using System;

using System.Collections.Generic;

using System.Linq;

using System.Reflection;

public class Program

{

    public static void Main()

    {

        var flags = EnumHelper.GetFlags(typeof(A), 3);

        Console.WriteLine(string.Join(", ", flags));

    }

}

​

[Flags]

enum A

{

    First = 1, Second = 2

}

​

static class EnumHelper

{

    public static IEnumerable<Enum> GetFlags(Type enumType, int value)

    {

        if (enumType == null)

            throw new ArgumentNullException(nameof(enumType));

        if (!enumType.IsEnum)

            throw new ArgumentException(string.Empty, nameof(enumType));

        var attribute = enumType.GetCustomAttribute<FlagsAttribute>();

        if (attribute == null)

            throw new ArgumentException(string.Empty, nameof(enumType));

        var enumValue = (Enum) Enum.ToObject(enumType, value);

        return from Enum enumVal in enumType.GetEnumValues()

               where enumValue.HasFlag(enumVal)

               select enumVal;

    }

}