Appropriate c# collection for fast search by multy keys

Question

Hello i'm working on refactoring some legacy code. There some code that represents something like "converter" from custom type to c# type.

          ...
           if (dataType == CustomType.Bit)
            {
                return typeof(bool);
            }
            else if (dataType == CustomType.Bittype ||
                dataType == CustomType.Char ||
                dataType == CustomType.Fromtotype ||
                dataType == CustomType.Mdcintervaltype ||
                dataType == CustomType.Nclob ||
                dataType == CustomType.Nchar ||
                dataType == CustomType.Ntext ||
                dataType == CustomType.Nvarchar ||
                dataType == CustomType.Nvarchar2 ||
                dataType == CustomType.Varchar ||
                dataType == CustomType.Varchar2)
            {
                return typeof(string);
            }
            else if (dataType == CustomType.Date ||
                dataType == CustomType.Datetime ||
                dataType == CustomType.Timestamp3 ||
                dataType == CustomType.Timestamp6)
            {
                return typeof(DateTime);
            }
            else if (dataType == CustomType.Decimal ||
                dataType == CustomType.Money ||
                dataType == CustomType.Number ||
                dataType == CustomType.Numeric)
            {
                return typeof(decimal);
            }

...

Q: I'm looking for some C# structure (collection or not) that would be able to help me with fast search with one to many relonships . (ie i would like something that look like { Collection possible keys} ==> { value }

Ps I think that simple Dictionary where each Custromtype is key and return the same type isn't "beatiful"

new Dictionary<string, Type>()(){
    { CustomType.Bittype, typeof(string)},
    { CustomType.Fromtotype, typeof(string)}
     ...
    { CustomType.Datetime,  typeof(DateTime)},
    { CustomType.Date, typeof(DateTime)}
    ....
}

Answer 1

You can make Dictionary a bit more beautiful if you put string as a default type; another suggestion is to implement it as an extension method

 public static class CustomTypeExtensions {
   private static Dictionary<CustomType, Type> s_Map = new Dictionary<CustomType, Type>() {
     {CustomType.Datetime,  typeof(DateTime)},
     {CustomType.Date, typeof(DateTime}, 
     ...
   }; 

   public static Type ToType(this CustomType value) {
     if (s_Map.TryGetValue(value, out var result))
       return result;
     else
       return typeof(string); // when not found, return string 
   }
 }

....

var custom = CustomType.Bittype;

...

Type t = custom.ToType();   

Answer 2

Dmitry's approach is fine, also you can use something like this:

class Example
{
    private readonly HashSet<CustomType> _stringCompatibleTypes = new HashSet<CustomType>
    {
        CustomType.Char, CustomType.Fromtotype, CustomType.Nclob, ...
    };

    private readonly HashSet<CustomType> _dateCompatibleTypes = new HashSet<CustomType>
    {
        CustomType.Datetime, CustomType.Timestamp3, CustomType.Timestamp6, ...
    };

    // Another type sets

    public Type Foo(CustomType dataType)
    {
        if (_stringCompatibleTypes.Contains(dataType))
        {
            return typeof(string);
        }

        if (_dateCompatibleTypes.Contains(dataType))
        {
            return typeof(DateTime);
        }

        ...
    }
}