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.

Answer 1

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))
{
   ...
}

Answer 2

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.

Answer 3

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;
}

Answer 4

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 5

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

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

Answer 6

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;
        }