Creating a constant Dictionary in C#
-
06-07-2019 - |
Question
What is the most efficient way to create a constant (never changes at runtime) mapping of string
s to int
s?
I've tried using a const Dictionary, but that didn't work out.
I could implement a immutable wrapper with appropriate semantics, but that still doesn't seem totally right.
For those who have asked, I'm implementing IDataErrorInfo in a generated class and am looking for a way to make the columnName lookup into my array of descriptors.
I wasn't aware (typo when testing! d'oh!) that switch accepts strings, so that's what I'm gonna use. Thanks!
Solution
Creating a truly compile-time generated constant dictionary in C# is not really a straightforward task. Actually, none of the answers here really achieve that.
There is one solution though which meets your requirements, although not necessarily a nice one; remember that according to the C# specification, switch-case tables are compiled to constant hash jump tables. That is, they are constant dictionaries, not a series of if-else statements. So consider a switch-case statement like this:
switch (myString)
{
case "cat": return 0;
case "dog": return 1;
case "elephant": return 3;
}
This is exactly what you want. And yes, I know, it's ugly.
OTHER TIPS
There are precious few immutable collections in the current framework. I can think of one relatively pain-free option in .NET 3.5:
Use Enumerable.ToLookup()
- the Lookup<,>
class is immutable (but multi-valued on the rhs); you can do this from a Dictionary<,>
quite easily:
Dictionary<string, int> ids = new Dictionary<string, int> {
{"abc",1}, {"def",2}, {"ghi",3}
};
ILookup<string, int> lookup = ids.ToLookup(x => x.Key, x => x.Value);
int i = lookup["def"].Single();
enum Constants
{
Abc = 1,
Def = 2,
Ghi = 3
}
...
int i = (int)Enum.Parse(typeof(Constants), "Def");
This is the closest thing you can get to a "CONST Dictionary":
public static int GetValueByName(string name)
{
switch (name)
{
case "bob": return 1;
case "billy": return 2;
default: return -1;
}
}
The compiler will be smart enough to build the code as clean as possible.
If using 4.5+ Framework I would use ReadOnlyDictionary (also ReadOnly Collection for lists) to do readonly mappings/constants. It's implemented in the following way.
static class SomeClass
{
static readonly ReadOnlyDictionary<string,int> SOME_MAPPING
= new ReadOnlyDictionary<string,int>(
new Dictionary<string,int>()
{
{ "One", 1 },
{ "Two", 2 }
}
)
}
Why not use namespaces or classes to nest your values? It may be imperfect, but it is very clean.
public static class ParentClass
{
// here is the "dictionary" class
public static class FooDictionary
{
public const string Key1 = "somevalue";
public const string Foobar = "fubar";
}
}
Now you can access .ParentClass.FooDictionary.Key1, etc.
There does not seem to be any standard immutable interface for dictionaries, so creating a wrapper seems like the only reasonable option, unfortunately.
Edit: Marc Gravell found the ILookup that I missed - that will allow you to at least avoid creating a new wrapper, although you still need to transform the Dictionary with .ToLookup().
If this is a need constrained to a specific scenario, you might be better off with a more business-logic-oriented interface:
interface IActiveUserCountProvider
{
int GetMaxForServer(string serverName);
}
Why not:
public class MyClass
{
private Dictionary<string, int> _myCollection = new Dictionary<string, int>() { { "A", 1 }, { "B", 2 }, { "C", 3 } };
public IEnumerable<KeyValuePair<string,int>> MyCollection
{
get { return _myCollection.AsEnumerable<KeyValuePair<string, int>>(); }
}
}