NameValueCollection で LINQ ref: http://qiita.com/wipiano/items/c70b84c5cc1b63db128b

file0.cs

public static NameValueCollection ParseQueryString(
    string query
)

NameValueCollectionExtensions.cs

public static class NameValueCollectionExtensions
{
    public static IEnumerable<KeyValuePair<string, IEnumerable<string>>> AsEnumerable(this NameValueCollection that)
    {
        return that.AllKeys
            .Select(key => new KeyValuePair<string, IEnumerable<string>>(key, that.GetValues(key)));
    }

    public static IEnumerable<KeyValuePair<string, string>> AsFlattenEnumerable(this NameValueCollection that)
    {
        return that.AllKeys
            .SelectMany(key => that.GetValues(key).Select(value => new KeyValuePair<string, string>(key, value)));
    }
}

Program.cs

class Program
{
    static void Main(string[] args)
    {
        var collection = new NameValueCollection()
        {
            {null, "nullValue1"},
            {null, "nullValue2"},
            {"", "emptyValue1"},
            {"hoge", "hogehoge"},
            {"fuga", "fugafuga"},
            {"fuga", "fugafuga2"}
        };

        // IEnumerable<KeyValuePair<string, IEnumerable<string>>>
        var enumerable = collection.AsEnumerable();
        Console.WriteLine(Utf8Json.JsonSerializer.ToJsonString(enumerable));

        // IEnumerable<KeyValuePair<string, string>>
        var flatten = collection.AsFlattenEnumerable();
        Console.WriteLine(Utf8Json.JsonSerializer.ToJsonString(flatten));
    }
}