ServerManagers/src/ServerManager.Discord/Attributes/RequireRoleAttribute.cs

using System;
using System.Linq;
using System.Threading.Tasks;
using Discord.Commands;
using Discord.WebSocket;

namespace ServerManagerTool.DiscordBot.Attributes
{
    public class RequireRoleAttribute : PreconditionAttribute
    {
        private readonly string _name;

        public RequireRoleAttribute(string name)
        {
            _name = name;
        }

        public override Task<PreconditionResult> CheckPermissionsAsync(ICommandContext context, CommandInfo command, IServiceProvider services)
        {
            // Check if this user is a Guild User, which is the only context where roles exist
            if (context.User is SocketGuildUser guildUser)
            {
                // If this command was executed by a user with the appropriate role, return a success
                if (guildUser.Roles.Any(r => r.Name == _name))
                {
                    // Since no async work is done, the result has to be wrapped with `Task.FromResult` to avoid compiler errors
                    return Task.FromResult(PreconditionResult.FromSuccess());
                }
                else
                {
                    // Since it wasn't, fail
                    return Task.FromResult(PreconditionResult.FromError($"You must have a role named '{_name}' to run this command."));
                }
            }
            else
            {
                return Task.FromResult(PreconditionResult.FromError("You must be in a guild to run this command."));
            }
        }
    }
}