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It feels like you're conflating the Identifier for the Channel with its position within a ChannelSet. The following is my visualisation of how your code/comments would look at the moment :

public sealed class ChannelSet
{
    private Channel[] channels;
    /// <summary>Retrieves the specified channel</summary>
    /// <param name="channelId">The id of the channel to return</param>
    Channel GetChannel(int channelId)
    {
        return channels[channelId-1];
    }
}

It does feel like you've decided that because Channels within a ChannelSet are identified by numbers that have an upper and lower bound they must be indexes and therefore as it's C#, 0 based. If the natural way to refer to each of the channels is by a number between 1 and X, refer to them by a number between 1 and X. Don't try and force them into being indexes.

If you really want to provide a way to access them by 0 based index (what benefit does this give your end user, or developers that consume the code?) then implement an Indexer:

public sealed class ChannelSet
{
    private Channel[] channels;
    /// <summary>Retrieves the specified channel</summary>
    /// <param name="channelId">The id of the channel to return</param>
    public Channel GetChannel(int channelId)
    {
        return channels[channelId-1];
    }
    
    /// <summary>Return the channel at the specified index</summary>
    public Channel this[int index]
    {
        return channels[index];
    }
}