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

Answer 1

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

Answer 2

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;
        }

Answer 3

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).

Answer 4

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.

Answer 5

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);
}

Answer 6

Use this method:

private static bool ContainsKey(this NameValueCollection collection, string key)
{
    if (collection.Get(key) == null)
    {
        return collection.AllKeys.Contains(key);
    }

    return true;
}

It is the most efficient for NameValueCollection and doesn't depend on does collection contain null values or not.