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DaveD
DaveD

Reputation: 2255

Building generic method for List<Enum>

In my application, I have something like:

public enum Locations {
  LocationA,
  LocationB,
  LocationC
}

private List<Locations> _myLocations;

public Int64 PackedLocations {
  get {
    return PackEnumList(this._myLocations);
  }
}

So: an enum (backed by int), a List of those enum values, and a read-only property which returns the result of a method I've left out so far.

That method, PackEnumList, is meant to give me a 64-bit integer where each BIT denotes whether the corresponding enum value was selected or not in a list of unique enum values. So in my example above, if _myLocations has only one item: {Locations.LocationA}, the result would be 1 (binary: ...00001), if we then add Locations.LocationC to that list, the result would be 5 (binary: ...000101). The implementation isn't important right now (but I'll include it below for completion/interest/feedback), but the signature of that method is:

public Int64 PackEnumList(List<Enum> listOfEnumValues) {
   ...
}

When I compile, I get an error that "the best overloaded method ... has some invalid arguments".

I'm guessing this is because _myLocations is being seen as a List of int values, but I'd like PackEnumList() to work even if the enumeration being used were backed by something else, if possible.

Is there a more appropriate way to make a method which will accept a List/Collection of any enumeration?

For completeness, here's the rest of what I'm trying to do (these are static because they're in a shared utility class). These are completely untested yet (since I can't get past the compile error when calling the pack method), so take them with a grain of salt. And there might be a better way to do this, I'm doing this half to solve an interesting problem, and half because I think it is an interesting problem.

    public static Int64 PackEnumList(List<Enum> listOfEnumValues) {
        BitArray bits = new BitArray(64, defaultValue: false);
        foreach (var value in listOfEnumValues) {
            // get integer value of each Enum in the List:
            int val = Convert.ToInt32(value);
            if (val >= 64) {
                // this enum has more options than we have bits, so cannot pack
                throw new Exception("Enum value out of range for packing: " + val.ToString());
            }
            bits[val] = true;
        }
        var res = new Int64[1];
        bits.CopyTo(res, 0);
        return res[0];
    }

    // (this method is a little farther from the ideal: the resulting list will need
    //     to be matched by the caller to the appropriate List of Enums by casting 
    //     each Int32 value to the Enum object in the list)
    public static List<Int32> UnpackEnumList(Int64 packedValue) {
        string binaryString = Convert.ToString(packedValue, 2);
        List<Int32> res = new List<Int32>();
        for (int pos = 0; pos < binaryString.Length; pos++) {
            if (binaryString[binaryString.Length - pos - 1] == '1') {
                // bit is on
                res.Add(pos);
            }
        }
        return res;
    }

Upvotes: 1

Views: 1125

Answers (4)

Dark Falcon
Dark Falcon

Reputation: 44201

A List<Enum> is not a List<Locations> nor a List<Int32>. Use a generic method to handle the list:

    public static void PackEnumList<T>(IEnumerable<T> list) where T : IConvertible
    {
        foreach (var value in list)
            int numeric = value.ToInt32();
        // etc.
    }

Upvotes: 1

hazzik
hazzik

Reputation: 13374

Change your method signature to public Int64 PackEnumList(IEnumerable<Enum> listOfEnumValues)

And then call it like following:

public Int64 PackedLocations 
{
    get { return PackEnumList(this._myLocations.Cast<Enum>()); }
}

Upvotes: 1

Psytronic
Psytronic

Reputation: 6113

I'd change your signature method to:

public Int64 PackEnumList<T>(IEnumerable<T> listOfEnumValues) where T : struct, IFormattable, IConvertible {
...
}

The where T : struct... constrains it to enum types only (any any other struct implementing both interfaces, which is probably very low)

Upvotes: 0

Jon Skeet
Jon Skeet

Reputation: 1504004

Is there a more appropriate way to make a method which will accept a List/Collection of any enumeration?

Within straight C#? Nope. But you can fudge it...

I have a project called Unconstrained Melody which allows you to make a generic method with a constraint of "T must be an enum type" or "T must be a delegate type". These are valid constraints at the IL level, but can't be expressed in C#

Basically Unconstrained Melody consists of two parts:

  • A library of useful methods with those constraints, where the source code is written using valid C# which doesn't actually represent those constraints, but uses a marker interface
  • An IL-rewriting project (ugly but servicable) which converts those constraints into the real "unspeakable" ones

(The expectation is that users of the library would just use the rewritten binary.)

It sounds like you could use the latter part of the project for your code here. It won't be terribly pleasant, but it would work. You might also find the library part useful.

As a side thought, you might want to consider using a [Flags]-style enum instead:

[Flags]
public enum Locations {
  LocationA = 1 << 0,
  LocationB = 1 << 1,
  LocationC = 1 << 2
}

Upvotes: 4

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