Handling precondition failures

A great feature of player preconditions is that they provide a centralized way to ensure that the player is in a certain state without having to write the same checks over and over again. However, if a precondition fails, the player will not be retrieved and an error will be returned. This error needs to be handled, otherwise the user will not know why the command failed.

Error handling

In the introduction of player preconditions, we used a central GetPlayerAsync method for the command module. We will now look at an example of how to handle precondition failures in this method which is done in the CreateErrorEmbed method, which we will have a look at.

private static Embed CreateErrorEmbed(PlayerResult<QueuedLavalinkPlayer> result)
{
    var title = result.Status switch
    {
        PlayerRetrieveStatus.UserNotInVoiceChannel => "You must be in a voice channel.",
        PlayerRetrieveStatus.BotNotConnected => "The bot is not connected to any channel.",
        PlayerRetrieveStatus.VoiceChannelMismatch => "You must be in the same voice channel as the bot.",

        PlayerRetrieveStatus.PreconditionFailed when result.Precondition == PlayerPrecondition.Playing => "The player is currently now playing any track.",
        PlayerRetrieveStatus.PreconditionFailed when result.Precondition == PlayerPrecondition.NotPaused => "The player is already paused.",
        PlayerRetrieveStatus.PreconditionFailed when result.Precondition == PlayerPrecondition.Paused => "The player is not paused.",
        PlayerRetrieveStatus.PreconditionFailed when result.Precondition == PlayerPrecondition.QueueEmpty => "The queue is empty.",

        _ => "Unknown error.",
    };

    return new EmbedBuilder().WithTitle(title).Build();
}

You can see that we check the Status property of the PlayerResult to determine the error message. If the status is PreconditionFailed, we can check the Precondition property to determine which precondition failed. If the status is not PreconditionFailed, we can check the Status property to determine the error message.