Why can't you use null as a key for a Dictionary?

Question

Apparently, you cannot use a null for a key, even if your key is a nullable type.

This code:

var nullableBoolLabels = new System.Collect         


        

Answer 1

Oftentimes you have to go back to C++ methodologies and techniques to fully understand how and why the .NET Framework works in a particular manner.

In C++, you oftentimes have to pick a key that will not be used - the dictionary uses this key to point to deleted and/or empty entries. For instance, you have a dictionary of , and after inserting an entry, you delete it. Rather than running the Garbage Cleanup right then and there, restructuring the dictionary, and leading to bad performance; the dictionary will just replace the KEY value with the key you previously selected, basically meaning "when you're traversing the dictionary memoryspace, pretend this pair does not exist, feel free to overwrite it."

Such a key is also used in dictionaries that pre-allocate space in buckets in a particular manner - you need a key to "initialize" the buckets with instead of having a flag for each entry that indicates whether or not its contents are valid. So instead of having a triple you would have a tuple with the rule being that if key == empty_key then it hasn't been initialized - and therefore you may not use empty_key as a valid KEY value.

You can see this sort of behavior in the Google hashtable (dictionary for you .NET people :) in the documentation here: http://google-sparsehash.googlecode.com/svn/trunk/doc/dense_hash_map.html

Look at the set_deleted_key and set_empty_key functions to get what I'm talking about.

I'd wager .NET uses NULL as either the unique deleted_key or empty_key in order to do these sort of nifty tricks that improve performance.