How do I check if a user exists in Dsharplus (C# discord bot)

Basically I want if someone types "whois (mention user)" the bot to send a message like "(user mentioned in first message) is (random message from array)" I just need to know 2 things How do I only send a message if it's an actual mentioned user and How do I set a variable something only if a certain user (my disc account) is mention

        [Description("Mention someone after and it will tell you all about them")]
        public async Task WhoIs(CommandContext ctx, [Description("Mention a User")] string name)
        {   string[] descriptions = {"", "", "", "", ""};
            Random rng = new Random();
            int rngOk = rng.Next(descriptions.Length);
            //if (name == "a certain user")
            //{
            //    rngOk = 3;
            //}
            await ctx.Channel.SendMessageAsync(name + descriptions[rngOk]).ConfigureAwait(false);
        }

This is my code so far If you need any additional information, just ask Any help would be appreciated