Discord.Net/docs/faq/samples/commands/DI.cs

public class MyService
{
	public string MyCoolString {get; set;}
}
public class Setup
{
	public IServiceProvider BuildProvider() => 
	new ServiceCollection()
	.AddSingleton<MyService>()
	.BuildServiceProvider();
}
public class MyModule : ModuleBase<SocketCommandContext>
{
	// Inject via public settable prop
	public MyService MyService {get; set;}
	
	// or via ctor
	private readonly MyService _myService;
	public MyModule (MyService myService) => _myService = myService;
	
	[Command("string")]
	public Task GetOrSetStringAsync(string input)
	{
		if (_myService.MyCoolString == null) _myService.MyCoolString = input;
		return ReplyAsync(_myService.MyCoolString);
	}
}