Creating a constant Dictionary in C#

What is the most efficient way to create a constant (never changes at runtime) mapping of strings to ints?

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!

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.

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.

I am not sure why no one mentioned this but in C# for things that I cannot assign const, I use static read-only properties.

Example:

public static readonly Dictionary<string, string[]> NewDictionary = new Dictionary<string, string[]>()
        {
            { "Reference1", Array1 },
            { "Reference2", Array2 },
            { "Reference3", Array3 },
            { "Reference4", Array4 },
            { "Reference5", Array5 }
        };

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.

Just another idea since I am binding to a winforms combobox:

public enum DateRange {
    [Display(Name = "None")]
    None = 0,
    [Display(Name = "Today")]
    Today = 1,
    [Display(Name = "Tomorrow")]
    Tomorrow = 2,
    [Display(Name = "Yesterday")]
    Yesterday = 3,
    [Display(Name = "Last 7 Days")]
    LastSeven = 4,
    [Display(Name = "Custom")]
    Custom = 99
    };

int something = (int)DateRange.None;

To get the int value from the display name from:

public static class EnumHelper<T>
{
    public static T GetValueFromName(string name)
    {
        var type = typeof(T);
        if (!type.IsEnum) throw new InvalidOperationException();

        foreach (var field in type.GetFields())
        {
            var attribute = Attribute.GetCustomAttribute(field,
                typeof(DisplayAttribute)) as DisplayAttribute;
            if (attribute != null)
            {
                if (attribute.Name == name)
                {
                    return (T)field.GetValue(null);
                }
            }
            else
            {
                if (field.Name == name)
                    return (T)field.GetValue(null);
            }
        }

        throw new ArgumentOutOfRangeException("name");
    }
}

usage:

var z = (int)EnumHelper<DateRange>.GetValueFromName("Last 7 Days");

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);
}

As of C# 8, the new switch expression is the cleanest way to achieve the result:

int value = inputString switch {
	"one" => 1,
	"two" => 2,
	_ => -1
};

or as a function

int GetValue(string inputString) => inputString switch {
    "one" => 1,
    "two" => 2,
    _ => -1
};

It is more concise that the "old" one:

int value = -1;
switch (inputString){
    case "one": value=1; break;
    case "two": value=2; break;
}

The difference in performance of the old and new one can vary across different compilers

Why not:

public class MyClass
{
private Dictionary<string, int> _myCollection = new Dictionary<string, int>() { { "A", 1 }, { "B", 2 }, { "C", 3 } };



public IEnumerable&lt;KeyValuePair&lt;string,int&gt;&gt; MyCollection
{
    get { return _myCollection.AsEnumerable&lt;KeyValuePair&lt;string, int&gt;&gt;(); }
}




}
}