What can you use as keys in a C# dictionary?

c# dictionary types custom-type
static_rtti picture static_rtti · Sep 6, 2013 · Viewed 7.6k times · Source

I come from a python world where only hashable objects may be used as keys to a dictionary. Is there a similar restriction in C#? Can you use custom types as dictionary keys?

Answer

Hans Passant picture Hans Passant · Sep 6, 2013

The requirement for a dictionary key is that it is comparable and hashable. That's turtles all the way down in .NET, every type (other than pointer types) derives from System.Object and it is always comparable thanks to its Equals() method. And hashable thanks to its GetHashCode() method. So any .NET type automatically can be used as a key.

If you want to use your own type as the key then you only need to do something special if you want to re-define object identity. In other words, if you need the ability for two distinct objects to be equal. You'd then override the Equals() method, typically comparing fields of the object. And then you must also override GetHashCode(), equal objects must generate the same hash code.

If the type cannot be changed or you want to customize the behavior especially for the Dictionary then you can pass a custom IEqualityComparer<> to the constructor. Keep in mind that the quality of the hash code you generate with your own GetHashCode() determines the dictionary efficiency.

Related questions

What is the best way to iterate over a dictionary?

I've seen a few different ways to iterate over a dictionary in C#. Is there a standard way?

Read More
Collection was modified; enumeration operation may not execute

I can't get to the bottom of this error, because when the debugger is attached, it does not seem to occur. Collection was modified; enumeration operation may not execute Below is the code. This is a WCF server in a …

Read More
How do you sort a dictionary by value?

I often have to sort a dictionary, consisting of keys & values, by value. For example, I have a hash of words and respective frequencies, that I want to order by frequency. There is a SortedList which is good for …

Read More