blocks|key|357297|text|问题是集合实现了IEnumerable+(而不是IEnumerable<T>)，枚举集合返回的是键，而不是对。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|357298|如果我是你，我会使用Dictionary<string,+string>，它是可枚举的，可以和LINQ一起使用。|357299|entityMap^0|8|B|O|E|0|A|Q|0^^$0|@$1|2|3|4|5|6|7|J|8|@$9|K|A|L|B|C]|$9|M|A|N|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|O|8|@$9|P|A|Q|B|C]]|D|@]|E|$]]|$1|H|3|-4|5|6|7|R|8|@]|D|@]|E|$]]]|I|$]]

The problem is that the collection implements <code>IEnumerable</code> (as opposed to <code>IEnumerable&lt;T&gt;</code>) and enumerating the collection returns the keys, not the pairs. 

If I were you, I'd use a <code>Dictionary&lt;string, string&gt;</code> which is enumerable and can be used with LINQ.

blocks|key|352179|text|AsQueryable必须接受泛型类型的IEnumerable<T>。NameValueCollection实现了IEnumerable，这是不同的。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|352180|而不是这样：|352181|{
++++NameValueCollection+nvc+=+RequestFields();
++++IQueryable+queryable+=+nvc.AsQueryable();
}|code-block|syntax|javascript|352182|试试OfType+(它接受非泛型接口)|352183|{
++++NameValueCollection+nvc+=+RequestFields();
++++IEnumerable<string>+canBeQueried+=+nvc.OfType<string>();
++++IEnumerable<string>+query+=
+++++++canBeQueried.Where(s+=>+s.StartsWith("abc"));
}|352184|entityMap|0|LINK|mutability|MUTABLE|url|http://msdn.microsoft.com/en-us/library/bb360913.aspx^0|0|B|K|E|Z|J|1L|B|0|0|0|2|6|0|0|0^^$0|@$1|2|3|4|5|6|7|Y|8|@$9|Z|A|10|B|C]|$9|11|A|12|B|C]|$9|13|A|14|B|C]|$9|15|A|16|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|17|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|18|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|19|8|@]|D|@$9|1A|A|1B|1|1C]]|E|$]]|$1|O|3|P|5|J|7|1D|8|@]|D|@]|E|$K|L]]|$1|Q|3|-4|5|6|7|1E|8|@]|D|@]|E|$]]]|R|$S|$5|T|U|V|E|$W|X]]]]

<code>AsQueryable</code> must take an <code>IEnumerable&lt;T&gt;</code>, a generic. <code>NameValueCollection</code> implements <code>IEnumerable</code>, which is different.

Instead of this:

<pre><code>{
 NameValueCollection nvc = RequestFields();
 IQueryable queryable = nvc.AsQueryable();
}
</code></pre>

Try <a href="http://msdn.microsoft.com/en-us/library/bb360913.aspx" rel="noreferrer">OfType</a> (it accepts the non-generic interface)

<pre><code>{
 NameValueCollection nvc = RequestFields();
 IEnumerable&lt;string&gt; canBeQueried = nvc.OfType&lt;string&gt;();
 IEnumerable&lt;string&gt; query =
 canBeQueried.Where(s =&gt; s.StartsWith("abc"));
}
</code></pre>

blocks|key|352204|text|您需要将非泛型IEnumerable“提升”为IEnumerable<string>。有人建议您使用OfType，但这是一种过滤方法。您所做的等同于强制转换，其中包含Cast运算符：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|352205|var+fields+=+RequestFields().Cast<string>();|code-block|syntax|javascript|352206|正如Frans所指出的，这只提供了对密钥的访问。您仍然需要为这些值索引到集合中。下面是一个从NameValueCollection中提取KeyValuePair的扩展方法|352207|public+static+IEnumerable<KeyValuePair<string,+string>>+ToPairs(this+NameValueCollection+collection)
{
++++if(collection+==+null)
++++{
++++++++throw+new+ArgumentNullException("collection");
++++}

++++return+collection.Cast<string>().Select(key+=>+new+KeyValuePair<string,+string>(key,+collection[key]));
}|352208|编辑：为了响应@Ruben+Bartelink的请求，下面是如何使用ToLookup访问每个键的全套值|BOLD|352209|public+static+ILookup<string,+string>+ToLookup(this+NameValueCollection+collection)
{
++++if(collection+==+null)
++++{
++++++++throw+new+ArgumentNullException("collection");
++++}

++++var+pairs+=
++++++++from+key+in+collection.Cast<String>()
++++++++from+value+in+collection.GetValues(key)
++++++++select+new+{+key,+value+};

++++return+pairs.ToLookup(pair+=>+pair.key,+pair+=>+pair.value);
}|352210|或者，使用C#+7.0元组：|352211|public+static+IEnumerable<(String+name,+String+value)>+ToTuples(this+NameValueCollection+collection)
{
++++if(collection+==+null)
++++{
++++++++throw+new+ArgumentNullException("collection");
++++}

++++return
++++++++from+key+in+collection.Cast<string>()
++++++++from+value+in+collection.GetValues(key)
++++++++select+(key,+value);
}|352212|entityMap^0|7|B|N|J|1E|6|2B|4|0|0|1A|J|1W|C|0|0|0|3|Y|8|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Z|8|@$9|10|A|11|B|C]|$9|12|A|13|B|C]|$9|14|A|15|B|C]|$9|16|A|17|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|18|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|19|8|@$9|1A|A|1B|B|C]|$9|1C|A|1D|B|C]]|D|@]|E|$]]|$1|M|3|N|5|H|7|1E|8|@]|D|@]|E|$I|J]]|$1|O|3|P|5|6|7|1F|8|@$9|1G|A|1H|B|Q]|$9|1I|A|1J|B|C]]|D|@]|E|$]]|$1|R|3|S|5|H|7|1K|8|@]|D|@]|E|$I|J]]|$1|T|3|U|5|6|7|1L|8|@]|D|@]|E|$]]|$1|V|3|W|5|H|7|1M|8|@]|D|@]|E|$I|J]]|$1|X|3|-4|5|6|7|1N|8|@]|D|@]|E|$]]]|Y|$]]

You need to "lift" the non-generic <code>IEnumerable</code> to an <code>IEnumerable&lt;string&gt;</code>. It has been suggested that you use <code>OfType</code> but that is a filtering method. What you're doing is the equivalent of a cast, for which there is the <code>Cast</code> operator:

<pre><code>var fields = RequestFields().Cast&lt;string&gt;();
</code></pre>

As Frans pointed out, this only provides access to the keys. You would still need to index into the collection for the values. Here is an extension method to extract <code>KeyValuePair</code>s from the <code>NameValueCollection</code>:

<pre><code>public static IEnumerable&lt;KeyValuePair&lt;string, string&gt;&gt; ToPairs(this NameValueCollection collection)
{
 if(collection == null)
 {
 throw new ArgumentNullException("collection");
 }

 return collection.Cast&lt;string&gt;().Select(key =&gt; new KeyValuePair&lt;string, string&gt;(key, collection[key]));
}
</code></pre>

Edit: In response to @Ruben Bartelink's request, here is how to access the full set of values for each key using <code>ToLookup</code>:

<pre><code>public static ILookup&lt;string, string&gt; ToLookup(this NameValueCollection collection)
{
 if(collection == null)
 {
 throw new ArgumentNullException("collection");
 }

 var pairs =
 from key in collection.Cast&lt;String&gt;()
 from value in collection.GetValues(key)
 select new { key, value };

 return pairs.ToLookup(pair =&gt; pair.key, pair =&gt; pair.value);
}
</code></pre>

Alternatively, using C# 7.0 tuples:

<pre><code>public static IEnumerable&lt;(String name, String value)&gt; ToTuples(this NameValueCollection collection)
{
 if(collection == null)
 {
 throw new ArgumentNullException("collection");
 }

 return
 from key in collection.Cast&lt;string&gt;()
 from value in collection.GetValues(key)
 select (key, value);
}
</code></pre>

blocks|key|354679|text|字典实际上可能更接近您想要使用的内容，因为它实际上会填充NameValueCollection所填充的更多角色。这是Bryan+Watts解决方案的一个变体：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|354680|public+static+class+CollectionExtensions
{
++++public+static+IDictionary<string,+string>+ToDictionary(this+NameValueCollection+source)
++++{
++++++++return+source.Cast<string>().Select(s+=>+new+{+Key+=+s,+Value+=+source[s]+}).ToDictionary(p+=>+p.Key,+p+=>+p.Value);+
++++}
}|code-block|syntax|javascript|354681|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 dictionary is probably actually closer to what you want to use since it will actually fill more of the roles that NameValueCollection fills. This is a variation of Bryan Watts' solution:

<pre><code>public static class CollectionExtensions
{
 public static IDictionary&lt;string, string&gt; ToDictionary(this NameValueCollection source)
 {
 return source.Cast&lt;string&gt;().Select(s =&gt; new { Key = s, Value = source[s] }).ToDictionary(p =&gt; p.Key, p =&gt; p.Value); 
 }
}
</code></pre>

blocks|key|3350235|text|对我来说，@Bryan+Watts‘(%2B1'd)+answer的ToLookup变体代表了迄今为止在只读基础上使用它的最清晰的方法。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|3350236|对于我的用例，我正在操作一个查询字符串以用于Linq2Rest，最后还需要将其转换回一个NameValueCollection，因此我有一组用于NameValueCollection的扩展方法，它们提供了更细粒度的操作(按参数名称(AsEnumerable)和按参数(AsKeyValuePairs)操作)，以及将其转换回ToNameValueCollection+(从任一表示形式)的反向操作。|3350237|消耗示例：|3350238|public+static+NameValueCollection+WithoutPagingOperators(+this+NameValueCollection+that+)
{
++++return+that.AsEnumerable()
++++++++.Where(+@param+=>+@param.Key+!=+OdataParameters.Skip+
++++++++++&&+@param.Key+!=+OdataParameters.Top+)
++++++++.ToNameValueCollection();
}|code-block|syntax|javascript|3350239|代码：|3350240|using+System.Collections.Generic;
using+System.Collections.Specialized;
using+System.Linq;

public+static+class+NamedValueCollectionExtensions
{
++++public+static+IEnumerable<KeyValuePair<string,+string[]>>+AsEnumerable(+this+NameValueCollection+that+)
++++{
++++++++return+that
++++++++++++.Cast<string>()+//+doesn't+implement+IEnumerable<T>,+but+does+implement+IEnumerable
++++++++++++.Select(+(+item,+index+)+=>+//+enable+indexing+by+integer+rather+than+string
++++++++++++++++new+KeyValuePair<string,+string[]>(+item,+that.GetValues(+index+)+)+);+//+if+you+use+the+indexer+or+GetValue+it+flattens+multiple+values+for+a+key,+Joining+them+with+a+','+which+we+don't+want
++++}

++++public+static+IEnumerable<KeyValuePair<string,+string>>+AsKeyValuePairs(+this+IEnumerable<KeyValuePair<string,+string[]>>+that+)
++++{
++++++++return+that
++++++++++++.SelectMany(+item+=>
++++++++++++++++item.Value.Select(+value+=>
++++++++++++++++++++new+KeyValuePair<string,+string>(+item.Key,+value+)+)+);
++++}

++++public+static+NameValueCollection+ToNameValueCollection(+this+IEnumerable<KeyValuePair<string,+string[]>>+that+)
++++{
++++++++return+that.AsKeyValuePairs().ToNameValueCollection();
++++}

++++public+static+NameValueCollection+ToNameValueCollection(+this+IEnumerable<KeyValuePair<string,+string>>+that+)
++++{
++++++++var+result+=+new+NameValueCollection();
++++++++foreach+(+KeyValuePair<string,+string>+item+in+that+)
++++++++++++result.Add(+item.Key,+item.Value+);
++++++++return+result;
++++}
}|3350241|entityMap|0|LINK|mutability|MUTABLE|url|http://nuget.org/packages/linq2rest^0|W|8|0|18|J|20|J|38|C|3Q|F|4H|L|M|9|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|10|8|@$9|11|A|12|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|13|8|@$9|14|A|15|B|C]|$9|16|A|17|B|C]|$9|18|A|19|B|C]|$9|1A|A|1B|B|C]|$9|1C|A|1D|B|C]]|D|@$9|1E|A|1F|1|1G]]|E|$]]|$1|H|3|I|5|6|7|1H|8|@]|D|@]|E|$]]|$1|J|3|K|5|L|7|1I|8|@]|D|@]|E|$M|N]]|$1|O|3|P|5|6|7|1J|8|@]|D|@]|E|$]]|$1|Q|3|R|5|L|7|1K|8|@]|D|@]|E|$M|N]]|$1|S|3|-4|5|6|7|1L|8|@]|D|@]|E|$]]]|T|$U|$5|V|W|X|E|$Y|Z]]]]

For me, @Bryan Watts' (+1'd) answer's <code>ToLookup</code> variant represents by far the clearest approach for using it on a read-only basis.

For my use case, I'm manipulating a query string for use with <a href="http://nuget.org/packages/linq2rest" rel="nofollow">Linq2Rest</a> and also need to turn it all back into a <code>NameValueCollection</code> at the end, so I have a set of extension methods for <code>NameValueCollection</code> which offer more granular operations (to operate both per parameter name (<code>AsEnumerable</code>) and per argument (<code>AsKeyValuePairs</code>)) and also the inverse operation of converting it back <code>ToNameValueCollection</code> (from either representation)).

Example consumption:

<pre><code>public static NameValueCollection WithoutPagingOperators( this NameValueCollection that )
{
 return that.AsEnumerable()
 .Where( @param =&gt; @param.Key != OdataParameters.Skip 
 &amp;&amp; @param.Key != OdataParameters.Top )
 .ToNameValueCollection();
}
</code></pre>

Code:

<pre><code>using System.Collections.Generic;
using System.Collections.Specialized;
using System.Linq;

public static class NamedValueCollectionExtensions
{
 public static IEnumerable&lt;KeyValuePair&lt;string, string[]&gt;&gt; AsEnumerable( this NameValueCollection that )
 {
 return that
 .Cast&lt;string&gt;() // doesn't implement IEnumerable&lt;T&gt;, but does implement IEnumerable
 .Select( ( item, index ) =&gt; // enable indexing by integer rather than string
 new KeyValuePair&lt;string, string[]&gt;( item, that.GetValues( index ) ) ); // if you use the indexer or GetValue it flattens multiple values for a key, Joining them with a ',' which we don't want
 }

 public static IEnumerable&lt;KeyValuePair&lt;string, string&gt;&gt; AsKeyValuePairs( this IEnumerable&lt;KeyValuePair&lt;string, string[]&gt;&gt; that )
 {
 return that
 .SelectMany( item =&gt;
 item.Value.Select( value =&gt;
 new KeyValuePair&lt;string, string&gt;( item.Key, value ) ) );
 }

 public static NameValueCollection ToNameValueCollection( this IEnumerable&lt;KeyValuePair&lt;string, string[]&gt;&gt; that )
 {
 return that.AsKeyValuePairs().ToNameValueCollection();
 }

 public static NameValueCollection ToNameValueCollection( this IEnumerable&lt;KeyValuePair&lt;string, string&gt;&gt; that )
 {
 var result = new NameValueCollection();
 foreach ( KeyValuePair&lt;string, string&gt; item in that )
 result.Add( item.Key, item.Value );
 return result;
 }
}
</code></pre>

blocks|key|3137360|text|我知道我来晚了，但我只是想补充一下我的答案，它不涉及.Cast扩展方法，而是使用AllKeys属性：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|3137361|var+fields+=+RequestFields().AllKeys;|code-block|syntax|javascript|3137362|这将允许使用以下扩展方法：|3137363|public+static+IEnumerable<KeyValuePair<string,+string>>+ToPairs(this+NameValueCollection+collection)
{
++++if(collection+==+null)
++++{
++++++++throw+new+ArgumentNullException("collection");
++++}

++++return+collection.AllKeys.Select(key+=>+new+KeyValuePair<string,+string>(key,+collection[key]));
}|3137364|希望这对任何未来的访问者有所帮助|3137365|entityMap^0|Q|5|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@$9|T|A|U|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|V|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|W|8|@]|D|@]|E|$]]|$1|M|3|N|5|H|7|X|8|@]|D|@]|E|$I|J]]|$1|O|3|P|5|6|7|Y|8|@]|D|@]|E|$]]|$1|Q|3|-4|5|6|7|Z|8|@]|D|@]|E|$]]]|R|$]]

I know I'm late to the party but just wanted to add my answer that doesn't involve the <code>.Cast</code> extension method but instead uses the AllKeys property:

<pre><code>var fields = RequestFields().AllKeys;
</code></pre>

This would allow the following extension method:

<pre><code>public static IEnumerable&lt;KeyValuePair&lt;string, string&gt;&gt; ToPairs(this NameValueCollection collection)
{
 if(collection == null)
 {
 throw new ArgumentNullException("collection");
 }

 return collection.AllKeys.Select(key =&gt; new KeyValuePair&lt;string, string&gt;(key, collection[key]));
}
</code></pre>

Hope this helps any future visitors

blocks|key|354744|text|我真的不明白为什么每个人都需要添加一个扩展方法。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|354745|这里有一些在VB.NET中实现的不同方法，包括4种不同的IEnumerable中间形式:数组、元组、匿名和KeyValuePair。对于等效的C#，请访问converter.telerik+.com并对其进行转换。|354746|Dim+nvc+As+New+NameValueCollection()+From+{{"E",+"55"},+{"A",+"11"},+{"D",+"44"},+{"C",+"33"},+{"G",+"66"},+{"B",+"22"}}

Dim+dictStrings+As+Dictionary(Of+String,+String)+=+nvc.Cast(Of+String).ToDictionary(Function(key)+key,+Function(key)+nvc(key))
Dim+Ints2Chars__+As+Dictionary(Of+Integer,+Char)+=+nvc.Cast(Of+Object).ToDictionary(Function(key)+CInt(nvc(CStr(key))),+Function(key)+CChar(key))

Dim+arrEnumerable__+=+From+x+In+nvc.Cast(Of+String)+Select+{x,+nvc(x)}
Dim+tupleEnumerable+=+From+x+In+nvc.Cast(Of+String)+Select+Tuple.Create(x,+nvc(x))
Dim+anonEnumerable_+=+From+X+In+nvc.Cast(Of+String)+Select+New+With+{X,+.Y+=+nvc(X)}
Dim+kvpEnumerable__+=+From+x+In+nvc.Cast(Of+String)+Select+New+KeyValuePair(Of+String,+String)(x,+nvc(x))

Dim+anonQuery+=+From+anon+In+anonEnumerable_+Let+n+=+CInt(anon.Y)+Order+By+n+Where+n+>+30+Select+New+With+{.num+=+n,+.val+=+anon.X}
Dim+dictQuery+=+anonQuery.ToDictionary(Of+Integer,+String)(Function(o)+o.num,+Function(o)+o.val)


Dim+dictArray_+=+arrEnumerable__.ToDictionary(Function(x)+x(0),+Function(x)+x(1))
Dim+dictTuples+=+tupleEnumerable.ToDictionary(Function(tuple)+tuple.Item1,+Function(tuple)+tuple.Item2)
Dim+dictAnon__+=+anonEnumerable_.ToDictionary(Function(anon)+anon.X,+Function(anon)+anon.Y)
Dim+dictKVPrs_+=+kvpEnumerable__.ToDictionary(Function(kvp)+kvp.Key,+Function(kvp)+kvp.Value)|code-block|syntax|javascript|354747|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|$]]

I don't really see why anyone would need to add an extension method. 
Here's some different ways to do it in VB.NET. It includes 4 different intermediate forms of IEnumerable: Array, Tuple, Anonymous, and KeyValuePair. For the C# equivalent go to converter.telerik dot com and convert it. 

<pre><code>Dim nvc As New NameValueCollection() From {{"E", "55"}, {"A", "11"}, {"D", "44"}, {"C", "33"}, {"G", "66"}, {"B", "22"}}

Dim dictStrings As Dictionary(Of String, String) = nvc.Cast(Of String).ToDictionary(Function(key) key, Function(key) nvc(key))
Dim Ints2Chars__ As Dictionary(Of Integer, Char) = nvc.Cast(Of Object).ToDictionary(Function(key) CInt(nvc(CStr(key))), Function(key) CChar(key))

Dim arrEnumerable__ = From x In nvc.Cast(Of String) Select {x, nvc(x)}
Dim tupleEnumerable = From x In nvc.Cast(Of String) Select Tuple.Create(x, nvc(x))
Dim anonEnumerable_ = From X In nvc.Cast(Of String) Select New With {X, .Y = nvc(X)}
Dim kvpEnumerable__ = From x In nvc.Cast(Of String) Select New KeyValuePair(Of String, String)(x, nvc(x))

Dim anonQuery = From anon In anonEnumerable_ Let n = CInt(anon.Y) Order By n Where n &gt; 30 Select New With {.num = n, .val = anon.X}
Dim dictQuery = anonQuery.ToDictionary(Of Integer, String)(Function(o) o.num, Function(o) o.val)


Dim dictArray_ = arrEnumerable__.ToDictionary(Function(x) x(0), Function(x) x(1))
Dim dictTuples = tupleEnumerable.ToDictionary(Function(tuple) tuple.Item1, Function(tuple) tuple.Item2)
Dim dictAnon__ = anonEnumerable_.ToDictionary(Function(anon) anon.X, Function(anon) anon.Y)
Dim dictKVPrs_ = kvpEnumerable__.ToDictionary(Function(kvp) kvp.Key, Function(kvp) kvp.Value)
</code></pre>

How to make <code>NameValueCollection</code> accessible to LINQ query operator such as where, join, groupby?

I tried the below:

<pre><code>private NameValueCollection RequestFields()
{
 NameValueCollection nvc = new NameValueCollection()
 {
 {"emailOption: blah Blah", "true"},
 {"emailOption: blah Blah2", "false"},
 {"nothing", "false"},
 {"nothinger", "true"}
 };
 return nvc;

}

public void GetSelectedEmail()
{
 NameValueCollection nvc = RequestFields();
 IQueryable queryable = nvc.AsQueryable();
}
</code></pre>

But I got an ArgumentException telling me that the source is not IEnumerable&lt;>.

Make NameValueCollection accessible to LINQ Query

如何使LINQ查询操作符，如where，join，groupby访问NameValueCollection？我尝试了以下几种方法：private NameValueCollection RequestFields(){    NameValueCollection nvc = new NameValueCollecti...

问使NameValueCollection可供链接查询访问
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