Delegate to an instance method cannot have null 'this' discord.net

Question

I am developing a C# program in .NET 8

I have a generic host with a bunch of services for a discord bot. In one I have code like this (simplified code)

public MessageService(DiscordSocketClient client, ILogger<DiscordClientService> logger)
    : base(client, logger)
{
    client.MessageUpdated += OnMessageUpdated;
}
private Task OnMessageUpdated(Cacheable<IMessage, ulong> oldMessage, SocketMessage newMessage, ISocketMessageChannel channel)
{
    Task.Run(async () =>
    {
        if (newMessage is not SocketUserMessage message) return;
        var context = new SocketCommandContext(Client, message);

        if (context.Channel.Id == _botOptions.ChannelId)
            await CheckChannelAsync(context);  // <--- Error here
    }
    return Task.CompletedTask;
}
private async Task<bool> CheckChannelAsync(SocketCommandContext context)
{
    if (context.Message.Attachments.Count == 0)
    {
        string[] allowedSites = File.ReadAllLines(Utilities.GetLocalFilePath("AllowedSites.txt"));
        if (!Array.Exists(allowedSites, context.Message.Content.Contains))
        {
            string errorMessage = $"Your message doesn't contain any allowed website";
            await MessageDeleterHandlerAsync(context, errorMessage);
            return false;
        }
    }
    else
    {
        var attachment = context.Message.Attachments.First();
        if (attachment.Width < 40 || attachment.Height < 40)
        {
            string errorMessage = $"The picture is too small!!";
            await MessageDeleterHandlerAsync(context, errorMessage);
            return false;
        }
    }
    return true;

}

sometimes I get a burst of the same error triggering multiple times per minute and then it goes away for some days

System.ArgumentException: Delegate to an instance method cannot have null 'this'.

   at System.MulticastDelegate.ThrowNullThisInDelegateToInstance()
   at MyBot.Services.MessageService.CheckChannelAsync(SocketCommandContext context)
   at MyBot.Services.MessageService.<>c__DisplayClass15_0.<<OnMessageUpdated>b__0>d.MoveNext() in /my/bot/Services/MessageService.cs:line 101

I don't know what the error message means. I'm not using delegates as far as I know so I don't know what to check to find out

I have tried logging if context is null but unsurprisingly it isn't as I dereference it fine before the error happens

This is using Discord.NET

EDIT: added the logic for CheckChannelAsync, also I'm not creating a delegate in the same way as the duplicate question so that one doesn't answer my question

Answer 1

This problem occurs when create a delegate directly from a method-group, i.e.

Action action = someObj.SomeMethod; // note not invoked

and it turns out that someObj is null; this isn't a NullReferenceException, but it is basically comparable. So: look for a method-group conversion (to a delegate), which could be an event subscription, and check that the target object is not null.

I can't see any of these in your code - the only candidate is:

client.MessageUpdated += OnMessageUpdated;

which is implicitly

client.MessageUpdated += this.OnMessageUpdated;

but we can we reasonably sure that this is not null; I'm guessing the real cause is in the code not shown. Note that the cause could be hidden in things like LINQ, for example:

.Where(filter.Clause)

if this is IEnumerable<T>, Clause is a method-group (most likely bool Clause(T value)), and filter turns out to be null.