Dictionary with null key? [duplicate]

Question

Firstly, why doesn't Dictionary<TKey, TValue> support a single null key?

Secondly, is there an existing dictionary-like collection that does?

I want to store an "empty" or "missing" or "default" System.Type, thought null would work well for this.

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More specifically, I've written this class:

class Switch {     private Dictionary<Type, Action<object>> _dict;      public Switch(params KeyValuePair<Type, Action<object>>[] cases)     {         _dict = new Dictionary<Type, Action<object>>(cases.Length);         foreach (var entry in cases)             _dict.Add(entry.Key, entry.Value);     }      public void Execute(object obj)     {         var type = obj.GetType();         if (_dict.ContainsKey(type))             _dict[type](obj);     }      public static void Execute(object obj, params KeyValuePair<Type, Action<object>>[] cases)     {         var type = obj.GetType();          foreach (var entry in cases)         {             if (entry.Key == null || type.IsAssignableFrom(entry.Key))             {                 entry.Value(obj);                 break;             }         }     }      public static KeyValuePair<Type, Action<object>> Case<T>(Action action)     {         return new KeyValuePair<Type, Action<object>>(typeof(T), x => action());     }      public static KeyValuePair<Type, Action<object>> Case<T>(Action<T> action)     {         return new KeyValuePair<Type, Action<object>>(typeof(T), x => action((T)x));     }      public static KeyValuePair<Type, Action<object>> Default(Action action)     {         return new KeyValuePair<Type, Action<object>>(null, x => action());     } } 

For switching on types. There are two ways to use it:

1. Statically. Just call Switch.Execute(yourObject, Switch.Case<YourType>(x => x.Action()))
2. Precompiled. Create a switch, and then use it later with switchInstance.Execute(yourObject)

Works great except when you try to add a default case to the "precompiled" version (null argument exception).

Answer 1

1) Why: As described before, the problem is that Dictionary requires an implementation of the Object.GetHashCode() method. null does not have an implementation, therefore no hash code associated.

2) Solution: I have used a solution similar to a NullObject pattern using generics that enables you to use the dictionary seamlessly (no need for a different dictionary implementation).

You can will use it, like this:

var dict = new Dictionary<NullObject<Type>, string>(); dict[typeof(int)] = "int type"; dict[typeof(string)] = "string type"; dict[null] = "null type";  Assert.AreEqual("int type", dict[typeof(int)]); Assert.AreEqual("string type", dict[typeof(string)]); Assert.AreEqual("null type", dict[null]); 

You just need to create this struct once in a lifetime :

public struct NullObject<T> {     [DefaultValue(true)]     private bool isnull;// default property initializers are not supported for structs      private NullObject(T item, bool isnull) : this()     {         this.isnull = isnull;         this.Item = item;     }      public NullObject(T item) : this(item, item == null)     {     }      public static NullObject<T> Null()     {         return new NullObject<T>();     }      public T Item { get; private set; }      public bool IsNull()     {         return this.isnull;     }      public static implicit operator T(NullObject<T> nullObject)     {         return nullObject.Item;     }      public static implicit operator NullObject<T>(T item)     {         return new NullObject<T>(item);     }      public override string ToString()     {         return (Item != null) ? Item.ToString() : "NULL";     }      public override bool Equals(object obj)     {         if (obj == null)             return this.IsNull();          if (!(obj is NullObject<T>))             return false;          var no = (NullObject<T>)obj;          if (this.IsNull())             return no.IsNull();          if (no.IsNull())             return false;          return this.Item.Equals(no.Item);     }      public override int GetHashCode()     {         if (this.isnull)             return 0;          var result = Item.GetHashCode();          if (result >= 0)             result++;          return result;     } } 

Answer 2

It just hit me that your best answer is probably to just keep track of whether a default case has been defined:

class Switch {     private Dictionary<Type, Action<object>> _dict;     private Action<object> defaultCase;      public Switch(params KeyValuePair<Type, Action<object>>[] cases)     {         _dict = new Dictionary<Type, Action<object>>(cases.Length);         foreach (var entry in cases)             if (entry.Key == null)                 defaultCase = entry.Value;             else                 _dict.Add(entry.Key, entry.Value);     }      public void Execute(object obj)     {         var type = obj.GetType();         if (_dict.ContainsKey(type))             _dict[type](obj);         else if (defaultCase != null)             defaultCase(obj);     }  ... 

The whole rest of your class would remain untouched.