What happens to C Dictionary int int lookup if the key does not exist

Question

I tried checking for null but the compiler warns that this condition will never occur   What should I be looking for

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You should check for Dictionary ContainsKey int key  before trying to pull out the value   Dictionary lt int  int gt  myDictionary   new Dictionary lt int  int gt     myDictionary Add 2 4   myDictionary Add 3 5    int keyToFind   7  if myDictionary ContainsKey keyToFind         myValueLookup   myDictionay keyToFind          do work      else          the key doesn t exist

User · Answer

The Dictionary throws a KeyNotFound exception in the event that the dictionary does not contain your key   As suggested  ContainsKey is the appropriate precaution   TryGetValue is also effective   This allows the dictionary to store a value of null more effectively   Without it behaving this way  checking for a null result from the    operator would indicate either a null value OR the non-existance of the input key which is no good

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int result  YourDictionaryName TryGetValue key  out int value    YourDictionaryName key    0    If the key is present in the dictionary  it returns the value of the key otherwise it returns 0   Hope  this code helps you

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Consider the option of encapsulating this particular dictionary and provide a method to return the value for that key   public static class NumbersAdapter       private static readonly Dictionary lt string  string gt  Mapping   new Dictionary lt string  string gt                  1      One             2      Two             3      Three              public static string GetValue string key                return Mapping ContainsKey key    Mapping key    key            Then you can manage the behaviour of this dictionary    For example here  if the dictionary doesn t have the key  it returns key that you pass by parameter

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If you re just checking before trying to add a new value  use the ContainsKey method   if   openWith ContainsKey  ht          openWith Add  ht    hypertrm exe        If you re checking that the value exists  use the TryGetValue method as described in Jon Skeet s answer

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Assuming you want to get the value if the key does exist  use Dictionary lt TKey  TValue gt  TryGetValue   int value  if  dictionary TryGetValue key  out value            Key was in dictionary   value  contains corresponding value    else           Key wasn t in dictionary   value  is now 0      Using ContainsKey and then the the indexer makes it look the key up twice  which is pretty pointless    Note that even if you were using reference types  checking for null wouldn t work - the indexer for Dictionary lt   gt  will throw an exception if you request a missing key  rather than returning null   This is a big difference between Dictionary lt   gt  and Hashtable

User · Answer

You should probably use   if myDictionary ContainsKey someInt          do something     The reason why you can t check for null is that the key here is a value type

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A helper class is handy   public static class DictionaryHelper       public static TVal Get lt TKey  TVal gt  this Dictionary lt TKey  TVal gt  dictionary  TKey key  TVal defaultVal   default TVal                 TVal val          if  dictionary TryGetValue key  out val                          return val                    return defaultVal

User · Answer

ContainsKey is what you re looking for

[c#] What happens to C# Dictionary<int, int> lookup if the key does not exist?

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