Check if Key Exists in NameValueCollection

Question

Is there a quick and simple way to check if a key exists in a NameValueCollection without looping through it?

Looking for something like Dictionary.ContainsKey() or similar.

There are many ways to solve this of course. Just wondering if someone can help scratch my brain itch.

Edea

just use Dictionary if you want to do a lookup based on the key .... BTW: you could use the indexer on this class but this will do the looping itself - so no gain – Bongbongo

Answer 1

From MSDN:

This property returns null in the following cases:

1) if the specified key is not found;

So you can just:

NameValueCollection collection = ...
string value = collection[key];
if (value == null) // key doesn't exist

---

2) if the specified key is found and its associated value is null.

collection[key] calls base.Get() then base.FindEntry() which internally uses Hashtable with performance O(1).

Bullfighter

This property returns null in the following cases: 1) if the specified key is not found; and 2) if the specified key is found and its associated value is null. This property does not distinguish between the two cases. – Salpinx

@Andreas that's why it's better to store empty string instead of null – Bullfighter

@Salpinx OP doesn't say anything about such collision. – Bullfighter

Right @abatishchev, however the OP says 'checking if a key exist'. Taking null as key doesn't exist is not true. At the end there is no answer without a compromise (no loop, use empty strings) – Salpinx

@Bullfighter that's like saying 0 equals null... sry – Alfilaria

@Andreas: Not, it's not. If you're using NameValueCollection and want to workaround its behavior with returning null in 2 cases, you need to workaround it. – Bullfighter

@Bullfighter saying 0 equals null is the same workaround-solution when not being able to use Nullable<int> ... it works ... but somehow unsatisfying :) – Alfilaria

I chose this as it's the simplest - and fit my needs at the time. I wasn't storing nulls. Do note the "compromise" comment by @Salpinx – Edea

Answer 2

This method handles the case when key is within the collection and its associated value is null.

private static bool ContainsKey(this NameValueCollection collection, string key) =>
    collection.Get(key) is not null || collection.AllKeys.Contains(key);

Starting from C# 9 you can use is not null, otherwise use != null

Millsaps

Remember to using System.Linq; when using this solution. – Balneal

Answer 3

Yes, you can use Linq to check the AllKeys property:

using System.Linq;
...
collection.AllKeys.Contains(key);

However a Dictionary<string, string[]> would be far more suited to this purpose, perhaps created via an extension method:

public static void Dictionary<string, string[]> ToDictionary(this NameValueCollection collection) 
{
    return collection.Cast<string>().ToDictionary(key => key, key => collection.GetValues(key));
}

var dictionary = collection.ToDictionary();
if (dictionary.ContainsKey(key))
{
   ...
}

Flanigan

That will loop over the whole collection which is O(n). While collection[key] internally uses Hashtable which is O(1) – Bullfighter

@Bullfighter Indeed, however collection[key] makes no distinction between the key not being present and a null value being stored against that key. – Virg

Also you can preform a dirty hack and retrieve private field of Hashtable using Reflection. – Bullfighter

I think this is a pretty silly solution. If someone is using NameValueCollection it's likely for reasons that dictionary isn't supported, such as having a null key. – Gazpacho

Answer 4

I don't think any of these answers are quite right/optimal. NameValueCollection not only doesn't distinguish between null values and missing values, it's also case-insensitive with regards to it's keys. Thus, I think a full solution would be:

public static bool ContainsKey(this NameValueCollection @this, string key)
{
    return @this.Get(key) != null 
        // I'm using Keys instead of AllKeys because AllKeys, being a mutable array,
        // can get out-of-sync if mutated (it weirdly re-syncs when you modify the collection).
        // I'm also not 100% sure that OrdinalIgnoreCase is the right comparer to use here.
        // The MSDN docs only say that the "default" case-insensitive comparer is used
        // but it could be current culture or invariant culture
        || @this.Keys.Cast<string>().Contains(key, StringComparer.OrdinalIgnoreCase);
}

Bebebebeerine

I like this solution. Looking at the NameValueCollectionBase source it defaults to using InvariantCultureIgnoreCase, however that doesn't mean that a different one isn't passed in to use instead by whatever class creates an instance of the NameValueCollection. – Officialese

Answer 5

queryItems.AllKeys.Contains(key)

Be aware that key may not be unique and that the comparison is usually case sensitive. If you want to just get the value of the first matching key and not bothered about case then use this:

        public string GetQueryValue(string queryKey)
        {
            foreach (string key in QueryItems)
            {
                if(queryKey.Equals(key, StringComparison.OrdinalIgnoreCase))
                    return QueryItems.GetValues(key).First(); // There might be multiple keys of the same name, but just return the first match
            }
            return null;
        }

Padang

Answer 6

You could use the Get method and check for null as the method will return null if the NameValueCollection does not contain the specified key.

See MSDN.

Odeen

You need to know index of the key to call the method. Don't you? – Bullfighter

Answer 7

If the collection size is small you could go with the solution provided by rich.okelly. However, a large collection means that the generation of the dictionary may be noticeably slower than just searching the keys collection.

Also, if your usage scenario is searching for keys in different points in time, where the NameValueCollection may have been modified, generating the dictionary each time may, again, be slower than just searching the keys collection.

Falange

Answer 8

This could also be a solution without having to introduce a new method:

    item = collection["item"] != null ? collection["item"].ToString() : null;

Arcograph

Having 2 lookups is not exactly efficient... – Pegeen

sure, assign collection["item"] to a variable then. – Arcograph

Answer 9

As you can see in the reference sources, NameValueCollection inherits from NameObjectCollectionBase.

So you take the base-type, get the private hashtable via reflection, and check if it contains a specific key.

For it to work in Mono as well, you need to see what the name of the hashtable is in mono, which is something you can see here (m_ItemsContainer), and get the mono-field, if the initial FieldInfo is null (mono-runtime).

Like this

public static class ParameterExtensions
{

    private static System.Reflection.FieldInfo InitFieldInfo()
    {
        System.Type t = typeof(System.Collections.Specialized.NameObjectCollectionBase);
        System.Reflection.FieldInfo fi = t.GetField("_entriesTable", System.Reflection.BindingFlags.Instance | System.Reflection.BindingFlags.NonPublic);

        if(fi == null) // Mono
            fi = t.GetField("m_ItemsContainer", System.Reflection.BindingFlags.Instance | System.Reflection.BindingFlags.NonPublic);

        return fi;
    }

    private static System.Reflection.FieldInfo m_fi = InitFieldInfo();


    public static bool Contains(this System.Collections.Specialized.NameValueCollection nvc, string key)
    {
        //System.Collections.Specialized.NameValueCollection nvc = new System.Collections.Specialized.NameValueCollection();
        //nvc.Add("hello", "world");
        //nvc.Add("test", "case");

        // The Hashtable is case-INsensitive
        System.Collections.Hashtable ent = (System.Collections.Hashtable)m_fi.GetValue(nvc);
        return ent.ContainsKey(key);
    }
}

for ultra-pure non-reflective .NET 2.0 code, you can loop over the keys, instead of using the hash-table, but that is slow.

private static bool ContainsKey(System.Collections.Specialized.NameValueCollection nvc, string key)
{
    foreach (string str in nvc.AllKeys)
    {
        if (System.StringComparer.InvariantCultureIgnoreCase.Equals(str, key))
            return true;
    }

    return false;
}

Carduaceous

Answer 10

In VB it's:

if not MyNameValueCollection(Key) is Nothing then
.......
end if

In C# should just be:

if (MyNameValueCollection(Key) != null) { }

Not sure if it should be null or "" but this should help.

Polaroid

Sorry that is in VB. C# should just be if (MyNameValueCollection(Key) != null) { } Not sure if it should be null or "" but this should help. – Polaroid

I believe the correct syntax is similar to a Dictionary data structure, as in MyNameValueCollection[Key], rather than MyNameValueCollection(Key), which expects a method call. – Fogy

Answer 11

I am using this collection, when I worked in small elements collection.

Where elements lot, I think need use "Dictionary". My code:

NameValueCollection ProdIdes;
string prodId = _cfg.ProdIdes[key];
if (string.IsNullOrEmpty(prodId))
{
    ......
}

Or may be use this:

 string prodId = _cfg.ProdIdes[key] !=null ? "found" : "not found";

Whitley

Answer 12

NameValueCollection n = Request.QueryString;

if (n.HasKeys())
   {
       //something
   }

Return Value Type: System.Boolean true if the NameValueCollection contains keys that are not null; otherwise, false. LINK

Ignorant

While this may answer the question, some explanation are often appreciated, especially to explain how this may be a good alternative to the many other answers. – Bostwick

This only checks if the collection contains any keys at all. The OP asked for the existence of a certain key. – Loftis