefault(x =&gt; x.Active == 1) ?? e.Contacts.FirstOrDefault();
}
</code></pre>

I'm trying to figure out a way of querying an object in my datamodel and include only those parameters that are not null. Like below:

<pre><code>public List&lt;Widget&gt; GetWidgets(string cond1, string cond2, string cond3)
{
 MyDataContext db = new MyDataContext();
 List&lt;Widget&gt; widgets = (from w in db.Widgets
 where 
 ... if cond1 != null w.condition1 == cond1 ...
 ... if cond2 != null w.condition2 == cond2 ...
 ... if cond3 != null w.condition3 == cond3 ...
 select w).ToList();
 return widgets;
}
</code></pre>

Since the widgets table can get very large, I'd like to avoid doing this:

<pre><code>public List&lt;Widget&gt; GetWidgets(string cond1, string cond2, string cond3)
{
 MyDataContext db = new MyDataContext();
 List&lt;Widget&gt; widgets = db.Widgets.ToList();

 if(cond1 != null)
 widgets = widgets.Where(w =&gt; w.condition1 == cond1).ToList();

 if(cond2 != null)
 widgets = widgets.Where(w =&gt; w.condition2 == cond2).ToList();

 if(cond3 != null)
 widgets = widgets.Where(w =&gt; w.condition3 == cond3).ToList();

 return widgets;
}
</code></pre>

I've looked at several example but don't really see anything that matches what I need to do.

Implementing conditional 'if' statements within a LINQ queries 'where' statement

我正在尝试找出一种在我的数据模型中查询对象的方法，并且只包含那些不为空的参数。如下所示：public List<Widget> GetWidgets(string cond1, string cond2, string cond3){ MyDataContext db = new MyDataContext();...

问在LINQ查询“where”语句中实现条件“if”语句
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