Dictionary<TKey,TValue> as a Dictionary key in C# 2.0

[dox]

Hello,

I'm having trouble using Dictionary classe. I want to use a Dictionary as a
key for another Dictionary. The following code gives a sample of what I'm
talking about.

Dictionary myDates = new Dictionary<Dictionary<string, string>, DateTime>();

The 'Add' or 'ContainsKey' methods need a Dictionary<string, string> as a
(first) parameter, but how to pass this kind of parameter without
instanciating another Dictionary<string, string> variable.

Any suggestion ?
Thanks

 

[Jon Skeet [C# MVP]]

I'm having trouble using Dictionary classe. I want to use a Dictionary as a
key for another Dictionary. The following code gives a sample of what I'm
talking about.

Dictionary myDates = new Dictionary<Dictionary<string, string>, DateTime>();

The 'Add' or 'ContainsKey' methods need a Dictionary<string, string> as a
(first) parameter, but how to pass this kind of parameter without
instanciating another Dictionary<string, string> variable.

What would you want to pass in as the parameter? What kind of key are
you wanting to add/test?

To be honest, Dictionary<TKey,TValue> makes a fairly unlikely hash key
- it doesn't override Equals or GetHashCode, so you have no equality
beyond identity.

Could you give more details about what you're trying to actually
achieve? What's the end goal?

 

[dox]

Here is my problem :
I have several server (identified by their names 'Server1', 'Server2'...)
and several message levels ('Information', 'Warning', 'Error'...).
I want for each couple 'server/message level' store the date of the last
message sent. I thougth about a structure like this :

"Server1"/"Information" -> 2008/03/12 14:00:00
"Server1"/"Error" -> 2008/03/11 09:55:00
"Server2"/"Error" -> 2008/02/27 10:11:00
....

In this structure, the key would be a Dictionary<string, string> identifying
the server and the message level. The value would be the DateTime. I know I
could achieve it in an easier way by concatenating the key
("Server1:Information") and using a Dictionary<string, DateTime>. I'm just
wondering if it is possible to use a Dictionary as a key, and how to use it.

 

[Jon Skeet [C# MVP]]

Here is my problem :
I have several server (identified by their names 'Server1', 'Server2'...)
and several message levels ('Information', 'Warning', 'Error'...).
I want for each couple 'server/message level' store the date of the last
message sent. I thougth about a structure like this :

"Server1"/"Information" -> 2008/03/12 14:00:00
"Server1"/"Error" -> 2008/03/11 09:55:00
"Server2"/"Error" -> 2008/02/27 10:11:00
...

In this structure, the key would be a Dictionary<string, string> identifying
the server and the message level. The value would be the DateTime. I know I
could achieve it in an easier way by concatenating the key
("Server1:Information") and using a Dictionary<string, DateTime>. I'm just
wondering if it is possible to use a Dictionary as a key, and how to use it.

You still wouldn't use a Dictionary as a key here. You'd use a
dictionary as the *value* of the outermost Dictionary:

Dictionary<string,Dictionary<string,DateTime>> foo;

Then you could do: Console.WriteLine(foo["Server1"]["Information"]).

However, you've already actually given the crux of a much better
solution, without realising it:

"I want for each couple 'server/message level'"

So, you want a type which encapsulates both the server and the message
level, and defines equality and hashcode based on that information
pair.

 

[dox]

You're right. After all, Dictionary as a key is probably not the best solution.
Let's write a custom Type, encapsulating both the server and message level.

Thanks a lot !

 

[Jon Skeet [C# MVP]]

You're right. After all, Dictionary as a key is probably not the best solution.

Not just that - it's not really a feasible one. Did you see what I
meant about using a dictionary as the first level *value* rather than
key?

Let's write a custom Type, encapsulating both the server and message level.

That would indeed be the best approach, IMO. Don't forget to override
both Equals and GetHashCode, in a consistent manner.