PrefixService.cs

class Program
{
    private readonly PrefixService _prefixService = new PrefixService();
    
    //.....
    private IServiceProvider ConfigureServices()
    {
        var map = new ServiceCollection()
            .AddSingleton(_prefixService)
            //....
            ;
        return map.BuildServiceProvider();
    }
    
    private async Task HandleCommand(SocketMessage msg)
    {
        //...
        var prefix = _prefixService.GetPrefix(guild) ?? "!"; //default prefix
        if (msg.HasStringPrefix(prefix, ref argPos))
        {
            //....
        }
    }
}

public class PrefixService
{
    private readonly Dictionary<ulong, string> _prefixes = new Dictionary<ulong, string>();
    
    public string GetPrefix(IGuild guild) => _prefixes.TryGetValue(guild.Id, out var p) ? p : null;
}