Check if Key Exists in NameValueCollection

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).