Dictionary returning a default value if the key does not exist

Question

I find myself using the current pattern quite often in my code nowadays  var dictionary   new Dictionary lt type  IList lt othertype gt  gt        Add stuff to dictionary  var somethingElse   dictionary ContainsKey key    dictionary key    new List lt othertype gt        Do work with the somethingelse variable   Or sometimes  var dictionary   new Dictionary lt type  IList lt othertype gt  gt        Add stuff to dictionary  IList lt othertype gt  somethingElse  if  dictionary TryGetValue key  out somethingElse        somethingElse   new List lt othertype gt         Both of these ways feel quite roundabout  What I really would like is something like   dictionary GetValueOrDefault key    Now  I could write an extension method for the dictionary class that does this for me  but I figured that I might be missing something that already exists  SO  is there a way to do this in a way that is more  easy on the eyes  without writing an extension method to dictionary

User · Answer

No  nothing like that exists   The extension method is the way to go  and your name for it  GetValueOrDefault  is a pretty good choice

User · Answer

I created a DefaultableDictionary to do exactly what you are asking for   using System  using System Collections  using System Collections Generic  using System Collections ObjectModel   namespace DefaultableDictionary       public class DefaultableDictionary lt TKey  TValue gt    IDictionary lt TKey  TValue gt            private readonly IDictionary lt TKey  TValue gt  dictionary          private readonly TValue defaultValue           public DefaultableDictionary IDictionary lt TKey  TValue gt  dictionary  TValue defaultValue                this dictionary   dictionary              this defaultValue   defaultValue                     public IEnumerator lt KeyValuePair lt TKey  TValue gt  gt  GetEnumerator                 return dictionary GetEnumerator                       IEnumerator IEnumerable GetEnumerator                 return GetEnumerator                       public void Add KeyValuePair lt TKey  TValue gt  item                dictionary Add item                      public void Clear                 dictionary Clear                       public bool Contains KeyValuePair lt TKey  TValue gt  item                return dictionary Contains item                      public void CopyTo KeyValuePair lt TKey  TValue gt    array  int arrayIndex                dictionary CopyTo array  arrayIndex                      public bool Remove KeyValuePair lt TKey  TValue gt  item                return dictionary Remove item                      public int Count               get   return dictionary Count                       public bool IsReadOnly               get   return dictionary IsReadOnly                       public bool ContainsKey TKey key                return dictionary ContainsKey key                      public void Add TKey key  TValue value                dictionary Add key  value                      public bool Remove TKey key                return dictionary Remove key                      public bool TryGetValue TKey key  out TValue value                if   dictionary TryGetValue key  out value                     value   defaultValue                             return true                     public TValue this TKey key                get                               try                                       return dictionary key                     catch  KeyNotFoundException                        return defaultValue                                               set   dictionary key    value                       public ICollection lt TKey gt  Keys               get   return dictionary Keys                       public ICollection lt TValue gt  Values               get                               var values   new List lt TValue gt  dictionary Values                        defaultValue                                    return values                                     public static class DefaultableDictionaryExtensions           public static IDictionary lt TKey  TValue gt  WithDefaultValue lt TValue  TKey gt  this IDictionary lt TKey  TValue gt  dictionary  TValue defaultValue                 return new DefaultableDictionary lt TKey  TValue gt  dictionary  defaultValue                       This project is a simple decorator for an IDictionary object and an extension method to make it easy to use   The DefaultableDictionary will allow for creating a wrapper around a dictionary that provides a default value when trying to access a key that does not exist or enumerating through all the values in an IDictionary   Example  var dictionary   new Dictionary lt string  int gt    WithDefaultValue 5    Blog post on the usage as well

User · Answer

I know this is an old post and I do favor extension methods  but here s a simple class I use from time to time to handle dictionaries when I need default values    I wish this were just part of the base Dictionary class    public class DictionaryWithDefault lt TKey  TValue gt    Dictionary lt TKey  TValue gt      TValue  default    public TValue DefaultValue       get   return  default        set    default   value          public DictionaryWithDefault     base         public DictionaryWithDefault TValue defaultValue    base          default   defaultValue        public new TValue this TKey key          get          TValue t        return base TryGetValue key  out t    t    default            set   base key    value            Beware  however  By subclassing and using new  since override is not available on the native Dictionary type   if a DictionaryWithDefault object is upcast to a plain Dictionary  calling the indexer will use the base Dictionary implementation  throwing an exception if missing  rather than the subclass s implementation

User · Answer

TryGetValue will already assign the default value for the type to the dictionary  so you can just use   dictionary TryGetValue key  out value     and just ignore the return value  However  that really will just return default TValue   not some custom default value  nor  more usefully  the result of executing a delegate   There s nothing more powerful built into the framework  I would suggest two extension methods   public static TValue GetValueOrDefault lt TKey  TValue gt       this IDictionary lt TKey  TValue gt  dictionary        TKey key       TValue defaultValue        TValue value      return dictionary TryGetValue key  out value    value   defaultValue     public static TValue GetValueOrDefault lt TKey  TValue gt       this IDictionary lt TKey  TValue gt  dictionary       TKey key       Func lt TValue gt  defaultValueProvider        TValue value      return dictionary TryGetValue key  out value    value            defaultValueProvider         You may want to put argument checking in  of course

[c#] Dictionary returning a default value if the key does not exist

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