Discord.Net/docs/guides/samples/require_owner.cs

// (Note: This precondition is obsolete, it is recommended to use the RequireOwnerAttribute that is bundled with Discord.Commands)

// Inherit from PreconditionAttribute
public class RequireOwnerAttribute : PreconditionAttribute
{
    // Override the CheckPermissions method
    public async override Task<PreconditionResult> CheckPermissions(CommandContext context, CommandInfo command, IDependencyMap map)
    {
        // Get the ID of the bot's owner
        var ownerId = (await map.Get<DiscordSocketClient>().GetApplicationInfoAsync()).Owner.Id;
        // If this command was executed by that user, return a success
        if (context.User.Id == ownerId)
            return PreconditionResult.FromSuccess();
        // Since it wasn't, fail
        else
            return PreconditionResult.FromError("You must be the owner of the bot to run this command.");
    }
}