EnumExtensions.cs

using System;
using System.Linq;
using System.Collections.Generic;

/// <summary>
/// Enum extensions.
/// </summary>
public static class EnumExtensions
{
    /// <inheritdoc cref="EnumExtensions{TEnum}.Values"/>
    public static IReadOnlyList<TEnum> GetValues<TEnum>()
        where TEnum : Enum => EnumExtensions<TEnum>.Values;

    /// <inheritdoc cref="EnumExtensions{TEnum}.Names"/>
    public static IReadOnlyList<string> GetNames<TEnum>()
        where TEnum : Enum => EnumExtensions<TEnum>.Names;
    
    /// <inheritdoc cref="EnumExtensions{TEnum}.GetFlags(TEnum)"/>
    public static IReadOnlyList<TEnum> GetFlags<TEnum>(this TEnum @enum)
        where TEnum : Enum => EnumExtensions<TEnum>.GetFlags(@enum);

    /// <inheritdoc cref="EnumExtensions{TEnum}.HasFlags(TEnum, TEnum)"/>
    public static bool HasFlags<TEnum>(this TEnum @enum, TEnum flags)
        where TEnum : Enum => EnumExtensions<TEnum>.HasFlags(@enum, @flags);
    
    /// <inheritdoc cref="EnumExtensions{TEnum}.HasFlags(TEnum, IEnumerable{TEnum})"/>
    public static bool HasFlags<TEnum>(this TEnum @enum, IEnumerable<TEnum> flags)
        where TEnum : Enum => EnumExtensions<TEnum>.HasFlags(@enum, @flags);
}

/// <summary>
/// <typeparamref name="TEnum"/> extensions.
/// </summary>
public static class EnumExtensions<TEnum>
    where TEnum : Enum
{
    public static readonly Type Type = typeof (TEnum);
    
    /// <summary>
    /// All values of <typeparamref name="TEnum"/>.
    /// </summary>
    public static readonly IReadOnlyList<TEnum> Values =
        Enum.GetValues(Type)
            .Cast<TEnum>()
            .ToList()
            .AsReadOnly();

    /// <summary>
    /// All names of values of <typeparamref name="TEnum"/>.
    /// </summary>
    public static readonly IReadOnlyList<string> Names =
        Enum.GetNames(Type)
            .ToList()
            .AsReadOnly();

    private static Dictionary<TEnum, IReadOnlyList<TEnum>> flagsCache = new();
    
    /// <summary>
    /// Get all flags of <paramref name="enum"/>.
    /// </summary>
    /// <param name="enum" />
    public static IReadOnlyList<TEnum> GetFlags(TEnum @enum)
    {
        if (!flagsCache.TryGetValue(@enum, out var flags))
            flags = Values.Where(value => @enum.HasFlag(value))
                          .ToList()
                          .AsReadOnly();

        return flags;
    }

    /// <summary>
    /// Determine if <paramref name="enum"/> has all flags.
    /// </summary>
    /// <param name="enum" />
    public static bool HasFlags(TEnum @enum,
                                TEnum flags) => HasFlags(@enum, GetFlags(flags));
    
    /// <summary>
    /// Determine if <paramref name="enum"/> has all flags.
    /// </summary>
    /// <param name="enum" />
    public static bool HasFlags(TEnum @enum, 
                                IEnumerable<TEnum> flags) => flags.All(flag => @enum.HasFlag(flag));

    /// <summary>
    /// All values of <typeparamref name="TEnum"/> casted to <typeparamref name="T"/>.
    /// </summary>
    /// <typeparam name="T" />
    public static IList<T> GetValues<T>() => Values.Cast<T>()
                                                   .ToList();
}