Question

如何使NameValueCollection可以访问LINQ查询运算符，例如where，join，groupby？

我尝试了以下内容：

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();
}

但我得到一个 ArgumentException 告诉我源不是IEnumerable＆lt;＆gt; 。

Answer 1

您需要将非通用IEnumerable“解除”为IEnumerable<string>。有人建议您使用OfType，但这是一种过滤方法。你正在做的是相当于一个演员，其中有Cast运算符：

var fields = RequestFields().Cast<string>();

正如Frans所指出的，这只能提供对密钥的访问。您仍然需要为值集合索引。以下是从KeyValuePair：

中提取NameValueCollection的扩展方法

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]));
}

修改：为响应@Ruben Bartelink的请求，以下是使用ToLookup访问每个密钥的完整值的方法：

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);
}

或者，使用C＃7.0元组：

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);
}

Answer 2

AsQueryable必须使用IEnumerable<T>，一般。 NameValueCollection实现IEnumerable，这是不同的。

而不是：

{
    NameValueCollection nvc = RequestFields();
    IQueryable queryable = nvc.AsQueryable();
}

尝试OfType（它接受非通用接口）

{
    NameValueCollection nvc = RequestFields();
    IEnumerable<string> canBeQueried = nvc.OfType<string>();
    IEnumerable<string> query =
       canBeQueried.Where(s => s.StartsWith("abc"));
}

Answer 3

字典实际上可能更接近您想要使用的字典，因为它实际上会填充NameValueCollection填充的更多角色。这是Bryan Watts解决方案的变体：

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); 
    }
}

Answer 4

我知道我迟到了，但只是想添加我的答案，不涉及.Cast扩展方法，而是使用AllKeys属性：

var fields = RequestFields().AllKeys;

这将允许以下扩展方法：

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]));
}

希望这可以帮助任何未来的访客

Answer 5

问题是集合实现IEnumerable（而不是IEnumerable<T>）并枚举集合返回键，而不是对。

如果我是你，我会使用一个可枚举的Dictionary<string, string>，可以与LINQ一起使用。

Answer 6

对我来说，@ Bryan Watts'（+ 1'd）答案的ToLookup变体代表了迄今为止最简单的以只读方式使用它的方法。

对于我的用例，我正在操作一个查询字符串以便与Linq2Rest一起使用，并且还需要将其全部重新转换为NameValueCollection，所以我有一组扩展方法for NameValueCollection提供更细粒度的操作（每个参数名称（AsEnumerable）和每个参数（AsKeyValuePairs））以及将其转换回ToNameValueCollection的逆操作（来自任何一种表述））。

消费示例：

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

代码：

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;
    }
}

Answer 7

我真的不明白为什么有人需要添加扩展方法这是在VB.NET中执行此操作的一些不同方法。它包括4种不同的IEnumerable中间形式：Array，Tuple，Anonymous和KeyValuePair。对于C＃等价物，请转到converter.telerik dot com并进行转换。

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)

使LINQ Query可以访问NameValueCollection

7 个答案: