使LINQ Query可以访问NameValueCollection

Question

如何使NameValueCollectionLINQ查询运算符可以访问,例如where,join,groupby？

我试过以下:

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 <>.

Answer 1

你需要的"升降机"的非通用IEnumerable到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,有Cast运营商:

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

正如弗兰斯指出的那样,这只能提供对密钥的访问.您仍然需要为值集合索引.这是一个KeyValuePair从以下方法中提取s 的扩展方法NameValueCollection:

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]));
}

编辑:为响应@Ruben Bartelink的请求,以下是如何使用以下方法访问每个密钥的完整值集ToLookup:

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必须采取IEnumerable<T>一个通用的.NameValueCollection工具IEnumerable,这是不同的.

而不是这个:

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

试试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,因为它实际上将填充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]));
}

希望这有助于未来的访客