Make NameValueCollection accessible to LINQ Query

How to make NameValueCollection accessible to LINQ query operator such as where, join, groupby?

I tried the below:

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();
}

But I got an ArgumentException telling me that the source is not IEnumerable<>.

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

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

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 KeyValuePairs from the 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]));
}

Edit: In response to @Ruben Bartelink's request, here is how to access the full set of values for each key using 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);
}

Alternatively, using C# 7.0 tuples:

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);
}