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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

Yes  you can use Linq to check the AllKeys property   using System Linq      collection AllKeys Contains key     However a Dictionary lt string  string   gt  would be far more suited to this purpose  perhaps created via an extension method   public static void Dictionary lt string  string   gt  ToDictionary this NameValueCollection collection         return collection Cast lt string gt    ToDictionary key   gt  key  key   gt  collection GetValues key       var dictionary   collection ToDictionary    if  dictionary ContainsKey key

User · Answer

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

User · Answer

This could also be a solution without having to introduce a new method       item   collection  item      null   collection  item   ToString     null

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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 lt string gt    Contains key  StringComparer OrdinalIgnoreCase

User · Answer

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

[c#] Check if Key Exists in NameValueCollection

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