Creating a constant Dictionary in C#

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");