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PODictionary with null key?
 

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    <p>Firstly, <em>why</em> doesn't <code>Dictionary&lt;TKey, TValue&gt;</code> support a single null key?</p> <p>Secondly, is there an existing dictionary-like collection that does?</p> <p>I want to store an "empty" or "missing" or "default" <code>System.Type</code>, thought <code>null</code> would work well for this.</p> <hr> <p>More specifically, I've written this class:</p> <pre><code>class Switch { private Dictionary&lt;Type, Action&lt;object&gt;&gt; _dict; public Switch(params KeyValuePair&lt;Type, Action&lt;object&gt;&gt;[] cases) { _dict = new Dictionary&lt;Type, Action&lt;object&gt;&gt;(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&lt;Type, Action&lt;object&gt;&gt;[] 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&lt;Type, Action&lt;object&gt;&gt; Case&lt;T&gt;(Action action) { return new KeyValuePair&lt;Type, Action&lt;object&gt;&gt;(typeof(T), x =&gt; action()); } public static KeyValuePair&lt;Type, Action&lt;object&gt;&gt; Case&lt;T&gt;(Action&lt;T&gt; action) { return new KeyValuePair&lt;Type, Action&lt;object&gt;&gt;(typeof(T), x =&gt; action((T)x)); } public static KeyValuePair&lt;Type, Action&lt;object&gt;&gt; Default(Action action) { return new KeyValuePair&lt;Type, Action&lt;object&gt;&gt;(null, x =&gt; action()); } } </code></pre> <p>For switching on types. There are two ways to use it:</p> <ol> <li>Statically. Just call <code>Switch.Execute(yourObject, Switch.Case&lt;YourType&gt;(x =&gt; x.Action()))</code></li> <li>Precompiled. Create a switch, and then use it later with <code>switchInstance.Execute(yourObject)</code></li> </ol> <p>Works great <em>except</em> when you try to add a default case to the "precompiled" version (null argument exception).</p>
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    1. COpossible duplicate of [C# -- Need an IDictionary implementation that will allow a null key](http://stackoverflow.com/questions/1916814/c-need-an-idictionary-implementation-that-will-allow-a-null-key)
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